Citation
Bioacoustics, maternal investment, and developmental strategies in the mole crickets, Scapteriscus acletus and vicinus

Material Information

Title:
Bioacoustics, maternal investment, and developmental strategies in the mole crickets, Scapteriscus acletus and vicinus
Creator:
Forrest, Timothy G. ( Dissertant )
Walker, Thomas J. ( Thesis advisor )
Lloyd, James E. ( Reviewer )
Crump, Martha L. ( Reviewer )
Place of Publication:
Gainesville, Fla.
Publisher:
University of Florida
Publication Date:
Copyright Date:
1986
Language:
English
Physical Description:
viii, 84 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Acoustics ( jstor )
Clutches ( jstor )
Eggs ( jstor )
Female animals ( jstor )
Financial investments ( jstor )
Insects ( jstor )
Mating behavior ( jstor )
Oviposition ( jstor )
Seasons ( jstor )
Sound pressure ( jstor )
Dissertations, Academic -- Entomology and Nematology -- UF
Entomology and Nematology thesis Ph. D
Mole crickets ( lcsh )
Scapteriscus ( lcsh )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )
theses ( local )

Notes

Abstract:
Three aspects of the reproductive ecology of the southern and tawny mole crickets, Scapteriscus acletus and vicinus. were investigated. Twenty-two sound fields of calling males were measured under field conditions. Total power output ranged from 1.9 to 22.4 uWatt and was positively correlated with male size and soil moisture. Efficiency of sound production was only 0.02 to 0.16 percent. The shape of the sound field approximates a hemisphere. Lifetime oviposition by caged, individual female mole crickets was monitored outdoors. Oviposition is highly seasonal, and females that mature in the fall wait until the following spring to oviposit. Most eggs are laid in May and June and percent hatch of clutches decreases later in the season. Dry weight of eggs, number of eggs per clutch, and percent hatch decrease with successive clutches from individual females. Average investment in eggs per clutch ranged from 10 to 24 percent of female body weight for acletus and from 5 to 16 percent for vicinus females. Lifetime investment (dry weight of eggs) was greater than 100 percent of female body weight (wet weight) for 6 of 12 acletus females, but only 1 of 8 vicinus females had a lifetime investment in eggs equaling its body weight. In acletus. but not vicinus, large females produced more offspring and the average size of their egg clutches was larger than those of smaller females. Insect size tactics and developmental strategies are discussed in relation to decisions individuals make about when to mature. Such decisions carry with them costs and benefits in terms of an individual's fitness. Whenever size affects reproductive success, selection should act such that individuals evaluate the costs and benefits due to changes in size and mature when the cost/benefit ratio is maximized. Predictions about seasonal changes in adult sizes within a natural population of mole crickets are tested. The changes in seasonal distributions of adult sizes of mole crickets support the predictions and suggest that individuals make decisions about when to mature based on costs and benefits associated with changes in size.
Thesis:
Thesis (Ph. D.)--University of Florida, 1986.
Bibliography:
Bibliography: leaves 78-83.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Timonthy G. Forrest.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Resource Identifier:
029847770 ( AlephBibNum )
AEK6937 ( NOTIS )
015504368 ( OCLC )

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Full Text











BIOACOUSTICS, MATERNAL INVESTMENT,
AND DEVELOPMENTAL STRATEGIES
IN THE MOLE CRICKETS,
SCAPTERISCUS ACLETUS AND VICINUS






By

TIMOTHY G. FORREST


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


























I believe in fires at midnight...
Silken mist outside the window-
Frogs and newts slip in the dark
Too much hurry ruins a'body
I'll sit easy, fan the spark
(Ian Anderson)



To Sue,
thank you for everything.













ACKNOWLEDGMENTS

Now is the solstice of the year.
Winter is the glad song that you hear.
Ring out, ring Solstice Bells.
(Ian Anderson)

I am most appreciative of the guidance that Tom Walker and Jim Lloyd have

given me. Their high standards of scholarship and science have been truly inspiring.

I thank them both for what I have received in terms of knowledge, opportunity, and

friendship. My work will always be a product of their teachings. I wish to thank

Marty Crump for adding a new dimension to my interest in animal sound

production. Her knowledge of anuran biology has been most rewarding and has

made me look at insects in a different light. All three gave helpful suggestions that

much improved this dissertation.

Henry Bennet-Clark, Dave Green, Les Bernstein, and Ginny Richards have

all been helpful in transmitting their knowledge of sound and acoustics, without

which I would never have completed Chapter I. Ken Prestwich provided

unpublished data and his help with the energetic of sound production was

invaluable. Frank Slansky was extremely helpful with comments on Chapters II and

III that much improved their content. Bill Oldacre provided the schematics for the

portable multiplexing unit and was always willing to give his time. John Allen

furnished a computer program that made the mentally impossible visually possible.

Sue Wineriter gave many hours of work on many drafts of many figures. Her

expertise in art and illustration is much appreciated. I would also like to thank my

friends Will Hudson and Steve Wing for making this time and degree an enjoyable

and enlightening one.









I also wish to acknowledge my parents for their help when help was needed

most. And finally I thank my family. Sue, I thank for her devotion and dedication

that have given me confidence in myself and the drive to finish. Her many hours of

assistance in the field will always be remembered. Justin, I thank you for making

Sue and me very happy.















TABLE OF CONTENTS





ACKNOWLEDGMENTS iii

LIST OF FIGURES. vi

ABSTRACT vii

INTRODUCTION 1

CHAPTERS

I BIOACOUSTICS 3

Introduction . 3
Materials and Methods 21
Results 27
Discussion 32

II OVIPOSITION AND MATERNAL INVESTMENT 36

Introduction . 36
Materials and Methods 37
Results 39
Discussion 50

III SIZE TACTICS AND DEVELOPMENTAL STRATEGIES 55

Introduction 55
Size Tactics in Mole Crickets 62
Materials and Methods 63
Results 64
Discussion 70

CONCLUSIONS 76

REFERENCES CITED 78

BIOGRAPHICAL SKETCH 84













LIST OF FIGURES


Figure

1. Pressure variation in a 2 Hz sine wave. 6

2. Phase relationship between velocity
and displacement of a pendulum. 10

3. Acoustic interference and the
summation of sinusoids. 14

4. Sound produced by a vibrating disc.. 19

5. Relationship between total power output,
male size and soil moisture in Scapteriscus. 26

6. Total power output as a function of
sound pressure above the calling burrows of
Scaptcriscus acltus. 29

7. Sound field of Scapteriscus acletus. 31

8. Scapteriscus egg laying seasons. 41

9. Relationship between female size and
total offspring produced in acletus.. 45

10. Relationship between female size and
average number of eggs per clutch in acletus. 47

11. Effect of female age on mass of eggs produced in
Scapteriscus. 49

12. Costs and benefits associated with maturing. 58

13. Hypothetical cost/benefit curve associated
with changes in an individual's size. 60

14. Relationship between adult pronotal length
and mass in Scapteriscus. .66

15. Seasonal size distributions in mole crickets. 69

16. Seasonal change in size distribution in
mole crickets.. 72














Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


BIOACOUSTICS, MATERNAL INVESTMENT,
AND DEVELOPMENTAL STRATEGIES
IN THE MOLE CRICKETS,
SCAPTERJSCUS ACLETUS AND VICINUS


By

Timothy G. Forrest

December 1986

Chairman: Thomas J. Walker
Major Department: Entomology and Nematology

Three aspects of the reproductive ecology of the southern and tawny mole

crickets, Scapteriscus acletus and vicinus, were investigated.

Twenty-two sound fields of calling males were measured under field

conditions. Total power output ranged from 1.9 to 22.4 uWatt and was positively

correlated with male size and soil moisture. Efficiency of sound production was

only 0.02 to 0.16 percent. The shape of the sound field approximates a hemisphere.

Lifetime oviposition by caged, individual female mole crickets was

monitored outdoors. Oviposition is highly seasonal, and females that mature in the

fall wait until the following spring to oviposit. Most eggs are laid in May and June

and percent hatch of clutches decreases later in the season. Dry weight of eggs,

number of eggs per clutch, and percent hatch decrease with successive clutches from

individual females. Average investment in eggs per clutch ranged from 10 to 24









percent of female body weight for acletus and from 5 to 16 percent for vicinus

females. Lifetime investment (dr weight of eggs) was greater than 100 percent of

female body weight (wet weight) for 6 of 12 aclctus females, but only 1 of 8 vicinus

females had a lifetime investment in eggs equaling its body weight. In acletus, but

not vicinus, large females produced more offspring and the average size of their egg

clutches was larger than those of smaller females.

Insect size tactics and developmental strategies are discussed in relation to

decisions individuals make about when to mature. Such decisions carry with them

costs and benefits in terms of an individual's fitness. Whenever size affects

reproductive success, selection should act such that individuals evaluate the costs

and benefits due to changes in size and mature when the cost/benefit ratio is

maximized. Predictions about seasonal changes in adult sizes within a natural

population of mole crickets are tested. The changes in seasonal distributions of

adult sizes of mole crickets support the predictions and suggest that individuals

make decisions about when to mature based on costs and benefits associated with

changes in size.














INTRODUCTION
The southern and tawny mole crickets, Scapteriscus aeletus and vicinus, were
accidentally introduced into the United States around the turn of the century

(Walker and Nickle 1981). Because they lack a specialized predator/parasite

complex their populations have gone unchecked, their numbers have increased, and

they are one of the major pests of turf grasses and pasturelands in the southeast.

Their economic importance has prompted large amounts of funding for research

aimed at controlling mole crickets and lessening the damage caused by them. Only

recently has there been progress toward this goal. The primary stumbling block has

been an insufficient knowledge of the basic biology of these pest species. During

the past decade numerous researchers have contributed to our understanding of the

animals and their elusive subterranean lifestyle.

Mole cricket biology. Scapteriscus spp. are usually associated with habitats

that are temporary and early successional. In the southeastern United States both

acletus and vicinus are common in pastures, cultivated fields, lawns, and other turf.

All adults have long hindwings and are capable of flight. Dispersal flights occur

during the spring and fall, consist mostly of females, and are important to the

crickets' ecology in that they allow individuals to colonize newly opened areas or to

leave unsuitable or deteriorating ones (Ulagaraj 1975, Walker et al. 1983). Flying

crickets land in response to their species-specific calling song (Ulagaraj 1975,

Ulagaraj and Walker 1973, 1975). Adults are easily collected at traps broadcasting

such songs. Both males and females respond to the calls and preferentially land






2

near males whose calls are louder than those of neighboring males (Forrest 1980,

1983).

The two species differ markedly in their food habits with acletus being

primarily carnivorous on soil arthropods, earthworms, etc., whereas vicinus is

mainly an herbivore, feeding on roots as well as grass blades (Matheny 1981).

In Gainesville both species are univoltine; however, they differ in that about

85 percent of the vicinus population overwinter as adults compared with only 25

percent of the acletus population (Hayslip 1943). The rest of the populations

overwinter as late-instar juveniles that mature the following spring when eggs are

laid. Egg clutches are laid during the spring in underground ovoid chambers (4 by 3

cm) constructed by females. Once the eggs are laid, the chamber is sealed and no

parental guarding of eggs occurs. Eggs within a clutch hatch synchronously (within a

24-h period), and after a few days the young tunnel to the surface.

The topics that follow are part of the ongoing effort to understand mole

cricket populations and biology. The subjects add to the understanding of the

reproductive ecology of mole crickets and should be useful in monitoring changes in

the population and in designing models concerning mole cricket population

dynamics. Chapter I covers the bioacoustics of mole crickets and is an investigation

in the efficiency of acoustic communication by the two crickets. Chapter II deals

with female oviposition and the maternal investment by females and how these are

influenced by season, female size and female age. The third chapter is a study of

how individuals evaluate costs and benefits associated with size and how these

relate to the decisions of whether or not to mature.














CHAPTER I
BIOACOUSTICS

Let me bring you songs from the wood:
To make you feel much better than you
could know-
(lan Anderson)

Introduction

Since man's beginnings the sounds of animals have frightened, amused, and

intrigued him. With the advent of the tape recorder and audiospectrograph, he

could accurately quantify the sounds of animals, and in the last two decades the

interest and research in the areas of animal communication and bioacoustics have

increased dramatically. A new generation of digital recording and measuring

devices has made endeavors into bioacoustics even more reliable and from an

empirical standpoint more profitable. It is from the world of insects that such

research has really profited. Here there is a wealth of species with a wide variety of

sound producing and sound receiving mechanisms, many hundreds of which have

evolved independently, each one having been shaped by natural selection for

efficient communication purposes. A thorough background of the physical nature

of vibration and sound will be given and this understanding will be the basis for a

discussion of mole cricket bioacoustics and sound production.

Vibration and Sound

In simple terms sound is a disturbance or vibration in some elastic medium. This

disturbance need not be uniform or repetitive. Such a disturbance transmits energy

to the particles of the medium; the particles move from their resting position,









collide with neighboring particles and transmit the energy to them. These neighbors

transmit the energy to other neighbors and so on. Because of the elasticity of the

medium, the particles tend to return to their resting position after being displaced.

Thus the energy is transferred in waves away from the source of the disturbance.

Waves can be of two types depending on the axis the particles move relative to the

movement of the wavefront. When the particle motion is perpendicular to the

movement of the wave it is said to be transverse, whereas particle motion parallel to

the motion of the wave is called longitudinal. Sound waves in air are longitudinal;

waves on the surface of water are transverse.

The characteristics of a sound will depend upon the type of disturbance as

well as the physical characteristics of the medium. In most instances the medium is

air, although sound can be transmitted through liquids and solids and many insects

use these media for communication. For simplicity I will restrict my discussion to

sound in air, although the principles generally apply to all sound.

Pressure, power and intensity. A disturbance in air causes a change in the

density of molecules and in the pressure of the gas. These fluctuations in the

pressure of a sound wave are easily detected and measured with instruments, such

as microphone, that are sensitive to such pressure changes. Because of their ease of

measurement, changes in instantaneous pressures are commonly used to describe

sound. Perhaps the most well known sounds are sinusoids in which the pressure

fluctuates in a sinusoidal manner and can be described by the equation

p(t)=A*sin((2*pi*f*t)+theta) 1
where pressure (p) is a function of time (t), A is the amplitude of the pressure

change, pi is the constant 3.14159..., f is the frequency of the sine wave and theta is

the pase angle. Since the instantaneous pressure changes sinusoidally with respect

to both time and space, p(x), the pressure at some distance (x) from the source, can



























Figure 1. Pressure variation in a 2 Hz sine wave. Pressure, p, is a sinusoidal
function of time, t, and the wave has an amplitude of A. Positive
values of p(t) arc compressions and negative values are rarefactions.

















A



PRESSURE
p (t)









also be described by substituting x/c for t in the above equation, where c is the

velocity of sound in the medium. In air, c is about 330 meters per second. Note

that the plus and minus signs in the space formula are used to describe sound

travelling in opposite directions from the source.

Figure 1 shows the instantaneous pressure changes for a 2 Hz (cycles per

second) wave with initial phase angle theta=0. At t=0 and x=0 the pressure is 0,

although the pressure may have been any value between -A and A depending on the

initial phase angle. With an initial phase angle of 900 the pressure at t=0 is equal to

A. When pressure, p(t), is positive, the particles of the medium are compressed.

This part of the sound wave is called the compression. When the pressure is

negative, a vacuum occurs and this part of the wave is the rarefaction. The sinusoid

in Fig. 1 is repetitive every 1/2 second because the sin(0)=sin(2*pi); i.e. the period is

1/2 second and is equal to the reciprocal of the frequency for any sinusoid. The

wavelength (lambda) of the sinusoid is the distance the wave travels in one period

and equals period*c or c/f. The 2 Hz wave has a period of 1/2 second and a

wavelength of 115 meters. Insects typically use much higher frequencies with

shorter wavelengths. For instance the calling song of the southern mole cricket,

Scapteriscus acletus, has a carrier frequency of 2.7 kHz with a wavelength of 12 cm.

In dealing with sound one often wishes to know the power or intensity (I) of

the sound. Intensity differs from the scalar quantity of pressure, in that intensity is a

vector quantity having both magnitude and direction and its units are given in

energy per time per unit area or Watts per square meter (W/m2) whereas units of

pressure are in force times area (Newton-meter2: Nm2). Sound power, the total

power produced by a source, is measured in the same units as electrical power, the

Watt. Since pressure is an easily measured aspect of sound it is important to

understand the relationship between the scalar pressure and the vector intensity.









Intensity is the product of the instantaneous pressure (p) and particle velocity (u)

I=p(t)*u(t) or p*u. 2

This equation is analogous to the power equation used in electrical systems where

power is voltage times current.

Particle velocity describes the fluid movement of the medium. Near the

source particle velocity and pressure are 900 out of phase. This relationship is

particularly important and has relevance to measurement of pressure, and therefore

intensity. To illustrate this phase difference consider a pendulum (Fig. 2). At its

resting position the pendulum hangs vertically and there is no movement, no kinetic

energy and no potential energy. However, suppose the pendulum is displaced some

distance, x, from the resting position. If it is held at this position, the pendulum has

obtained some potential energy but has no kinetic energy. Releasing the pendulum

will cause it to swing toward its resting position and there is an exchange between

potential and kinetic energy. When the pendulum has reached its resting position

its velocity is maximum, its displacement a minimum. The pendulum then swings

past the resting position, and because of gravity the kinetic energy begins to change

back to potential energy. At the end of the swing the pendulum reaches its

maximum displacement (now -x) and its velocity has slowed to a minimum of zero.

It can be seen that the phases of displacement (=pressure) and velocity (=particle

velocity) are shifted by 900. The phase relationship changes as the sound moves

away from the source and can be described by

Tangent theta=lambda/r. 3

Lambda is the wavelength of sound, r is the distance from the source, and theta is

the phase angle (in radians) between particle velocity and pressure (Bennet-Clark

1971 and refs). It can be seen that the phase shift approaches 900 as r becomes zero

and the phase difference between particle velocity and pressure decreases to zero as

















() ....****








xD




DISPLACEMENT









the distance r increases. Though the boundaries are not well defined, the near field

is where pressure and particle velocity are not in phase, whereas in the far field

particle velocity and pressure are in phase. For practical purposes the far field

begins at distances greater than about 1 wavelength of the lowest frequency sound

produced by the source. In the far field the sound field is said to be purely active

(Gade 1982). This is useful because in the active field particle velocity can be

calculated from pressure using the acoustical equivalent to Ohm's law, current

equals the ratio of voltage to resistance:

u=p/poc, 4

where poc is the characteristic impedance of the medium, about 40 dyne*sec/cm3

for air (Gade 1982). Substituting p/poc for particle velocity in the intensity equation

(2) it can be seen that intensity is proportional to the square of the pressure

I=p2/Poe 5
and can thus be calculated using measurements of pressure-- provided these

measurements are taken in the far field.

Decibels. Pressure, power and intensity levels are often given in decibel

units. These units are logarithmic expressions of the ratio of any two sound

pressures or powers. Normally the levels are described relative to some standard

reference. For instance, the commonly used reference for power level is 10-12 W,

that for intensity is 10-12 W/m2, and that for pressure is 2x10-5 Nm2 = 2x10-5 Pa =

.0002 dyne/cm2 = .0002 ubar. All of these standards represent 0 decibels (dB).

Sound intensity, I in dB, can be calculated using the equation

I=10 log W/Wr 6

where W is the intensity and Wr is the reference or 10-12 W/m2. An intensity ratio

of 2 (i.e. W/Wr = 2) is 3 dB relative to the reference. A power level, PWL, can be






12


calculated using the same equation except that the reference is 10-12 W. Often the

terms power and intensity are interchanged although each has a specific meaning.

Similarly sound pressure level, SPL in dB, can be calculated by

SPL= 20 log P/Pr. 7

In this equation P is the pressure and Pr is the reference. A pressure ratio of 2

equals 6 dB relative to the reference. Note from the equations that for any given

level in dB the power ratio and intensity ratio, W/Wr, equals the square of the

pressure ratio, P/Pr. Since 0 dB corresponds to both references, a sound pressure

level can be converted into an intensity level directly, provided, of course, the

pressure is measured in the far field.

Acoustic interference. Consider two sound waves travelling in opposite

directions. One logical question to ask is: What happens when the two waves

meet? The resultant pressures are simply the sum of the pressures of the two waves.

This is graphically shown for a pair of equal-amplitude sinusoids in Figure 3. In

Fig. 3a the waves are approaching each other from opposite directions. In 3b the

waves are positioned in space so that a portion of the waves occupying the same

space are in phase (i.e. the compressions of one wave occurs with the compression

of the other) and the pressures add to produce pressure that is greater than the

pressure of each individual wave. This is called constructive interference. In 3c the

pressures sum to zero since the phases of the individual waves are opposite and they

exactly cancel. This condition is destructive interference. As each wave continues

on its way they are unchanged even though there were great pressure changes when

the two waves met. When two sounds interfere destructively the energy of the

sound is not lost. When the pressure decreases, the particle velocity increases and



























Figure 3. Acoustic interference and the summation of sinusoids.

a. Two equal-amplitude sine waves of the same frequency travelling
in opposite directions toward each other.
b. During this instant in time the two occupy the same space and the
pressure of the resultant sinusoid (solid line) is the sum of the two
individual waves. The resultant has an amplitude greater than either
of the individual waves; this condition is termed constructive
interference.

c. At this instant the individual waves are opposite in phase and their
pressures sum to zero (solid line). This is destructive interference.














PREURE


PRESSURE


c





PRESSURE


I
-----------
-- -- -- - .. .. .. .. .


I'r
: :
:I: : ::
:
:: :: -:I
: II '
:: /
I :; :: :: ::
: ::
I : : : i : : :
: : : :
i : : : :
::
\i :~?









there is a conservation of energy. The sum of any two sinusoids that have the same

frequency is another sinusoid of the same frequency but with a different amplitude.

Propagation and the inverse square law. In an ideal environment sound

waves radiate at the same rate in all directions from the source, and thus, the

wavefront is spherical in nature. Because the same acoustic energy must occupy a

greater surface area as the distance from the source increases, sound intensity

decreases with distance from the source, since intensity is measured in Watts per

unit area. The surface area of a sphere increases proportionately with the square of

its radius (4*pi*r2), and thus the intensity of a sound is inversely proportional to the

square of the distance from the source--the inverse square law.

Suppose the intensity is measured at some distance from the source. The

intensity at twice that distance will be one-fourth or about 6 dB less [10*log(0.25)].

If the power ratio is 0.25, the pressure ratio is 0.5. For the same doubling of

distance the pressure will also decrease by 6 dB [20*log(0.5)]. Pressure, the scalar

quantity, is proportional to the distance from the source. For every doubling of

distance the pressure is halved and it is this proportionality that will be used to

calculate sound fields of crickets.

Insect Bioacoustics

The study of insect bioacoustics, the special problems that insects have in

using acoustic signals to communicate, and the biophysical aspects of sound

production began more than 15 years ago and have been reviewed by

Bennet-Clark 1971, 1975, Michelsen and Nocke 1974, Michelsen 1983 and Elsner

and Popov 1978. The interest in the field has continued to grow primarily because

the animals are easily maintained, they are easily manipulated for experimental

purposes, and they offer a host of sound producing mechanisms to study.









Many insect groups use acoustic signals to communicate: ants do it, cicadas

do it, lacewings do it, and moths and beetles do it. The complexity of the

communication varies from very simple sounds of hitting a part of the body on the

substrate to sounds that are extremely complex with distinct signals for special

contexts.- For instance crickets have different types of calls classified as calling,

courtship, or aggressive (Alexander 1961). The different life stages of passalid

beetles use acoustic signals to communicate, adult to young and young to adult.

These beetles have a repertoire of as many as seven different sounds used in 13

different behavioral contexts (Schuster 1975). Some species of insects may use

acoustic signals to communicate to other species of animals--for example, many of

the tiger moths that use sounds to confuse and deceive bat predators (Fullard et al.

1979).

However, insects are very inefficient at producing sound because the

efficiency of sound production is dependent on the relationship between the size of

the oscillator or sound source, the frequency of the sound produced by the oscillator

and the immediate surroundings of the oscillator. To be an efficient radiator, the

diameter of the sound source must be about the size of, or larger than, the

wavelength of sound produced by the source. Because insects are small, and thus

their sound producing organs are small, they must produce a sound with very small

wavelengths to be efficient. That is, they must use high frequency sounds. However,

there is a disadvantage. High frequencies are attenuated by air at a faster rate than

low frequencies (Wiley and Richards 1978, Michelsen 1978). Therefore, given the

same power output for a high and low frequency signal, the intensity at the same

distance from the source will be greater for the low frequency than the high

frequency sound. Because of this, low frequency sounds are better for long distance









communication and insects must make a trade-off between the effective distance of

communication and the efficiency of signal production.

Insect muscles cannot contract at the high rate needed to produce

frequencies in the kHz range. Therefore, insects usually use some mechanism for

frequency multiplication. Most often this is accomplished by stridulation (Elsner

1983). During stridulation one part of the insect body is rubbed against another.

One part usually has a row of teeth, and during each stroke a number of teeth are

struck, each adding to the sound. The frequency of the muscle contraction is

multiplied by the number of teeth hit.

One of the most familiar groups of singing insects are the Orthoptera. This

group contains the grasshoppers, which produce song in at least three different ways

(Otte 1970). It includes cockroaches, some of which produce a hissing sound by the

expulsion of air from the spiracles (Nelson and Fraser 1980). But above all, the

crickets and katydids are the most noted and familiar singers of the group. Few

people have not heard a cricket.

Sound production in the Ensifera. The Ensifera are an ancient insect group

and include the Tettigoniidae (katydids), the Gryllidae (crickets), the Gryllotalpidae

(mole crickets) and the Haglidac. The forewings are similarly modified for sound

production in the four families. While calling the wings are raised above the

abdomen and moved back and forth one on top of the other. Located on the

underside of the Cu2 vein of each wing is a row of teeth called the pars stridens or

file. On the posterior margin of each wing is the plectrum or scraper. Stridulation

occurs during the closing stroke of the wings when the plectrum is pulled against the

grain across the teeth of the file. In katydids the left wing is usually over the right,

in crickets the right is usually over the left, while in haglids and mole crickets each

configuration occurs with equal frequency (Masaki et al. in press, Forrest in press).



























Figure 4. Sound produced by a vibrating disc.

a. As the disc moves to one side it compresses the particles of air and
at the same time produces a vacuum on the other side of the disc.
The shading of the circles represents the degree of displacement of
the particles (ie. black circles are compressions and no circles are
rarefactions).

b. Sound output and phase relationships of a dipole source.

c. Sound field of the dipole source. If the vibrating disc is small
relative to the wavelength of sound produced, the outputs from each
side of the disc will destructively interfere along the edges of the
disc. The result is a dumbbell shaped sound field.


















-,-/I I IIII
I II









In some species the wings have become specialized for their tasks and there is an

asymmetry between the left and right wings.

The file and scraper are closely associated with specific membranous areas of

the wings. The cell or cells are caused to vibrate at the frequency of the

tooth-impact rate of the scraper on the file. The cells are resonant near the

impact frequency, and the vibrations of the cells produce the sound.

The vibration of the wing membrane can be likened to the movement of a

piston or disc. As the disc moves back and forth in air, it produces a compression

and a rarefaction simultaneously on opposite sides of the disc. That is, a sound

wave is generated from each side of the disc, each being equal in amplitude and

frequency, but opposite in phase (Fig. 4). When the diameter of this doublet or

dipole source is small relative to the wavelength of sound produced, the sounds

radiating from the two sides of the disc destructively interfere along the edges of the

disc (Fig. 4b). The result is a dumbbell shaped sound field with a maximum

pressure perpendicular to the disc surface and a minimum pressure along its edges

(Fig. 4c). Of course the particle velocity at the edges is high and the intensity is

equal in all directions.

The membranes of crickets are less than one-tenth the wavelength of sound

produced, and they are inefficient primarily because of their size. Energetically,

producing sound may cost a male ten times as much as resting (Prestwich and

Walker 1981). However, because calling contributes importantly to a male's

reproductive success, selection should be strong on males to increase signaling

efficiency and effectiveness. Crickets have evolved a number of behaviors that use

baffles to increase calling efficiency (see Forrest 1982 and refs.). The study of these

baffles and their effect on sound production is one important aspect of cricket

mating systems and reproductive biology.









One such baffle system is used by the southern and tawny mole crickets,

Scapteriscus acletus and vicinus. Both species construct a calling burrow shortly

after sunset, prior to the female flight period. The burrow is shaped by the cricket

and expands exponentially from the throat to the opening at the soil surface

(Nickerson et al. 1979).

Because the males call from within a burrow, their sound fields are very

suitable for measuring. The position of the male in the burrow is constant, as is the

position of the burrow opening. The calling songs are continuous trills that change

less than 1 dB in sound pressure during the 1 to 1.5 hours of calling (Forrest 1981,

1983), and an observer can move around outside the burrow with little disturbance

to the calling male. If the animal is disturbed, he runs down into the burrow system

but returns in a short period of time to the same calling position and resumes

advertising for females.

I measured 22 sound fields of Scapteriscus males (17 acletus, 5 vicinus) under

field conditions to determine the power output of the crickets, the efficiency of

sound production and the directional properties of the sound field.

Materials and Methods

Sound pressure level readings were taken using nine Bruel & Kjaer, model

4125 condenser microphones and type 2642 preamplifier. The typical frequency

response for these microphones is flat (1 dB) from 200 Hz to 10 kHz for a free

field response and an angle of incidence of 00. Directional characteristics show less

than 1 dB loss for +300 deviations from a 00 incidence angle for signal frequencies

at 2, 4, and 8 kHz (Bruel & Kjaer specifications).

The output of each microphone could be adjusted independently, and each

microphone was calibrated using a model 4230 B&K pistonphone calibrator.

Initially, each microphone was calibrated after each sound field was measured.









Later the calibration was done every two weeks. The stability of the microphones

was such that later they were calibrated only every month. Never was the level of

any of the microphones more than 0.5 dB from the calibration level of 94 dB.

The outputs of the microphones were multiplexed to a single input of a

model 2219 Brucl & Kjaer sound level meter that measured the sound pressure re

0.0002 dyne/cm2 (dB A). The entire system was battery operated and field portable.

Cables from the microphones were long enough so that the observer and equipment

were more than a meter from the sound source and nearest microphone. This

avoided reflection of sound waves off the observer's body, and interference in the

pressure readings was reduced to a minimum.

The nine microphones were equally spaced around a semicircle of tubular

aluminum (every 22.50, +20) such that the diaphragm of each microphone was 25

cm from the center of the semicircle and the angle of incidence was zero (+2). An

angle of incidence of 00 is recommended when the direction of the sound source is

known (Brucl 1983). At 25 cm the microphones were always more than 1

wavelength of the carrier frequency of the calling song of the mole crickets (acletus

2.7 Khz or 12 cm wavelength, vicinus 3.3 Khz or 10 cm wavelength). Thus sound

pressure measurements were taken in the far field (Michelscn 1978).

By rotating the semicircle in 22.50 steps along one of the axes, sound

pressure levels could be taken at every 22.50 latitude and longitude of a hemisphere

surrounding the sound source located on the ground in the center of the semicircle.

A stand anchored one end of the hemisphere to the ground and a handle that

moved the semicircle locked every 22.50. Axis of rotation was always with the

longitudinal axis of the cricket so that on every 22.50 rotation the two microphones

at the ends of the semicircle remained at the same anterior and posterior position

relative to the cricket burrow. This arrangement provided a check for changes in









the crickets' output and added reliability to the measurements. Altogether, pressure

measurements were taken at 65 different positions surrounding the opening of the

cricket's burrow with the anterior and posterior positions each having 9 separate

measurements. Sound fields with more than +0.5 dB variation in these 9

measurements were not used in analysis. These measurements represent the most

detailed measurements of cricket sound fields.

All sound fields were measured outdoors at the Horticultural Farm of the

University of Florida. Sound fields of crickets were measured while they called in

the field or while they called from soil-filled, 12-liter buckets buried so that the

bucket lip and the soil in the bucket were at ground level. Only crickets calling on

level ground were measured and all obstacles that might cause interference or

reflections were removed from the area.

To determine the influence of male size on acoustic output, the males were

captured and the lengths of their pronota along the midline were measured to the

nearest 0.1 mm using vernier calipers. The mass of each male was measured to the

nearest 1.0 mg with a model AC100 Mettlcr balance. A soil sample (2 cm dia X 4.5

cm deep) was taken within 5 cm of the burrow opening. This sample was dried and

moisture content determined. Soil temperature at a depth of 2.5 cm was also taken.

Calculation of sound fields and power output. Each of the 65 sound pressure

levels measured at 25 cm was converted to a radial distance from the cricket at

which the pressure would be 70 dB. For instance a pressure that read 73 dB at 25

cm would be 70 dB at about 35 cm. The points were converted to polar coordinates

and the areas of all triangles defined by adjacent triads of points were summed to

obtain the surface area of the sound field. A test of this procedure revealed that it

underestimates the surface areas of 24-200 cm dia hemispheres by only 2-3%.









Table 1. Power input, output and efficiency of calling male Scaptcriscus.

PNL MASS TEMP PULSEa SOIL POWER PWL POWERb EFF.
RATE MOIST OUT IN
mm mg 0C p/sec uW dB mW %


Scapteriscus acletus

7.7 574 26.8 56.2
7.7 574 25.2 54.0
7.7 574 25.4 54.3
7.7 574 30.7 61.7
7.7 574 27.2 56.8

8.1 614 29.1 59.4
8.1 614 25.7 54.7

8.6 737 25.8 54.8
8.6 737 30.2 61.0
8.6 737 30.8 61.8

8.8 776 20.4 47.3

8.9 676 28.8 59.0
8.9 676 27.7 57.5

9.4 772 22.5 50.2

9.5 875 22.4 50.1

9.9 1172 19.1 45.4

10.0 1162 20.9 48.0

S. vicinus

8.3 890 N.A.
8.3 890 25.4 139.0

8.4 889 26.6 145.3

8.4 834 21.4 118.0

8.4 N.A. 21.2 116.9


0.081
0.099
N.A.
0.036
0.109

0.026
0.103

0.076
0.030
0.039

0.155

0.114
0.034

0.138

0.112

0.219

0.099


0.047
0.082

0.187

0.100

0.078


3.310
4.403
2.355
1.920
4.056

2.481
6.115

8.430
2.391
4.408

3.247

4.456
5.958

8.983

11.771

22.386

4.618


2.259
3.458

6.540

9.639


65.2 9.145
66.4 8.749
63.7 8.798
62.8 10.111
66.1 9.244

63.9 10.391
67.9 9.491

69.3 11.424
63.8 12.823
66.4 13.014

65.1 10.221

66.5 11.353
67.8 11.032

69.5 10.867

70.7 12.279

73.5 14.779

66.6 15.555


63.5
65.4 36.883

68.2 38.571

69.8 29.155


4.175 66.2


0.04
0.05
0.03
0.02
0.05

0.02
0.07

0.08
0.02
0.03

0.03

0.04
0.06

0.09

0.10

0.16

0.03



0.01

0.02

0.03


a Based on relationship to soil temperature (Ulagaraj 1976).
b Based on data from Prestwich and Walker 1981.
N.A.- Not available or data not taken.



























Figure 5. Relationship between total power output and male size and total power
and soil moisture in Scapteriscus.

a. Total power (dB re 10-12 W) as a function of male size (pronotal
length mm). Power (in Watts) of males with more than one sound
field measurement were averaged for regression analysis. Vertical
lines at these points show the range in power for individual qales.
Regression equation is y = 2.4x + 46.4 (acletus only p<0.05, r =0.45).
Open circles are acletus males, filled circles are vicinus males.

b. Total power as a function of soil moisture (percent of dry weight of
soil sample). Connected points are for sound fields by the same
male at different soil moistures. Percent soil moisture was log
transformed for regression analysis y = 6.2log(x) + 61.5 (acletus only
p<0.05, r =0.40).

























8.5 9.0 9.5
Pronotal Length (mm)


5.0 10.0 15.0 20.0
Soil Moisture


PWL (dB)


65

60 5
7.5


PWL (dB)


60 L
0.


10.5


25. 0


= = * * * * *


0









Since the measurements were taken in the far field, the 70 dB isobar

represents a 0.01 mW/m2 intensity level and the total power, in mW, is given by the

intensity level times the surface area.

Results

The acoustic power produced by calling male mole crickets ranged from 1.9 to 22.4

uW (62.8 73.5 dB re 10-12 W) for the 17 acletus sound fields and from 2.3 to 9.6

uW (63.5 69.8 dB) for the 5 vicinus sound fields (Table 1). Part of this variation in

the power output was due to the size of the male and the moisture in the soil

surrounding the male's burrow. There was a significant relationship between male

size and power level (p<0.05, r2=0.45; Fig. 5a, Note that regression was done only on

sound fields from acletus males and that the acoustic powers for individuals for

which more than one sound field was measured were averaged and used for

computing the regression). There was also a significant relationship between the

soil moisture and power level (acletus only p<0.05, r2=0.40; Fig. 5b, Percent soil

moisture was log transformed for regression analysis). The data from individual

males also showed the same trend. That is, their power output increased with

increasing soil moisture (5 out of 7 acletus and 1 of 1 vicinus). However, a check to

see if soil moisture and male size were correlated revealed no significant

relationship and large males did not produce more power simply because they

called from moister soil.

The power output of a male can be estimated by a single measurement of the

sound pressure level directly above the burrow. There was a direct relationship

between the sound pressure directly above the burrow and the power level of the

sound field produced (p<0.05, r2=0.62; Fig. 6).

Using soil temperature, the pulse rate of each cricket (wing stroke rate) was

estimated (Ulagaraj 1976). Since the power used by the cricket is directly related to


























Figure 6. Total power outpuA (dB re 10-12 W) as a function of sound pressure (dB
re 2x10l- Nm ) directly above the calling burrow. Sound pressures
are measured at 25 cm from the burrow opening. The regression is
significant (p<0.05, r-=.62) y = 0.58x + 26.0.




















a
0




a a


00
S 0 0
0 y .58x + 26.0

I I I


55 70 75
Sound Pressure (dB)


PWL (dB)


Rfn


60



























Figure 7. Sound field of Scapteriscus acletus. Sound field is base on an average of
17 radial distances at each of 65 radii. Standard errors around the 65
points are always less than 4 cm. Anterior is designated by ant, posterior
by pst, right by rit and left by Ift.

a. View from above the sound field.

b. Looking from the crickets' left.

c. A perspective view.













.p si. . . . ...







lit
/ /








ant





b 2


















ant pSI










7









the wing stroke rate while calling (Prestwich and Walker 1981), I estimated the

percent efficiency of sound production for each sound field. Subtracting an

estimated resting metabolism jf a)out 370 uliter O2/g h (Prestwich unpublished,

personal communication 1986) from the metabolic rates while calling revealed that

the mole crickets were very inefficient. Percent efficiency ranged from 0.02 to 0.16

percent (Table 1). The data for estimated power used while calling are similar to

actual measurements of calling metabolism of male acletus (Prestwich, personal

communication 1986).

Because the individual sound fields were variable, an average sound field

based on the average radial distances along each of the 65 radii was computed for

the 17 sound fields from acletus males. The standard errors of the 65 means were

about 2.5 and always less than 4 cm. The shape of the sound field most closely

resembled a hemisphere and the mean distances did not differ significantly from the

radius of a 26 cm hemisphere. At ground level (X-Y plane, Fig. 7) the transverse

axis of the sound field was 48.4 cm and the longitudinal axis was 53.7 cm across.

Height of the sound field was 26.6 cm. Unlike the sound fields of other crickets, the

burrow opening acts as a point source and radiates equally in all directions. The

total power of the averaged sound field was 4.33 uW or 66.4 dB Using 26 cm as the

radius for a hemisperical sound field, the power should be 4.25 uW or 66.3 dB.

Discussion

The acoustic power produced by male Scapteriscus is less than one-

hundredth of the power produced by the French mole cricket, Gryllotalpa vineae.

Bennet-Clark (1970) calculated a mean power output of 1.2 mW. Assuming that

this was produced by a total muscle power of 3.5 mW, Bennet-Clark calculated an

efficiency of 28-35 percent. However, the power used was probably underestimated

since he estimated power used from data on grasshopper flight muscles. Using data









from Prestwich and Walker (1981) Gryllotalpa vineac should use about 70 mW of

power for an average 3.3 g cricket calling with a pulse rate of 66 pulses/sec. The

efficiency then becomes 1.7 percent, a little less efficient than a typical loudspeaker.

This is still more than ten times more efficient than Scapteriscus. Since female

Scapteriscus preferentially land at louder males (Forrest 1980, 1983), males are

expected to call at their maximum output, but because Scapteriscus are one-sixth

to one-third the size of Gryllotalpa it seems likely that they are unable to produce

as much acoustical power because of their much smaller muscle mass. Note that the

power produced by Scapteriscus acletus was directly related to the size of the male

(Fig. 5a). Soil moisture also influenced output probably because it enables the

animal to pack the sandy soil better, and thus, it absorbs less of the acoustic energy.

Bennet-Clark (1970) also noted that moist soil increased output in Gryllotalpa

vineae.

The efficiency of sound production is somewhat lower in Scapteriscus spp.

than that reported for other animals that use acoustic signals to communicate.

Hylid frogs are about 1 to 3 percent efficient (Prestwich and Bruger, unpublished).

The frogs are probably more efficient because the calls are produced by vibration of

a vocal sac that acts much like a pulsating sphere. Such a source does not have the

acoustic short circuit a dipole source has, and if the vocal sacs of the frogs are the

proper dimensions relative to the wavelength of their calling songs they may have

resonant properties that would also increase the efficiency. Male bladder cicadas,

Cystosoma saundersii, produce about 0.35 mW of acoustic power and are about 0.82

percent efficient. Like the frogs, the abdomen of the cicada acts as a baffle and

resonating system (MacNally and Young 1981). Other crickets have only slightly

higher calling efficiencies than those of Scaptcriscus. Anurogryllus arboreus are









about 0.2 percent efficient and Oecanthus celerinictus are from 0.3 to 1.3 percent

efficient (Prcstwich unpublished).

The sound fields of individual males were often irregular although they were

similar in overall shape to the average distribution pattern (Fig. 7). Since a new

burrow is constructed each evening, the openings may be very different from cricket

to cricket and from night to night for the same cricket and this probably causes the

irregularities. The distribution pattern of sound from a mole cricket burrow differs

considerably from the directional patterns of other crickets. The burrow acts as an

infinite baffle around the vibrating membranes raised in the throat of the horn

(Bennet-Clark 1970, Nickerson et al. 1979). The burrow opening then produces a

point source of sound and the waves radiate equally in all directions. The result is a

spherical shaped distribution pattern.

The sound fields of other crickets usually have some directional properties.

Anurogryllus arboreus (Paul and Walker 1979) and Gryllus campestris (Nocke

1971) have directional characteristics due to the destructive interference along the

edges of the wings. Probably the most noticeable sound fields for their directional

quality are those of tree crickets (Williams 1945). The sound pressure may be more

than 20 dB less at the sides of the cricket compared to anterior and posterior

pressures, and males often turn while calling seemingly to direct sound in different

directions (Toms 1984, Forrest unpublished). Male Gryllotalpa vineae also produce

a directional pattern because the two burrow openings act like dual speakers, and

there is a characteristic radiation pattern dependent upon the relationship of

distance between the two openings and the wavelength of the sound produced. This

radiation pattern of Gryllotalpa vineae is such that it should increase the probability

that a female will intercept the sound fields (see Bennet-Clark 1975 and Forrest

1982 for a discussion of selection on directional sound fields).







35

In summary, the power output of male Scapteriscus is relatively small

compared to other species of acoustically signaling animals, and male mole crickets

are very inefficient at producing sound (Table 1). Large males produce greater

sound power than smaller males, and moist calling sites increase the total output

(Fig. 5). The radiation pattern of sound from the calling burrow is hemispherical in

shape (Fig. 7), and the total power can be estimated from a single measurement of

sound pressure above the burrow (Fig. 6).














CHAPTER II
OVIPOSITION AND MATERNAL INVESTMENT

It's only the giving that makes you what you are.
(Ian Anderson)

Introduction

Females of some insects may lay as many as 600,000 eggs during their

lifetime although the norm is usually a few hundred (Hinton 1981). These nutrient-

filled eggs are a major investment in offspring, and parental investment, especially

the differences in investment between the sexes, is a predominant force in selection

on reproductive strategies (Trivers 1972), life history tactics, and resource allocation

(Boggs 1981). In most instances, investment by males ends at copulation or

fertilization (see Thornhill [1976] on paternal investment in insects), and females

generally do not invest after oviposition. Unlike paternal investment, which is

frequently difficult to assess, maternal investment can often be measured simply by

determining the mass of eggs produced.

In crickets oviposition and oviposition sites are diverse and eggs are

extremely variable in size, number laid, and whether or not they are diapausing.

While a great deal of attention has been given to nuptial feeding and paternal

investment in katydids (Gwynne 1981, 1983), crickets (Sakaluk 1984), and

cockroaches (Mullins and Keil 1980), few studies have involved orthopteran egg

deposition and measurement of maternal investment in eggs. This study reports the

oviposition of southern and tawny mole crickets. I followed the reproductive output

of females caged under field conditions and studied the influence of size, season,









and aging on female investment in eggs and female fitness as measured by the

number of hatchlings produced. Egg laying in these species was earlier described by

Hayslip (1943), but his study concerned mole cricket life histories and lacked

long-term monitoring of individual females. Walker and Nation (1982) studied

fall mating, egg maturation, and sperm storage of individuals but did not examine

maternal investment in offspring.

Materials and Methods

All studies of oviposition were done in Gainesville, Florida, and involved

females that were collected after they were attracted to the conspecific male calling

song: either an artificial song (Walker 1982) or the natural song of a male calling

from a soil-filled bucket surrounded by a trapping device (Forrest 1983). Females

were placed in buckets of soil (19-liter) and were fed 10 cm3 of ground, dry dog

food each week, enough to ensure that food was not a limiting factor in egg

production. Dog food was chosen because it had proven to be an adequate food in

maintaining other crickets in the laboratory. The buckets were kept outdoors under

two conditions. 1) The buckets were on the ground and screen lids prevented the

crickets' escape. 2) Buckets were buried flush with the soil surface, surrounded by a

doughnut-shaped pitfall, and covered with a cylindrical, hardware cloth cage.

Females had to fly to leave the buckets and in so doing they hit the cage, fell, and

were trapped. Trapped females were then placed in new buckets and the old

buckets were checked for eggs. If females could not leave because of screen lids, or

if they did not fly from a bucket, their buckets were examined for oviposition and

mortality about every 21 days (13 to 24 days depending on temperature).

Examination of buckets consisted of carefully scraping thin layers of soil (0.5-

1 cm) from the bucket using a spatulalike piece of sheet metal. This allowed the egg

cells to be excavated without damaging the eggs. The depth and the number of eggs









of each egg cell were noted and the eggs were placed on moist soil in numbered,

aluminum containers. To estimate investment in each clutch, a sample of eggs was

dried (at 30-370C) and weighed to the nearest 1.0 mg using a model AC 100 Mettler

balance. Remaining eggs were kept in the aluminum containers at room

temperature and monitored daily until hatched.

Oviposition Season

Females of each species (n= 11 vicinus, 9 acletus) were collected before

oviposition during the 1982 fall flight season and placed singly in buckets with a

male (condition 1 above). During the following spring, surviving females and spring

females collected before oviposition were kept (condition 2) throughout the flight

and reproductive seasons. Because females were found to lay no eggs in the fall,

data from spring 1982 were included in analysis of reproductive season.

Oviposition Cycles and Flight

During the springs of 1982 and 1983, females were kept in buckets (condition

2) and their flights monitored to determine the relationship between oviposition

and flight activity. The duration between hatching dates of successive egg clutches

from individual females was used as an estimate of time between egg clutches.

Effects of Size and Age on Fecundity

Females varying in size were kept singly in buckets (condition 2) with a male

and monitored for oviposition throughout their reproductive lifetime. Only females

collected prior to any reproductive activities or that were known virgins were used.

The wet weight (to the nearest 1.0 mg) of females was measured only once prior to

their use in the experiment with a model AC100 Mettler balance. Pronotal length

measured to the nearest 0.1 mm with vernier calipers was used as an indication of

female size. In most cases, pronotal length was used because it has a direct









relationship to female weight (r2=0.72 aclctus, 0.66 vicinus, p < 0.001 see Chapter

III), is less variable than weight, and is easily measured in the field.

Insect eggs of many species absorb water during development, thus

increasing both volume and weight (Hinton 1981). Because of differences in water

content among eggs, dry weights were used as a measure of investment. Investment

in individual eggs (mean dry weight per egg of samples), investment in clutches

(mean dry weight of egg times number of eggs), and number of offspring produced

(percent hatch times number of eggs) were measured for successive clutches of each

female.

Results

Mortality. Longevity, and Seasonal Oviposition

Of 9 acletus and 11 vicinus monitored during the fall, only 3 (33%) and 4

(36%) survived the winter. Since mortality was concentrated in colder months, this

high mortality was probably due to buckets being placed above ground, increasing

their exposure to cold temperatures during the winter. There was no apparent

influence of size on mortality. No eggs were laid by any of the fall females and

when females were found while examining buckets for eggs they were motionless or

extremely sluggish. Of all females monitored during 1982 and 1983, 54% (n = 24) of

vicinus and 41% (n = 22) of acletus females died before reproducing (including one

vicinus and one acletus that were killed during examination of their buckets).

Longevity of females in buckets was highly variable and ranged from 16 to 278 days

(mean 91) for acletus and from 7 to 254 days (mean 89) for vicinus females.

Egg-laying season was similar for the two species during the springs of 1982

and 1983 (Fig. 8). Oviposition started in late April and stopped in late July or early

August, with most egg clutches produced in May and June. The proportion of

























Figure 8. Oviposition season in mole crickets (a) S aclctus and (b) S. vicinus.
Graphs show the monthly proportion of egg clutches laid. For each
month, bar on the left represents data for spring 1982 and the bar on
right data for spring 1983. Note greater proportion of unsuccessful
clutches (0-50% hatch) late in the season.





















0.4





u 0.3

0
I

O
, 0.2
0
z
0
I-
cr
0 0.1
CLo.
0
cr
oC


0






0.4





o 0.3
I
O
o

LL 0.2
0
Z
0
a
0 0.1
IO
0
cc
a_


S. vicinus
S0-50% hatch
S> 50% hatch


JUL AUG


APR MAY JUN JUL AUG


MAY JUN















Table 2. Correlation matrix of paired comparisons for S. acletus (top half of table)
and S. vicinus (bottom half, underlined).

Clutch % Hatch Egg No. of eggs
no. wt per clutch


Clutch -0.40 -0.72 -0.32
no.


% Hatch -0.61 0.38 0.32


Egg
wt -0.66 0.49 0.30

No. of eggs
per clutch -0.61 0.65 0.57

a Spearman's rank correlation (rho). All correlations are significant (p < 0.01).









unsuccessful clutches (<50% hatch) increased as the reproductive season progressed

(Fig. 8). This was probably the effect of decreased hatching in later clutches of

females (Table 2) rather than an influence by season alone.

Oviposition Cycles and Flight

The average interval between clutches, as estimated by the hatching dates of

successive clutches, was 10 days for acletus (n = 38, 95% CL 8-11 days) and 9 days

for vicinus (n = 32, 95% CL 7-12 days).

Females rarely flew between egg clutches. In only 1 of 68 instances did a

vicious female (n = 9) take flight between the production of egg clutches. Of all

females producing eggs, seven vicinus never left buckets, three flew prior to egg

production but not after, and one flew after egg production but not before. Female

acletus (n = 13) were more likely to take flight between clutches (13 of 43

occasions). For acletus females producing eggs, six never flew from their buckets,

two flew after they began laying eggs, and five flew both before and after they

started ovipositing.

Reproduction and Investment by Individuals

Females produced variable numbers of eggs and clutches; some died before

ever reproducing, while others produced more than 450 eggs and as many as 10

clutches. Part of the variation in fecundity was due to size differences among the

females. Excluding females that produced fewer than three clutches, there was a

significant positive relationship between pronotal length of acletus females and the

total number of offspring produced (sum of the number of eggs per clutch times

percent hatch of the clutch, Fig. 9, p < 0.05) and between pronotal length and

average number of eggs per clutch (Fig. 10, p < 0.01). These relationships were not

significant for vicinus females. There was no relationship between female size and

the number of clutches or the size of eggs produced for either species.


























Figure 9. Relationship of size and total number of offspring produced for 12
S. acletus females eaged outdoors. Line is calculated using the
regression equation (r = 0.33, p < 0.05).



































y 4.5(x) 264.3


280

240

200

160

120

80

40 I

0-
6.6


Total
Offspring


7.0 7.4 7.8 8.2 6 9.0
Pronotal Length (mm)






























Figure 10. Mean number of eggs per clutch for 12 S. acletus females varying in
size caged outdoors. Line is calculated using the regression
equation (r = 0.50, p <0.01).


1





























Mean Number
of
Eggs/Clutch


60


50


40


30


20


10
6.


5


7.0 7.4 7.8 8.2
Pronotal Length (mm)


47


0

0


o 0
0 0

*0



0
y 7.9 W -24.6


8.6 9.0


-


























Figure 11. Plot of ranked egg weight for successive clutches of female mole
crickets. Egg weight is the mean dry weight per egg of a sample
from each clutch. Clutches with the same mean weight were given
averaged ranks. Points connected by lines represent points at the
same position on the graph. The number of females producing a
specific number of clutches can be found by counting the points for
each clutch number. Note that females invest less (smaller weight =
higher rank) in later clutches (higher clutch numbers). (a) S. acletus,
(b) vicinus.




































a




*


**

r

.*1 *

-r


S. acletus


1 2 3 4 5 6
CLUTCH


r

*


S S vicinus


1 2 3 4 5 6 7 8 9 10
CLUTCH


7 8 9


't~


-









Average investment per clutch was from 10 to 24 percent of female body
weight for aclctus and ranged from 5 to 16 percent of female body for vicinus

females. Total lifetime investment was greater than 100 percent of female body

weight for 6 of 12 acletus females and for 1 of 8 vicinus females. Since dry weight of

eggs was measured and female body weight was measured while the crickets were

alive (i.e., they do not represent dry weight of females), the investments in eggs are

actually substantially larger.

In both species, investment per egg and per clutch decreased as females

produced successive clutches. There was a significant inverse correlation between

weight of eggs and clutch number (Fig. 11) as well as an inverse correlation between

the number of eggs per clutch and clutch number (Table 2). Also, the percent hatch

for clutches and mass per egg were significantly and negatively correlated with

clutch number (Table 2). Later clutches of a female had fewer eggs, smaller eggs,

and lower percent hatch.

Discussion

My observations on oviposition did not differ from Hayslip's (1943) Plant

City, Florida, observations with regard to depth and average number of eggs found

for egg cells of acletus or vicinus. Oviposition season was also similar for vicinus;

however, acletus in Plant City extended oviposition into September. This may be

the result of warmer temperatures and a longer growing season farther south (ca.

190 km), conditions that result in a bivoltine life cycle for acletus in South Florida

(Walker et al. 1983). I did not find fall oviposition in buckets by females of either

species, but flush samples taken during November in Gainesville have contained

hatchling acletus suggesting that fall oviposition may occur, although young









juveniles apparently do not survive the winter (Stackhouse W., E. L. Matheny and

W. G. Hudson unpublished data).

Even though oviposition seasons of acletus and vicinus are similar, flight

seasons are not. Most of the flight season of vicinus is over by the start of

oviposition, while acletus flights are concurrent with oviposition (compare Fig. 2 of

Walker et al. [1983] with Fig. 8). This agrees with differences in flight from buckets

in the two species. That acletus females are more likely to fly between clutches may

have been one factor contributing to the more rapid spread of this species after its

introduction into the United States (Walker and Nickle 1981).

Clutch cycles run between 7 and 12 days as evidenced by the average time

between hatching of successive egg clutches of females. This agrees with earlier

work that estimated a 9 day period for a female to mature an egg clutch based upon

the measurement of primary oocytes of females (Forrest 1981). That flying females

have small oocytes suggests that they recently matured or laid a clutch and are

starting a new clutch cycle (Forrest 1981). Ngo and Beck (1982), working on

dispersal flights of acletus, noticed a significant 10-day peak in recaptures of

marked and released females. Oviposition cycles may have accounted for this peak.

They also found 12-day cycles in overall flight activity and attributed it to female

oviposition cycles. This assumes that females are synchronous in their egg laying. I

found no such synchrony. Because acletus females in my tests flew between only

30% of the egg clutches produced, it seems unlikely that peaks in flight activity are

due to oviposition cycles. Such peaks are more likely caused by environmental

factors making flights favorable or synchronizing maturation.

Larger eggs generally produce higher quality, more vigorous progeny that

may be more mobile, may suffer less mortality during early stages, may develop

faster, or may require fewer instars (Capinera 1979). But given finite resources,









producing larger eggs may not always increase the fitness of the female and a trade-

off should exist between number of eggs produced and their size (Smith and

Frctwell 1974, Parker and Begon 1986). Producing small eggs is apparently

disadvantageous for mole cricket females; because egg size and hatching success

were positively correlated in both species. Producing fewer eggs may also hamper

escape from the egg cell if siblings must cooperate while digging to the surface.

Large clutches of large eggs then should maximize a female's fitness. However, as

females aged they produced smaller clutches with smaller eggs. If clutch size is

important and resources are limited as females get older, they may divert some of

their resources away from egg size, at the cost of hatching success, to increase the

clutch size. That later clutches are smaller with smaller eggs could have also been

the effect of longer lifespans than would normally occur under natural conditions.

Organisms rarely survive their full potential life span and theories on the evolution

of senescence suggest that selection should act to increase reproductive output early

in life at its expense later (Hamilton 1966). Females kept in buckets were protected

from hazards, and, thus, their becoming senescent may have influenced the size of

their later clutches. Another orthopteroid, the stick insect Diapheromera vclii,

shows a similar decline in the rate of egg production and a decrease in investment in

eggs later in life (Sivinski 1977).

Gwynne (1983) related sexual differences in mating behaviors to differences

in parental investment in Tettigoniidae and other Orthoptcra. As sexual selection

theory predicted (Trivers 1972), when males invest more they are more selective of

mates than females, whereas females are more selective and males more

competitive when maternal investment is greater than paternal investment.

Investment in progeny by female mole crickets is far greater than that for males.

Males provide no paternal investment except the possible energy a female receives









from consuming the small spermatophore (<1 mm diam, estimated 0.1% of male

body weight) a male passes to the female during mating. On the other hand, a

female's average investment per clutch may be as much as 24% of her body weight.

Behavioral differences between the sexes support predictions based on differences

in parental investment. Male mole crickets display acoustically, are aggressive, and

often fight near calling burrows, whereas females are discriminating in their choice

of mates (Forrest 1983).

In a model of optimum egg size and clutch size in insects, Parker and Begon

(1986) showed that if larval success was determined solely by the size of the egg,

then selection would act such that females all produced eggs that were the same

size. A consequence of this is that large females can produce larger clutches than

smaller females. Data from mole crickets fit the model. There was a positive

correlation between egg size (investment) and hatching success of clutches for mole

crickets. Mean clutch-specific egg size did not differ between large and small

females nor did the rate of decline of investment in eggs or egg clutches. Also,

larger females produced larger clutches.

Although a direct influence of size on fecundity is common in insects, few

studies have been conducted under natural conditions and few data are available for

Orthoptcra. I found a significant relationship between female size and mean clutch

size (Fig. 10) and total offspring produced (Fig. 9) for acletus but not vicinus

females. That only one of eight vicinus females invested more than her body weight

in lifetime egg output compared with six of twelve acletus females suggests that this

difference may be due to the inability of vicinus, primarily a herbivore, to assimilate

the dog food provided as food. Female vicinus would then have to rely on juvenile

reserves for egg production, and although a larger female may carry greater reserves

these would probably not be enough to produce more than a few clutches. Dog









food, however, was able to keep the females alive for an average of 90 days and in

some cases for more than 250 days, indicating that this diet was reasonably adequate

for the crickets. Female vicinus might simply consume more of the poor quality

food, and, thus, expend more energy to compensate for its lack of nutritional value

(see Slansky [1982] for a review of insect nutritional ecology).

Large acletus females can produce almost 3 times as many offspring and

more than 1.5 times as many eggs per clutch as small females. Gwynne (1984)

found that female size and nuptial feeding by males directly affected fecundity in the

bushcricket, Requena verticalis. Females feeding on seven spermatophylaxes more

than doubled egg production, and the size of eggs produced also increased. There is

also a relationship between the number of ovariolcs (and presumably reproductive

output) and size in betwecn-species comparisons of melanopline grasshoppers

(Bellinger and Pienkowski 1985).

Female investment and fecundity and their relationship with size tactics and

developmental strategies in insects have received almost no attention, though such

selective forces shape population size structure and influence population dynamics.

Assuming a direct relationship between fecundity and size, Masaki (1967) explained

the latitudinal size variation in the Emma field cricket, Telcogryllus emma, on the

basis of a balance between the opposing selection pressures of increasing fecundity

with increasing size and decreasing size with shortened growing season of more

northern latitudes. I have used similar explanations for within-population size

variation and changes in the seasonal distributions of sizes of both male and female

mole crickets (see Chapter III). These types of data should improve models of

population dynamics and help explain the life-history strategies in insects from an

individual, evolutionary perspective.














CHAPTER III
SIZE TACTICS AND DEVELOPMENTAL STRATEGIES

From early days of infancy,
through trembling years of youth
Long murky middle age
and final hours long in the tooth.
(Ian Anderson)

Introduction

Size is one of the most important attributes of any organism, and within any

species there is often a great deal of variation in size among individuals. An

animal's size will profoundly affect the prey it can cat, what predators it has, its

physiology and energetic, and its reproductive success (Peters 1983). Thus an

organism's size and life history are intimately related. However, research on life

history strategies has virtually ignored the relationship between size and

development, and has focused primarily on tactics of diapause, migration and

dispersal (Blakley 1981; Denno and Dingle 1981).

Animals with complex life cycles are interesting with regard to size tactics

because certain life stages are specifically evolved for growth and increasing body

size, while other stages are adapted for dispersal and/or reproduction. Amphibians

and insects are perhaps the best known groups with complex life cycles. Of the two,

insects offer a simpler system in which to study size tactics because growth occurs

only in juvenile and non-reproductive stages (see Wilbur and Collins 1973; Smith-

Gill and Berven 1979; Werner 1986 for a discussion of amphibian metamorphosis).

Unlike amphibians, there is usually no growth once an insect becomes adult (i.e., no

increase in size although abdominal stretching may occur especially in females), and









adult size is determined by what happens during juvenile stages. Individuals can

make decisions about their size by using certain cues to determine when to molt to

adult.

Costs and Benefits

Regarding decisions, I am not implying that the animals make a conscious

decision but rather these are evolved responses to cues the individual has available.

The decisions an individual makes about changes in size carry with them costs and

benefits. Although difficult to assess, these costs and benefits are ultimately

measured in terms of fitness. Accordingly, selection should act such that individuals

evaluate the costs and benefits and make those decisions that will maximize fitness.

For insects the decision an individual makes about its size can be simplified to a

decision of whether or not to become an adult. Figure 12 shows some of the costs

and benefits associated with a yes or no decision to mature, that is to molt to pupa

or adult. For instance, at a point in the development of an individual the decision

to continue as a juvenile (no-decision Fig. 12) has associated with it costs due to

delayed reproduction with the risk of dying before maturing and reproducing.

There is also the cost of limited mobility since juveniles cannot fly. Benefits to an

individual remaining a juvenile are due to an increase in fitness because of a longer

period of growth and thus a further increase in size, assuming there is a direct effect

of adult size on fitness (see below).

Graphically the cost/benefit curves might look like Figure 13. Early in its

development an individual might benefit a great deal because of small increases in

size while the costs of the same size change increase only slightly. Later on small

changes in size increase fitness (benefits) very little while costs for the same amount

of change become increasingly more expensive. The individual is expected to adjust

its development such that it maximizes the difference between benefits and costs


























Figure 12. Costs and benefits to an individual associated with yes or no
decisions of whether or not to become an adult.






















BECOME ADULT?


BENEFITS attain larger size



COSTS delayed reproduction
no flight


longer/earlier reproduction
flight


size restricted


NO


YES


























Figure 13. Hypothetical cost/benefit curves for changes in an individual's size.
Selection is expected to act such that individuals weigh costs and
benefits and become adult when the difference between the curves is
maximized (dashed line).














Cost
or
Benefit


Size









(i.e. the individual is expected to make the decision to molt to adult at the size

indicated by the vertical line). Cues such as population density, availability and

suitability of food and mates, size of competitors, sex ratio, length of the remaining

growing season, and "expected" temperature (particularly important for

poikilotherms) should be used in making the decision of when to molt.

Although some authors have focused on an optimal body size within a

species (sce Roff 1981), it is the variation in sizes among individuals and the

deviation from the optimal that needs to be investigated. Previous work on size

variation in insects dealt with variation between populations. Primarily latitudinal or

altitudinal differences among the populations were used to explain the variation

(Masaki 1967, 1978; Dearn 1977), and Roff (1980,1983) has proposed models that

use season length, = generation length, which varies with latitude and altitude, as a

tool in predicting insect size and life histories. Proximate causes of size variation in

insect populations include temperature gradients (Sweeney and Vannote 1978),

larval diet (Slansky 1982; Andersen and Nilssen 1983; Palmer 1984) and population

density (Peters and Barbosa 1977). Borgia (1979) discussed variation in male size

and the decisions males should make in weighing costs and benefits of changes in

size. Borgia was primarily interested in male size and its importance in a male's

ability to control specific resources, but such tactics and decisions should be

applicable whenever size influences an individual's fitness.

Size and fitness in insects

For poikilotherms, reproductive success or output is almost always

influenced by size (Peters 1983). It is well known that for many insects there are

positive relationships between female size and the number of eggs, size of eggs, and

size of clutches produced (Hinton 1981). Only recently have there been attempts to

correlate male reproductive success with size. In species where males compete for









females (female defense polygyny) or for resources used by females (resource

defense polygyny) large males often have a reproductive advantage over smaller

males that may have to adopt alternative mating strategies to acquire mates. Where

female choice is involved, females often choose males on the basis of cues

correlated with male size (Thornhill and Alcock 1983 and refs.) Thus it is well

documented among insects that large size confers some advantage in fitness for

both sexes. For this reason insects are appropriate for investigating the occurrences

of size tactics and developmental strategies.

Size Tactics in Mole Crickets

Costs and benefits associated with size are almost impossible to measure

directly since they may differ from one individual to the next and may change at any

time during an individual's life. However, developmental strategies should be

reflected in adult sizes, and certain predictions about changes in size distribution for

a species can often be tested. To examine whether individuals weigh costs and

benefits associated with changes in size I tested predictions about seasonal

differences and within-season changes in size distributions for two species of mole

cricket.

Size is an important component in the sexual success of an individual. For

males, the intensity of the calling song is correlated with male size--larger males

produce louder calls (Forrest 1980,1983 and Chapter I). Flying females selectively

respond to and land at louder calls, and all things being equal, larger males will

attract and mate with more females than smaller males (Forrest 1983). Larger

females also have a reproductive advantage over smaller females. Large females

may produce as many as 3 times as many offspring and more than 1.5 times as many

eggs per clutch than smaller females (see Chapter II).









Predictions

Since reproduction occurs only during a brief period the following spring,

during the fall the cost to an individual due to delayed reproduction associated with

remaining a juvenile is zero (see Fig. 12). One would predict that individuals that

are smaller than average would overwinter as juveniles to increase their size, while

those that are larger than average would gain relatively little by increasing in size,

but more by becoming adult, dispersing to favorable overwintering sites, and

preparing for reproduction early the following spring. There may be a cost of

delayed maturation due to limited movement. For instance, if an individual found

itself in an unfavorable site where the likelihood of surviving the winter is very low

(i.e., the costs are very high), that individual should mature and disperse to a more

favorable site. However, this cost would be borne by large individuals as well as

small, while the additional cost of maturing at a small size is borne only by small

individuals. Therefore I expected a greater proportion of larger than average

individuals in fall flights compared to spring flights.

The second prediction involves an expected change in the size distribution

during the spring reproductive season. Because the egg laying (reproductive)

season is brief, individuals that remain juveniles incur greater and greater costs as

the season progresses. Because the chance of surviving to the following

reproductive season (the next spring) is very small, the costs of delaying

reproduction increase dramatically and soon outweigh the benefits of increasing in

size. I predicted that as the spring season progressed smaller and smaller

individuals should mature.

Materials and Methods

Quarter-monthly samples (up to 50 individuals for 1981, 25 for 1982) of adult

male and female mole crickets, acletus and vicinus, were obtained from a routine









sound trapping station (Walker 1982) on the Agronomy Farm of the University of

Florida. Samples were taken during the spring and fall flight seasons of both years.

Since fall adults are of the same generation as adults of the following spring,

measurements spanned three generations, and samples from spring 1981 and fall

1982 represented only part of the 80-81 and 82-83 generations respectively.

To prevent desiccation, adults were placed in 13 dram snap cap vials

containing moist soil but no food. Measurements of pronotal length (to nearest

0.1mm measured along the midline using vernier calipers) and mass (to nearest 1.0

mg using a model AC100 Mettler balance) were taken for each individual within 24

hr of capture.

Results

There was a significant positive relationship between mass and pronotal length (Fig.

14). A linear model is shown because this model explained as much of the variation

in the data as did the more usual model of mass equals the cube of a linear

dimension. The regressions differed between species, whereas within a species the

sexes had similar slopes that did not differ significantly. In both species the

intercept was smaller for males than females (significant for acletus but not vicinus),

thus for a given pronotal length males weighed less even though males on average

are the larger sex (Fig. 14).

Figure 15 shows the size distributions for each sex during spring and fall

seasons (combined for both years). Distributions were approximately normal and

male and female sizes were broadly overlapping. On average males were larger

than females (Fig. 15, Table 3). For acletus the distributions changed seasonally.

Fall adults were larger than spring adults (Fig. 15a & b). The distributions for

vicinus females suggest the opposite trend (though not significant Table 3), but the

exaggeration of the fall proportions is the result of a small sample of fall vicinus

























Figure 14. Linear relationship between pronotal length and mass for male and
female S. acletus (circles) and vicinus (squares). Lines are plotted
using regression equations and the endpoints represent the extreme
values of pronotal length for each group. All regressions are
significant p<.001. N=1222 male and 1431 female acletus; 284 male
and 714 female vicinus.











































* acletus d y=168x -848 r2=.85
O acletus9 y=163x -760 r2=.72
* vicinus dr y238x -1147 r2=.78
O vicinus 9 y=249x -1173 r2= .66


1200 -


1000


800 -


600 -


400 I-


6.0 7.0 8.0 9.0 10.0 11.0 12.
Pronotal Length (mm)


1400r














Table 3. Mean pronotal length (mm), standard deviation, and number of adult
mole crickets trapped during spring and fall flight of 1981 and 1982.


SPECIES SPRING FALL
sex 1981 1982 1981 1982
S. acletus

males 8.8, 7a 8.8, .7a 9.5, .6b 9.3, .5b
(n=594) (369) (32) (227)
females 7.9, .5a 7.9, .5a 8.4, .4c 8.2, .4b
(709) (371) (102) (249)

S. vicinus

males 8.4, .5b 8.2, .5a ---- 8.3, .4ab
(162) (119) (0) (3)
females 7.9, .4b 7.8, .4a 8.0, .6ab 7.8, .4ab
(379) (273) (3) (59)
aMeans in the same row followed by different letters are significantly different
(p<.05).


























Figure 15. Frequency distributions (expressed as a proportion of season total)
of adult mole crickets captured during 1981 and 1982 flights at
Gainesville, Florida.









S aletus s0
or Srn n=l60a
n 'all n.35;


jij


~.miIIflmfl LA Eli ______________


65 70 75 80 85 90 95
Pronotal Length (mm)


altillfJ


100 105 110 115


S ademus d
. sano n=259
Olall n=259


lhhR tja


65 70 75 80 85 9.0 95 100 105 110 115
Pronotal Length [mm)
C

| B* sg n-652
0 fa n=62





65 70 75 80 85 90 95 100 105 110 115
Pronotal Length (mm)


[.r


S -cmnus dlf
* sprng n-281


H ill


65 70 75 80 85 90 95 100 105 110 115
Pronolal Length [mm)


Y-t----~--~ll


mmlmm


.11N









(n=62). Only three vicinus males were captured during the two fall seasons and

were not included in the figure.

Mean pronotal lengths were significantly larger for fall aclctus than for

spring individuals, whereas vicinus did not differ in average pronotal lengths

between the two seasons. -Pronotal lengths were similar for seasons compared

between years (Table 3). Note also that fall individuals typically showed less

variation in size than those in the spring.

Mean pronotal lengths of quarter-monthly samples decreased during the

spring flight season and reached a minimum toward the end of the reproductive

season (Fig. 16). During the spring there was a significant relationship between

pronotal length and the week of the season, but this relationship was not significant

for fall individuals. This relationship explained less than 10 percent of the variation

in the data when individual measurements were used in the regression analysis (Fig.

16). A regression using sample means (for samples with more than 10 individuals)

and date of sample explained 37-89% of the variation in the mean pronotal length

(Fig. 16). Data for vicinus males are not shown because few samples contained

more than 10 males (1981 n=4, 1982 n=5). The relationship was significant for

individual measurements of male vicinus for spring of 1982 (p<.05) and both years

combined (p=.05); however no significant relationships were found between the

nine sample means and date of capture.
Discussion

The data on seasonal and distributional shifts in mole cricket sizes support

the predictions and suggest that individuals weigh costs and benefits associated with

changes in size (Fig. 12). There was one major difference between the two species

that may indicate different overwintering strategies. There was a significant

difference between fall and spring adult sizes for aclctus but not vicinus. According

























Figure 16. Plot of mean pronotal length of quarter-monthly samples (where
N>10) for adult mole crickets captured during 1981 and 1982 flights.
For all means shown standard errors are about 0.1mm. Trend lines
plotted from regression equations for individual measurements for
1981 and 1982 combined (solid) or separately in the case of vicinus
(dashed). Spring declines in sample means were significant--

A) 1981:r2=.38, p<.03. 1982: .89, <.001. 1981+82: .70, <.001.

B) 1981:r2=.89, p<.001. 1982: .72, <.001. 1981+82: .79, <.001.

C) 1981:r2=.45, p<.03. 1982: .62, <.004. 1981+82- NS.














S. acletus


0



o o
@00

0 00
0 0C


9.6


9.4
E
E 9.2


S9.0

S8.
8.8
0
S8.6


8.4

8.2



8.6


S8.4
E
8.2
o0
C
a, 8.0
-J

I 7.8
o
o
S7.6


7.4


A A A


A A
A


FEMALES
A 81
A 82


A A sA A A



na


Sep Oct I Nov I Dec 1 Feb Mar Apr I May Jun Jul
Fall Spring





S. vicinus


A A A a a

A A A-A

A A a


Sep I Oct I Nov Dec Feb I Mar Apr I May I Jun Jul
Fall Spring


o

- *-- -


MALES
0 81
S82


Sep I Oct I Nov I Dec 1 1 Feb I Mar I Apr I May I Jun I Jul
Fall Spring


8.4


Y 8.2

0 8.0
-1
- 7.8
o
S7.6

7.4


FEMALES
a 81
A 82












to the predictions made, individuals that are smaller than average in the fall should

wait until the following spring to mature. That adult vicinus arc less variable in size

than acletus (Fig. 15) may have made the detection of seasonal differences more

difficult. It is interesting that a greater percentage (85%) of the vicinus population

overwinter as adults than in the acletus population (25%). Forage quantity and

quality are greatly reduced during the winter months and, since vicinus is mainly

herbivorous, this may limit the potential for growth during that part of the year.

Benefits due to overwintering as a juvenile would be lessened and it may be more

important to locate overwintering sites that are favorable for survival and allow

early reproduction the following spring.

I found that as the reproductive season progressed there was a decline in the

pronotal lengths of individuals for successive samples. That is, a significant

relationship was found between pronotal length and capture date. Although the

model did not explain much of the variation in measurements of individuals the

overall trend in the data (sample means Fig. 16) was highly significant. This might

be expected since all sizes of individuals should be maturing throughout the season,

but the proportion of small individuals increases dramatically toward the end of the

season. Small individuals have probably found themselves in areas where growing

conditions are poor and they mature at the smaller size in order to makes the 'best

of a bad situation' (Dawkins 1980).

Rogers (1942), working with a number of tipulid species, noticed a decline

(as much as 20%) in the size of late season specimens and attributed the decline to

lack of food and moisture. Similar seasonal changes were also found in the

milkweed leaf beetle, Labidomera clivicollis (Palmer 1984). There was a significant

gradual decline in male and female sizes during the growing season caused by a

decline in the amount of available food. Food quality was not discussed and may be









even more important. Slansky (1982) pointed to the growing interest in insect

nutritional ecology and discussed the decisions individuals must make relative to

nutrition and their final adult size.

For mole crickets, the similarity from year to year in the time at which the

smallest sample means occur suggests that photoperiod may be one proximate cue

used in the molting decision. However, cold weather during the spring of 1981

caused a delay in the vicinus flights (Walker et al. 1983) and probably delayed

maturation and caused the shift seen in the seasonal decline in size for 1981

(dashed lines Fig. 16c). Since acletus flights occur later in the spring, cold weather

had no affect on delaying maturation or the decline in size during the spring

months. Other proximate causes for the observed decline, such as food and

temperature, seem unlikely. Food quantity and quality as well as temperature are

increasing during the months of May, June, and July.

Insect size variation, though environmentally induced, may still be under the

influence of natural selection. The mechanisms can be simple. It is well known that

in many insects different numbers of molts precede adulthood. For instance, mole

crickets may have from 6 to 10 molts before becoming an adult. Molting is brought

about by changes in hormone liters of the individual. These hormone levels, while

under genetic control, are known to be influenced by many environmental cues.

Therefore even though variation in size can be caused environmentally, tle

decisions to molt have a genetic basis that can be acted on by natural selection.

One pervasive point in most studies of development is that an individual

must reach a 'critical size' before becoming adult (Blakley and Goodner 1978,

Blakley 1981, Palmer 1984). Starvation of juveniles before they attain this size

results in death. On the other hand, some cockroaches apparently determine their

ultimate size at a specific instar and adjust the number of instars to reach this size at









adulthood (Tanaka 1981). Slansky (1982) suggested that this minimum size is

adjusted by selection and the balance between costs of prolonged development and

the costs of reduced size and fitness.

The understanding of size variation as it relates to developmental strategies

should broaden our understanding of an organism's life history. An individual's size

is the direct result of its development. Size has been implicated as the factor

affecting a number of alternative reproductive behaviors in insects (Thornhill and

Alcock 1983) and is a major determinant in the sex change of sequential

hermaphrodites (Charnov 1979). Size tactics may even be one generation removed.

For instance females that provision broods and female parasites may adjust the

amount of investment in their offspring, thus influencing their offspring's adult size

and reproductive behaviors (Alcock 1979). Once size tactics and the developmental

strategies of an organism are explained from an individual's standpoint, new light

will be shed onto other aspects of that organism's life history.














CONCLUSIONS

Male mole crickets produce sound. Like most crickets these acoustic signals

function to attract females for mating purposes. Even though males use their

burrows as a baffle and they call from within an acoustic horn, the production of

sound is an inefficient process. Males are less than 0.2 percent efficient in

producing from 2 to 22 uWatts of acoustic power. The power produced is

dependent on male size and the moisture of the soil surrounding the calling burrow.

The sound field is hemispherical, and thus, has no directional characteristics. By

preferentially landing at louder males, females may use the calling songs to locate

large mates or moist soil for oviposition.

Oviposition occurs during the spring, primarily in May and June. The

investment in eggs, the numbers of eggs, and the percent hatch of eggs decline as

females age. Large females produce more hatchlings and have larger egg clutches

than smaller females.

Because size is such an important aspect of the reproductive success of both

sexes, males and females should make decisions about when to mature that will

maximize fitness. Each decision about whether or not to mature has certain costs

and benefits that must be evaluated. During the fall, individuals that are smaller

than average overwinter as juveniles in order to increase in size before spring

reproduction. As the reproductive season progresses the cost of delaying

maturation increases and smaller individuals mature.






77


It is important to note that the observations are made from the standpoint of

the individual. As such, they can be used to predict aspects of mole cricket life

history and will be useful in the models concerning the population dynamics of these

pest species.














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BIOGRAPHICAL SKETCH

Timothy G. Forrest was born Friday, the 13th of April, 1956, in St. John's,

Newfoundland, Canada. He received his Bachelor of Science degree in 1978 from

Florida Southern College where he majored in biology and mathematics. He

received his Master of Science degree in entomology from the University of Florida

during the spring of 1981. Since that time he has been working toward his

doctorate. In January of 1982 he married Susan Lynn Mashke. They became

parents on June 25, 1986 when their son, Justin Broc, was born.




Along the coast road, by the headland
the early lights of winter glow
I'll pour a cup to you....
Raise it up- say Cheerio.
(Ian Anderson)













I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.



Thomas J. Walker, Chairman
Professor of Ehtomology and Nematology

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.



James E. Lloyd
Professor of Entomology and Nematology

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.



Martha L. Crump
Associate Professor of Zoology

This dissertation was submitted to the Graduate Faculty of the College of
Agriculture and to the Graduate School and was accepted as partial fulfillment of
the requirements for the degree of Doctor of Philosophy.

December 1986 ---
Dean, College of Agriculture


Dean, Graduate School




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BIOACOUSTICS, MATERNAL INVESTMENT,
AND DEVELOPMENTAL STRATEGIES
IN THE MOLE CRICKETS,
SCAPTERISCIJS ACLETUS AND VICINUS
By
TIMOTHY G. FORREST
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1986

I believe in fires at midnight...
Silken mist outside the window-
Frogs and newts slip in the dark
Too much hurry ruins a'body
I’ll sit easy, fan the spark
(Ian Anderson)
To Sue,
thank you for everything.

ACKNOWLEDGMENTS
Now is the solstice of the year.
Winter is the glad song that you hear.
Ring out, ring Solstice Bells.
(Ian Anderson)
1 am most appreciative of the guidance that Tom Walker and Jim Lloyd have
given me. Their high standards of scholarship and science have been truly inspiring.
I thank them both for what I have received in terms of knowledge, opportunity, and
friendship. My work will always be a product of their teachings. I wish to thank
Marty Crump for adding a new dimension to my interest in animal sound
production. Her knowledge of anuran biology has been most revvarding and has
made me look at insects in a different light. All three gave helpful suggestions that
much improved this dissertation.
Henry Bennet—Clark, Dave Green, Les Bernstein, and Ginny Richards have
all been helpful in transmitting their knowledge of sound and acoustics, without
which I would never have completed Chapter I. Ken Prestwich provided
unpublished data and his help with the energetics of sound production was
invaluable. Frank Slansky was extremely helpful with comments on Chapters II and
III that much improved their content. Bill Oldacre provided the schematics for the
portable multiplexing unit and was always willing to give his time. John Allen
furnished a computer program that made the mentally impossible visually possible.
Sue Wineritcr gave many hours of work on many drafts of many figures. Her
expertise in art and illustration is much appreciated. I would also like to thank my
friends Will Hudson and Steve Wing for making this time and degree an enjoyable
and enlightening one.
iii

I also wish to acknowledge my parents for their help when help was needed
most. And finally I thank my family. Sue, I thank for her devotion and dedication
that have given me confidence in myself and the drive to finish. Her many hours of
assistance in the field will always be remembered. Justin, I thank you for making
Sue and me very happy.
IV

TABLE OF CONTENTS
ACKNOWLEDGMENTS
iii
LIST OF FIGURES
vi
ABSTRACT
vii
INTRODUCTION
1
CHAPTERS
I
BIOACOUSTICS
3
Introduction .......
3
Materials and Methods .....
21
Results .......
27
Discussion .......
32
II
OVIPOSITION AND MATERNAL INVESTMENT .
36
Introduction .......
36
Materials and Methods .....
37
Results .......
39
Discussion .......
50
III
SIZE TACTICS AND DEVELOPMENTAL STRATEGIES
55
Introduction .......
55
Size Tactics in Mole Crickets ....
62
Materials and Methods .....
63
Results .......
64
Discussion .......
70
CONCLUSIONS
76
REFERENCES CITED
78
BIOGRAPHICAL SKETCH
84
v

LIST OF FIGURES
Figure
1. Pressure variation in a 2 Hz sine wave. ... 6
2.
Phase relationship between velocity
and displacement of a pendulum. .
10
3.
Acoustic interference and the
summation of sinusoids. ....
14
4.
Sound produced by a vibrating disc..
19
5.
Relationship between total power output,
male size and soil moisture in Scapteriscus.
26
6.
Total power output as a function of
sound pressure above the calling burrows of
Scapteriscus acletus......
29
7.
Sound field of Scapteriscus acletus.
31
8.
Scapteriscus epp laving seasons.
41
9.
Relationship between female size and
total offspring produced in acletus..
45
10.
Relationship between female size and
average number of epps per clutch in acletus.
47
11.
Effect of female age on mass of eggs produced in
Scapteriscus. ......
49
12.
Costs and benefits associated with maturing.
58
13.
Hypothetical cost/bencfit curve associated
with changes in an individual’s size.
60
14.
Relationship between adult pronotal length
and mass in Scapteriscus. ....
66
15.
Seasonal size distributions in mole crickets.
69
16.
Seasonal change in size distribution in
mole crickets.. .....
72
vi

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
BIOACOUSTICS, MATERNAL INVESTMENT,
AND DEVELOPMENTAL STRATEGIES
IN THE MOLE CRICKETS,
SCAPTF-RISCUS ACLETUS AND VICINUS
By
Timothy G. Forrest
December 1986
Chairman: Thomas J. Walker
Major Department: Entomology and Nematology
Three aspects of the reproductive ecology of the southern and tawny mole
crickets, Scapteriscus acletus and vicinus. were investigated.
Twenty-two sound fields of calling males were measured under field
conditions. Total power output ranged from 1.9 to 22.4 «Watt and was positively
correlated with male size and soil moisture. Efficiency of sound production was
only 0.02 to 0.16 percent. The shape of the sound field approximates a hemisphere.
Lifetime oviposition by caged, individual female mole crickets was
monitored outdoors. Oviposition is highly seasonal, and females that mature in the
fall wait until the following spring to oviposit. Most eggs are laid in May and June
and percent hatch of clutches decreases later in the season. Dry weight of eggs,
number of eggs per clutch, and percent hatch decrease with successive clutches from
individual females. Average investment in eggs per clutch ranged from 10 to 24
vii

percent of female body weight for acletus and from 5 to 16 percent for vicinus
females. Lifetime investment (dry weight of eggs) was greater than 100 percent of
female body weight (wet weight) for 6 of 12 acletus females, but only 1 of 8 vicinus
females had a lifetime investment in eggs equaling its body weight. In acletus. but
not vicinus. large females produced more offspring and the average size of their egg
clutches was larger than those of smaller females.
Insect size tactics and developmental strategies are discussed in relation to
decisions individuals make about when to mature. Such decisions carry with them
costs and benefits in terms of an individual’s fitness. Whenever size affects
reproductive success, selection should act such that individuals evaluate the costs
and benefits due to changes in size and mature when the cost/bencfit ratio is
maximized. Predictions about seasonal changes in adult sizes within a natural
population of mole crickets are tested. The changes in seasonal distributions of
adult sizes of mole crickets support the predictions and suggest that individuals
make decisions about when to mature based on costs and benefits associated with
changes in size.

INTRODUCTION
The southern and tawny mole crickets, Scapteriscus aeletus and vicinus. were
accidentally introduced into the United States around the turn of the century
(Walker and Nickle 1981). Because they lack a specialized predator/parasite
complex their populations have gone unchecked, their numbers have increased, and
they are one of the major pests of turf grasses and pasturelands in the southeast.
Their economic importance has prompted large amounts of funding for research
aimed at controlling mole crickets and lessening the damage caused by them. Only
recently has there been progress toward this goal. The primary stumbling block has
been an insufficient knowledge of the basic biology of these pest species. During
the past decade numerous researchers have contributed to our understanding of the
animals and their elusive subterranean lifestyle.
Mole cricket biology. Scapteriscus spp. are usually associated with habitats
that are temporary and early successional. In the southeastern United States both
aeletus and vicinus are common in pastures, cultivated fields, lawns, and other turf.
All adults have long hindwings and are capable of flight. Dispersal flights occur
during the spring and fall, consist mostly of females, and are important to the
crickets’ ecology in that they allow individuals to colonize newly opened areas or to
leave unsuitable or deteriorating ones (Ulagaraj 1975, Walker ct al. 1983). Flying
erickets land in response to their species—specific calling song (Ulagaraj 1975,
Ulagaraj and Walker 1973, 1975). Adults are easily collected at traps broadcasting
such songs. Both males and females respond to the calls and preferentially land
1

2
near males whose calls are louder than those of neighboring males (Forrest 19S0,
1983).
The two species differ markedly in their food habits with acletus being
primarily carnivorous on soil arthropods, earthworms, etc., whereas vicinus is
mainly an herbivore, feeding on roots as well as grass blades (Matheny 1981).
In Gainesville both species are univoltine; however, they differ in that about
85 percent of the vicinus population overwinter as adults compared with only 25
percent of the acletus population (Hayslip 1943). The rest of the populations
overwinter as late —instar juveniles that mature the following spring when eggs are
laid. Egg clutches are laid during the spring in underground ovoid chambers (4 by 3
cm) constructed by females. Once the eggs are laid, the chamber is sealed and no
parental guarding of eggs occurs. Eggs within a clutch hatch synchronously (within a
24-h period), and after a few days the young tunnel to the surface.
The topics that follow are part of the ongoing effort to understand mole
cricket populations and biology. The subjects add to the understanding of the
reproductive ecology of mole crickets and should be useful in monitoring changes in
the population and in designing models concerning mole cricket population
dynamics. Chapter I covers the bioacoustics of mole crickets and is an investigation
in the efficiency of acoustic communication by the two crickets. Chapter II deals
with female oviposition and the maternal investment by females and how these are
influenced by season, female size and female age. The third chapter is a study of
how individuals evaluate costs and benefits associated with size and how these
relate to the decisions of whether or not to mature.

CHAPTER I
BIOACOUSTICS
Let me bring you songs from the wood:
To make you feel much better than you
could know-
flan Anderson)
Introduction
Since man’s beginnings the sounds of animals have frightened, amused, and
intrigued him. With the advent of the tape recorder and audiospectrograph, he
could accurately quantify the sounds of animals, and in the last two decades the
interest and research in the areas of animal communication and bioacoustics have
increased dramatically. A new generation of digital recording and measuring
devices has made endeavors into bioacoustics even more reliable and from an
empirical standpoint more profitable. It is from the world of insects that such
research has really profited. Here there is a wealth of species with a wide variety of
sound producing and sound receiving mechanisms, many hundreds of which have
evolved independently, each one having been shaped by natural selection for
efficient communication purposes. A thorough background of the physical nature
of vibration and sound will be given and this understanding will be the basis for a
discussion of mole cricket bioacoustics and sound production.
Vibration and Sound
In simple terms sound is a disturbance or vibration in some elastic medium. This
disturbance need not be uniform or repetitive. Such a disturbance transmits energy
to the particles of the medium; the particles move from their resting position,
3

4
collide with neighboring particles and transmit the energy to them. These neighbors
transmit the energy to other neighbors and so on. Because of the elasticity of the
medium, the particles tend to return to their resting position after being displaced.
Thus the energy is transferred in waves away from the source of the disturbance.
Waves can be of two types depending on the axis the particles move relative to the
movement of the wavefront. When the particle motion is perpendicular to the
movement of the wave it is said to be transverse, whereas particle motion parallel to
the motion of the wave is called longitudinal. Sound waves in air are longitudinal;
waves on the surface of water are transverse.
The characteristics of a sound will depend upon the type of disturbance as
well as the physical characteristics of the medium. In most instances the medium is
air, although sound can be transmitted through liquids and solids and many insects
use these media for communication. For simplicity I will restrict my discussion to
sound in air, although the principles generally apply to all sound.
Pressure, power and intensity. A disturbance in air causes a change in the
density of molecules and in the pressure of the gas. These fluctuations in the
pressure of a sound wave are easily detected and measured with instruments, such
as microphone, that are sensitive to such pressure changes. Because of their ease of
measurement, changes in instantaneous pressures are commonly used to describe
sound. Perhaps the most well known sounds are sinusoids in which the pressure
fluctuates in a sinusoidal manner and can be described by the equation
p(t)=A*sin((2*pi*f*t)+theta) 1
where pressure (p) is a function of time (t), A is the amplitude of the pressure
change, pi is the constant 3.14159..., f is the frequency of the sine wave and theta is
the phase angle. Since the instantaneous pressure changes sinusoidally with respect
to both time and space, p(x), the pressure at some distance (x) from the source, can

Figure 1. Pressure variation in a 2 Hz sine wave. Pressure, p, is a sinusoidal
function of time, t, and the wave has an amplitude of A. Positive
values of p(t) arc compressions and negative values are rarefactions.

6

7
also be described by substituting +x/c for t in the above equation, where c is the
velocity of sound in the medium. In air, c is about 330 meters per second. Note
that the plus and minus signs in the space formula are used to describe sound
travelling in opposite directions from the source.
Figure 1 shows the instantaneous pressure changes for a 2 Hz (cycles per
second) wave with initial phase angle theta=0. At t=0 and x=0 the pressure is 0,
although the pressure may have been any value between - A and A depending on the
initial phase angle. With an initial phase angle of 90° the pressure at t=0 is equal to
A. When pressure, p(t), is positive, the particles of the medium are compressed.
This part of the sound wave is called the compression. When the pressure is
negative, a vacuum occurs and this part of the wave is the rarefaction. The sinusoid
in Fig. 1 is repetitive every 1/2 second because the sin(0)=sin(2*pi); i.e. the period is
1/2 second and is equal to the reciprocal of the frequency for any sinusoid. The
wavelength (lambda) of the sinusoid is the distance the wave travels in one period
and equals period*c or c/f. The 2 Hz wave has a period of 1/2 second and a
wavelength of 115 meters. Insects typically use much higher frequencies with
shorter wavelengths. For instance the calling song of the southern mole cricket,
Scaptcriscus acletus. has a carrier frequency of 2.7 kHz with a wavelength of 12 cm.
In dealing with sound one often wishes to know the power or intensity (I) of
the sound. Intensity differs from the scalar quantity of pressure, in that intensity is a
vector quantity having both magnitude and direction and its units are given in
energy per time per unit area or Watts per square meter (W/m^) whereas units of
pressure are in force times area (Newton-meter^: Nm^). Sound power, the total
power produced by a source, is measured in the same units as electrical power, the
Watt. Since pressure is an easily measured aspect of sound it is important to
understand the relationship between the scalar pressure and the vector intensity.

8
Intensity is the product of the instantaneous pressure (p) and particle velocity (u)
I=p(t)*u(t) or p*u. 2
This equation is analogous to the power equation used in electrical systems where
power is voltage times current.
Particle velocity describes the fluid movement of the medium. Near the
source particle velocity and pressure are 90° out of phase. This relationship is
particularly important and has relevance to measurement of pressure, and therefore
intensity. To illustrate this phase difference consider a pendulum (Fig. 2). At its
resting position the pendulum hangs vertically and there is no movement, no kinetic
energy and no potential energy. However, suppose the pendulum is displaced some
distance, x, from the resting position. If it is held at this position, the pendulum has
obtained some potential energy but has no kinetic energy. Releasing the pendulum
will cause it to swing toward its resting position and there is an exchange between
potential and kinetic energy. When the pendulum has reached its resting position
its velocity is maximum, its displacement a minimum. The pendulum then swings
past the resting position, and because of gravity the kinetic energy begins to change
back to potential energy. At the end of the swing the pendulum reaches its
maximum displacement (now -x) and its velocity has slowed to a minimum of zero.
It can be seen that the phases of displacement (=pressure) and velocity (=particle
velocity) are shifted by 90°. The phase relationship changes as the sound moves
away from the source and can be described by
Tangent theta=lambda/r. 3
Lambda is the wavelength of sound, r is the distance from the source, and theta is
the phase angle (in radians) between particle velocity and pressure (Bennet—Clark
1971 and refs). It can be seen that the phase shift approaches 90° as r becomes zero
and the phase difference between particle velocity and pressure decreases to zero as


10
X
DISPLACEMENT
-X
TIME

11
the distance r increases. Though the boundaries are not well defined, the near field
is where pressure and particle velocity are not in phase, whereas in the far field
particle velocity and pressure are in phase. For practical purposes the far field
begins at distances greater than about 1 wavelength of the lowest frequency sound
produced by the source. In the far field the sound field is said to be purely active
(Gade 1982). This is useful because in the active field particle velocity can be
calculated from pressure using the acoustical equivalent to Ohm’s law, current
equals the ratio of voltage to resistance:
u=p/p0c, 4
where p0c is the characteristic impedance of the medium, about 40 dyne*sec/cm2
for air (Gade 1982). Substituting p/p0c for particle velocity in the intensity equation
(2) it can be seen that intensity is proportional to the square of the pressure
I=p2/p0c 5
and can thus be calculated using measurements of pressure-- provided these
measurements are taken in the far field.
Decibels. Pressure, power and intensity levels are often given in decibel
units. These units are logarithmic expressions of the ratio of any two sound
pressures or powers. Normally the levels are described relative to some standard
reference. For instance, the commonly used reference for power level is 10'*2 W,
that for intensity is 10’*2 W/m2, and that for pressure is 2x10"-’ Nm2 = 2x10’^ Pa =
.0002 dyne/cm2 = .0002 ubar. All of these standards represent 0 decibels (dB).
Sound intensity, I in dB, can be calculated using the equation
1=10 log W/Wr 6
where W is the intensity and Wf is the reference or 10 W/m . An intensity ratio
of 2 (i.e. W/Wr = 2) is 3 dB relative to the reference. A power level, PWL, can be

12
17
calculated using the same equation except that the reference is 10 W. Often the
terms power and intensity are interchanged although each has a specific meaning.
Similarly sound pressure level, SPL in dB, can be calculated by
SPL= 20 log P/Pr. 7
In this equation P is the pressure and Pr is the reference. A pressure ratio of 2
equals 6 dB relative to the reference. Note from the equations that for any given
level in dB the power ratio and intensity ratio, W/Wr, equals the square of the
pressure ratio, P/Pf. Since 0 dB corresponds to both references, a sound pressure
level can be converted into an intensity level directly, provided, of course, the
pressure is measured in the far field.
Acoustic interference. Consider two sound waves travelling in opposite
directions. One logical question to ask is: What happens when the two waves
meet? The resultant pressures are simply the sum of the pressures of the two waves.
This is graphically shown for a pair of equal—amplitude sinusoids in Figure 3. In
Fig. 3a the waves are approaching each other from opposite directions. In 3b the
waves are positioned in space so that a portion of the waves occupying the same
space are in phase (i.e. the compressions of one wave occurs with the compression
of the other) and the pressures add to produce pressure that is greater than the
pressure of each individual wave. This is called constructive interference. In 3c the
pressures sum to zero since the phases of the individual waves are opposite and they
exactly cancel. This condition is destructive interference. As each wave continues
on its way they are unchanged even though there were great pressure changes when
the two waves met. When two sounds interfere destructively the energy of the
sound is not lost. When the pressure decreases, the particle velocity increases and

Figure 3. Acoustic interference and the summation of sinusoids.
a. Two equal—amplitude sine waves of the same frequency travelling
in opposite directions toward each other.
b. During this instant in time the two occupy the same space and the
pressure of the resultant sinusoid (solid line) is the sum of the two
individual waves. The resultant has an amplitude greater than either
of the individual waves; this condition is termed constructive
interference.
c. At this instant the individual waves are opposite in phase and their
pressures sum to zero (solid line). This is destructive interference.

14

15
there is a conservation of energy. The sum of any two sinusoids that have the same
frequency is another sinusoid of the same frequency but with a different amplitude.
Propagation and the inverse square law. In an ideal environment sound
waves radiate at the same rate in all directions from the source, and thus, the
wavefront is spherical in nature. Because the same acoustic energy must occupy a
greater surface area as the distance from the source increases, sound intensity
decreases with distance from the source, since intensity is measured in Watts per
unit area. The surface area of a sphere increases proportionately with the square of
its radius (4*pi*r^), and thus the intensity of a sound is inversely proportional to the
square of the distance from the source--the inverse square law.
Suppose the intensity is measured at some distance from the source. The
intensity at twice that distance will be one-fourth or about 6 dB less [10*log(0.25)].
If the power ratio is 0.25, the pressure ratio is 0.5. For the same doubling of
distance the pressure will also decrease by 6 dB [20*log(0.5)]. Pressure, the scalar
quantity, is proportional to the distance from the source. For every doubling of
distance the pressure is halved and it is this proportionality that will be used to
calculate sound fields of crickets.
Insect Bioacoustics
The study of insect bioacoustics, the special problems that insects have in
using acoustic signals to communicate, and the biophysical aspects of sound
production began more than 15 years ago and have been reviewed by
Bennet—Clark 1971, 1975, Miehelsen and Nocke 1974, Michelsen 1983 and Eisner
and Popov 1978. The interest in the field has continued to grow primarily because
the animals are easily maintained, they are easily manipulated for experimental
purposes, and they offer a host of sound producing mechanisms to study.

16
Many insect groups use acoustic signals to communicate: ants do it, cicadas
do it, lacewings do it, and moths and beetles do it. The complexity of the
communication varies from very simple sounds of hitting a part of the body on the
substrate to sounds that are extremely complex with distinct signals for special
contexts.- For instance crickets have different types of calls classified as calling,
courtship, or aggressive (Alexander 1961). The different life stages of passalid
beetles use acoustic signals to communicate, adult to young and young to adult.
These beetles have a repertoire of as many as seven different sounds used in 13
different behavioral contexts (Schuster 1975). Some species of insects may use
acoustic signals to communicate to other species of animals-for example, many of
the tiger moths that use sounds to confuse and deceive bat predators (FuIIard et al.
1979).
However, insects are very inefficient at producing sound because the
efficiency of sound production is dependent on the relationship between the size of
the oscillator or sound source, the frequency of the sound produced by the oscillator
and the immediate surroundings of the oscillator. To be an efficient radiator, the
diameter of the sound source must be about the size of, or larger than, the
wavelength of sound produced by the source. Because insects are small, and thus
their sound producing organs are small, they must produce a sound with very small
wavelengths to be efficient. That is, they must use high frequency sounds. However,
there is a disadvantage. High frequencies are attenuated by air at a faster rate than
low frequencies (Wiley and Richards 1978, Michelsen 1978). Therefore, given the
same power output for a high and low frequency signal, the intensity at the same
distance from the source will be greater for the low frequency than the high
frequency sound. Because of this, low frequency sounds are better for long distance

17
communication and insects must make a trade-off between the effective distance of
communication and the efficiency of signal production.
Insect muscles cannot contract at the high rate needed to produce
frequencies in the kHz range. Therefore, insects usually use some mechanism for
frequency multiplication. Most often this is accomplished by stridulation (Eisner
1983). During stridulation one part of the insect body is rubbed against another.
One part usually has a row of teeth, and during each stroke a number of teeth are
struck, each adding to the sound. The frequency of the muscle contraction is
multiplied by the number of teeth hit.
One of the most familiar groups of singing insects are the Orthoptera. This
group contains the grasshoppers, which produce song in at least three different ways
(Otte 1970). It includes cockroaches, some of which produce a hissing sound by the
expulsion of air from the spiracles (Nelson and Fraser 1980). But above all, the
crickets and katydids are the most noted and familiar singers of the group. Few
people have not heard a cricket.
Sound production in the Ensifera. The Ensifera are an ancient insect group
and include the Tettigoniidae (katydids), the Gryllidae (crickets), the Giyllotalpidae
(mole crickets) and the Haglidae. The forewings arc similarly modified for sound
production in the four families. While calling the wings are raised above the
abdomen and moved back and forth one on top of the other. Located on the
underside of the Cu2 vein of each wing is a row of teeth called the pars stridens or
file. On the posterior margin of each wing is the plectrum or scraper. Stridulation
occurs during the closing stroke of the wings when the plectrum is pulled against the
grain across the teeth of the file. In katydids the left wing is usually over the right,
in crickets the right is usually over the left, while in haglids and mole crickets each
configuration occurs with equal frequency (Masaki et al. in press, Forrest in press).

Figure 4. Sound produced by a vibrating disc.
a. As the disc moves to one side it compresses the particles of air and
at the same time produces a vacuum on the other side of the disc.
The shading of the circles represents the degree of displacement of
the particles (ie. black circles are compressions and no circles are
rarefactions).
b. Sound output and phase relationships of a dipole source.
c. Sound field of the dipole source. If the vibrating disc is small
relative to the wavelength of sound produced, the outputs from each
side of the disc will destructively interfere along the edges of the
disc. The result is a dumbbell shaped sound field.

19
I

20
In some species the wings have become specialized for their tasks and there is an
asymmetry between the left and right wings.
The file and scraper are closely associated with specific membranous areas of
the wings. The cell or cells are caused to vibrate at the frequency of the
tooth—impact rate of the scraper on the file. The cells are resonant near the
impact frequency, and the vibrations of the cells produce the sound.
The vibration of the wing membrane can be likened to the movement of a
piston or disc. As the disc moves back and forth in air, it produces a compression
and a rarefaction simultaneously on opposite sides of the disc. That is, a sound
wave is generated from each side of the disc, each being equal in amplitude and
frequency, but opposite in phase (Fig. 4). When the diameter of this doublet or
dipole source is small relative to the wavelength of sound produced, the sounds
radiating from the two sides of the disc destructively interfere along the edges of the
disc (Fig. 4b). The result is a dumbbell shaped sound field with a maximum
pressure perpendicular to the disc surface and a minimum pressure along its edges
(Fig. 4c). Of course the particle velocity at the edges is high and the intensity is
equal in all directions.
The membranes of crickets are less than one-tenth the wavelength of sound
produced, and they are inefficient primarily because of their size. Energetically,
producing sound may cost a male ten times as much as resting (Prestwich and
Walker 1981). However, because calling contributes importantly to a male’s
reproductive success, selection should be strong on males to increase signaling
efficiency and effectiveness. Crickets have evolved a number of behaviors that use
baffles to increase calling efficiency (see Forrest 1982 and refs.). The study of these
baffles and their effect on sound production is one important aspect of cricket
mating systems and reproductive biology.

21
One such baffle system is used by the southern and tawny mole crickets,
Scapteriscus acletus and vicinus. Both species construct a calling burrow shortly
after sunset, prior to the female flight period. The burrow is shaped by the cricket
and expands exponentially from the throat to the opening at the soil surface
(Nickerson et al. 1979).
Because the males call from within a burrow, their sound fields are very
suitable for measuring. The position of the male in the burrow is constant, as is the
position of the burrow opening. The calling songs are continuous trills that change
less than 1 dB in sound pressure during the 1 to 1.5 hours of calling (Forrest 1981,
1983), and an observer can move around outside the burrow with little disturbance
to the calling male. If the animal is disturbed, he runs down into the burrow system
but returns in a short period of time to the same calling position and resumes
advertising for females.
I measured 22 sound fields of Scapteriscus males (17 acletus. 5 vicinusl under
field conditions to determine the power output of the crickets, the efficiency of
sound production and the directional properties of the sound field.
Materials and Methods
Sound pressure level readings were taken using nine Brucl & Kjaer, model
4125 condenser microphones and type 2642 preamplifier. The typical frequency
response for these microphones is flat (+1 dB) from 200 Hz to 10 kHz for a free
field response and an angle of incidence of 0°. Directional characteristics show less
than 1 dB loss for +30° deviations from a 0° incidence angle for signal frequencies
at 2, 4, and 8 kHz (Bruel & Kjaer specifications).
The output of each microphone could be adjusted independently, and each
microphone was calibrated using a model 4230 B&K pistonphone calibrator.
Initially, each microphone was calibrated after each sound field was measured.

22
Later the calibration was done every two weeks. The stability of the microphones
was such that later they were calibrated only every month. Never was the level of
any of the microphones more than 0.5 dB from the calibration level of 94 dB.
The outputs of the microphones were multiplexed to a single input of a
model 2219 Brucl & Kjaer sound level meter that measured the sound pressure re
0.0002 dync/cirr (dB A). The entire system was battery operated and field portable.
Cables from the microphones were long enough so that the observer and equipment
were more than a meter from the sound source and nearest microphone. This
avoided reflection of sound waves off the observer’s body, and interference in the
pressure readings was reduced to a minimum.
The nine microphones were equally spaced around a semicircle of tubular
aluminum (every 22.5°, +2°) such that the diaphragm of each microphone was 25
cm from the center of the semicircle and the angle of incidence was zero (±2°). An
angle of incidence of 0° is recommended when the direction of the sound source is
known (Brucl 1983). At 25 cm the microphones were always more than 1
wavelength of the carrier frequency of the calling song of the mole crickets (aclctus
2.7 Khz or 12 cm wavelength, vicinus 3.3 Khz or 10 cm wavelength). Thus sound
pressure measurements were taken in the far field (Michelscn 1978).
By rotating the semicircle in 22.5° steps along one of the axes, sound
pressure levels could be taken at every 22.5° latitude and longitude of a hemisphere
surrounding the sound source located on the ground in the center of the semicircle.
A stand anchored one end of the hemisphere to the ground and a handle that
moved the semicircle locked every 22.5°. Axis of rotation was always with the
longitudinal axis of the cricket so that on every 22.5° rotation the two microphones
at the ends of the semicircle remained at the same anterior and posterior position
relative to the cricket burrow. This arrangement provided a check for changes in

23
the crickets’ output and added reliability to the measurements. Altogether, pressure
measurements were taken at 65 different positions surrounding the opening of the
cricket’s burrow with the anterior and posterior positions each having 9 separate
measurements. Sound fields with more than +0.5 dB variation in these 9
measurements were not used in analysis. These measurements represent the most
detailed measurements of cricket sound fields.
All sound fields were measured outdoors at the Horticultural Farm of the
University of Florida. Sound fields of crickets were measured while they called in
the field or while they called from soil—filled, 12-liter buckets buried so that the
bucket lip and the soil in the bucket were at ground level. Only crickets calling on
level ground were measured and all obstacles that might cause interference or
reflections were removed from the area.
To determine the influence of male size on acoustic output, the males were
captured and the lengths of their pronota along the midline were measured to the
nearest 0.1 mm using vernier calipers. The mass of each male was measured to the
nearest 1.0 mg with a model AC100 Mettlcr balance. A soil sample (2 cm dia X 4.5
cm deep) was taken within 5 cm of the burrow opening. This sample was dried and
moisture content determined. Soil temperature at a depth of 2.5 cm was also taken.
Calculation of sound fields and power output. Each of the 65 sound pressure
levels measured at 25 cm was converted to a radial distance from the cricket at
which the pressure would be 70 dB. For instance a pressure that read 73 dB at 25
cm would be 70 dB at about 35 cm. The points were converted to polar coordinates
and the areas of all triangles defined by adjacent triads of points were summed to
obtain the surface area of the sound field. A test of this procedure revealed that it
underestimates the surface areas of 24-200 cm dia hemispheres by only 2-3%.

24
Table 1. Power input, output and efficiency of calling male Scaptcriscus.
PNL
mm
MASS
mg
TEMP
°C
PULSE'
RATE
p/sec
1 SOIL
MOIST
POWER
OUT
uW
PWL
dB
POWERb
IN
mW
EFF.
%
Scaptcriscus acletus
7.7
574
26.8
56.2
0.081
3.310
65.2
9.145
0.04
7.7
574
25.2
54.0
0.099
4.403
66.4
8.749
0.05
7.7
574
25.4
54.3
N.A.
2.355
63.7
8.798
0.03
7.7
574
30.7
61.7
0.036
1.920
62.8
10.111
0.02
7.7
574
27.2
56.8
0.109
4.056
66.1
9.244
0.05
8.1
614
29.1
59.4
0.026
2.481
63.9
10.391
0.02
8.1
614
25.7
54.7
0.103
6.115
67.9
9.491
0.07
8.6
737
25.8
54.8
0.076
8.430
69.3
11.424
0.08
8.6
737
30.2
61.0
0.030
2.391
63.8
12.823
0.02
8.6
737
30.8
61.8
0.039
4.408
66.4
13.014
0.03
8.8
776
20.4
47.3
0.155
3.247
65.1
10.221
0.03
8.9
676
28.8
59.0
0.114
4.456
66.5
11.353
0.04
8.9
676
27.7
57.5
0.034
5.958
67.8
11.032
0.06
9.4
772
22.5
50.2
0.138
8.983
69.5
10.867
0.09
9.5
875
22.4
50.1
0.112
11.771
70.7
12.279
0.10
9.9
1172
19.1
45.4
0.219
22.386
73.5
14.779
0.16
10.0
1162
20.9
48.0
0.099
4.618
66.6
15.555
0.03
S. vicinus
8.3
890
N.A.
0.047
2.259
63.5
8.3
890
25.4
139.0
O.OS2
3.458
65.4
36.883
0.01
8.4
889
26.6
145.3
0.187
6.540
68.2
38.571
0.02
8.4
834
21.4
118.0
0.100
9.639
69.8
29.155
0.03
8.4
N.A.
21.2
116.9
0.078
4.175
66.2
? Based on relationship to soil temperature (Ulagaraj 1976).
13 Based on data from Prcstwich and Walker 1981.
N.A.- Not available or data not taken.

5. Relationship between total power output and male size and total power
and soil moisture in Scaptcriscus.
1 r)
a. Total power (dB re 10 W) as a function of male size (pronotal
length mm). Power (in Watts) of males with more than one sound
field measurement were averaged for regression analysis. Vertical
lines at these points show the range in power for individual males.
Regression equation is y = 2.4x + 46.4 (acletus only p<0.05, r -0.45).
Open circles are acletus males, filled circles are vicinus males.
b. Total power as a function of soil moisture (percent of dry weight of
soil sample). Connected points are for sound fields by the same
male at different soil moistures. Percent soil moisture was log
transformed for regression analysis y = 6.21og(x) + 61.5 (acletus only
p<0.05, r -0.40).

26
a
PWL (dB)
Pronotal Length (mm)

27
Since the measurements were taken in the far field, the 70 dB isobar
represents a 0.01 mW/m^ intensity level and the total power, in mW, is given by the
intensity level times the surface area.
Results
The acoustic power produced by calling male mole crickets ranged from 1.9 to 22.4
uW (62.8 - 73.5 dB re 10—^ W) for the 17 acletus sound fields and from 2.3 to 9.6
«W (63.5 - 69.8 dB) for the 5 vicinus sound fields (Table 1). Part of this variation in
the power output was due to the size of the male and the moisture in the soil
surrounding the male’s burrow. There was a significant relationship between male
size and power level (p<0.05, r^=0.45; Fig. 5a, Note that regression was done only on
sound fields from acletus males and that the acoustic powers for individuals for
which more than one sound field was measured were averaged and used for
computing the regression). There was also a significant relationship between the
soil moisture and power level (acletus only p<0.05, r^=0.40; Fig. 5b, Percent soil
moisture was log transformed for regression analysis). The data from individual
males also showed the same trend. That is, their power output increased with
increasing soil moisture (5 out of 7 acletus and 1 of 1 vicinus). However, a check to
see if soil moisture and male size were correlated revealed no significant
relationship and large males did not produce more power simply because they
called from moister soil.
The power output of a male can be estimated by a single measurement of the
sound pressure level directly above the burrow. There was a direct relationship
between the sound pressure directly above the burrow and the power level of the
sound field produced (p<0.05, r^=0.62; Fig. 6).
Using soil temperature, the pulse rate of each cricket (wing stroke rate) was
estimated (Ulagaraj 1976). Since the power used by the cricket is directly related to

Figure 6. Total power output (dB re 10"^ W) as a function of sound pressure (dB
re 2xl0"3 Nm^) directly above the calling burrow. Sound pressures
are measured at 25 cm from the burrow opening. The regression is
significant (p<0.05, r -.62) y = 0.58x + 26.0.

29

Figure 7. Sound field of Scapteriscus acletus. Sound field is base on an average of
17 radial distances at each of 65 radii. Standard errors around the 65
points are always less than 4 cm. Anterior is designated by ant, posterior
by pst, right by rit and left by 1ft.
a. View from above the sound field.
b. Looking from the crickets’ left.
c. A perspective view.

31

32
the wing stroke rate while calling (Prestwich and Walker 1981), I estimated the
percent efficiency of sound production for each sound field. Subtracting an
estimated resting metabolism of a 3out 370 i/liter O^g h (Prestwich unpublished,
personal communication 1986) from the metabolic rates while calling revealed that
the mole crickets were very inefficient. Percent efficiency ranged from 0.02 to 0.16
percent (Table 1). The data for estimated power used while calling are similar to
actual measurements of calling metabolism of male acletus (Prestwich, personal
communication 1986).
Because the individual sound fields were variable, an average sound field
based on the average radial distances along each of the 65 radii was computed for
the 17 sound fields from acletus males. The standard errors of the 65 means were
about 2.5 and always less than 4 cm. The shape of the sound field most closely
resembled a hemisphere and the mean distances did not differ significantly from the
radius of a 26 cm hemisphere. At ground level (X-Y plane, Fig. 7) the transverse
axis of the sound field was 48.4 cm and the longitudinal axis was 53.7 cm across.
Height of the sound field was 26.6 cm. Unlike the sound fields of other crickets, the
burrow opening acts as a point source and radiates equally in all directions. The
total power of the averaged sound field was 4.33 uW or 66.4 dB Using 26 cm as the
radius for a hemisperical sound field, the power should be 4.25 uW or 66.3 dB.
Discussion
The acoustic power produced by male Scapteriscus is less than one-
hundredth of the power produced by the French mole cricket, Orvliotalpa vmeae.
Bennet—Clark (1970) calculated a mean power output of 1.2 mW. Assuming that
this was produced by a total muscle power of 3.5 mW, Bennet—Clark calculated an
efficiency of 28-35 percent. However, the power used was probably underestimated
since he estimated power used from data on grasshopper flight muscles. Using data

33
from Prestwich and Walker (1981) Grvllotalpa vineac should use about 70 mW of
power for an average 3.3 g cricket calling with a pulse rate of 66 pulses/sec. The
efficiency then becomes 1.7 percent, a little less efficient than a typical loudspeaker.
This is still more than ten times more efficient than Scapteriscus. Since female
Scapteriscus preferentially land at louder males (Forrest 1980, 1983), males are
expected to call at their maximum output, but because Scapteriscus are one—sixth
to one—third the size of Grvllotalpa it seems likely that they are unable to produce
as much acoustical power because of their much smaller muscle mass. Note that the
power produced by Scapteriscus acletus was directly related to the size of the male
(Fig. 5a). Soil moisture also influenced output probably because it enables the
animal to pack the sandy soil better, and thus, it absorbs less of the acoustic energy.
Bennet—Clark (1970) also noted that moist soil increased output in Grvllotalpa
vineae.
The efficiency of sound production is somewhat lower in Scapteriscus spp.
than that reported for other animals that use acoustic signals to communicate.
Hylid frogs are about 1 to 3 percent efficient (Prestwich and Bruger, unpublished).
The frogs are probably more efficient because the calls are produced by vibration of
a vocal sac that acts much like a pulsating sphere. Such a source does not have the
acoustic short circuit a dipole source has, and if the vocal sacs of the frogs are the
proper dimensions relative to the wavelength of their calling songs they may have
resonant properties that would also increase the efficiency. Male bladder cicadas,
Cvstosoma saundersii. produce about 0.35 mW of acoustic power and are about 0.82
percent efficient. Like the frogs, the abdomen of the cicada acts as a baffle and
resonating system (MacNally and Young 1981). Other crickets have only slightly
higher calling efficiencies than those of Scapteriscus. Anurogrvllus arboreus are

34
about 0.2 percent efficient and Oecamhus cclcnnictus are from 0.3 to 1.3 percent
efficient (Prcstwich unpublished).
The sound fields of individual males were often irregular although they were
similar in overall shape to the average distribution pattern (Fig. 7). Since a new
burrow is constructed each evening, the openings may be very different from cricket
to cricket and from night to night for the same cricket and this probably causes the
irregularities. The distribution pattern of sound from a mole cricket burrow differs
considerably from the directional patterns of other crickets. The burrow acts as an
infinite baffle around the vibrating membranes raised in the throat of the horn
(Bennct—Clark 1970, Nickerson et al. 1979). The burrow opening then produces a
point source of sound and the waves radiate equally in all directions. The result is a
spherical shaped distribution pattern.
The sound fields of other crickets usually have some directional properties.
Anurogrvllus arboreus (Paul and Walker 1979) and Grvllus campestris (Nocke
1971) have directional characteristics due to the destructive interference along the
edges of the wings. Probably the most noticeable sound fields for their directional
quality are those of tree crickets (Williams 1945). The sound pressure may be more
than 20 dB less at the sides of the cricket compared to anterior and posterior
pressures, and males often turn while calling seemingly to direct sound in different
directions (Toms 1984, Forrest unpublished). Male Grvllotalpa vineae also produce
a directional pattern because the two burrow openings act like dual speakers, and
there is a characteristic radiation pattern dependent upon the relationship of
distance between the two openings and the wavelength of the sound produced. This
radiation pattern of Grvllotalpa vineae is such that it should increase the probability
that a female will intercept the sound fields (sec Bennct—Clark 1975 and Forrest
1982 for a discussion of selection on directional sound fields).

35
In summary, the power output of male Scapteriscus is relatively small
compared to other species of acoustically signaling animals, and male mole crickets
are very inefficient at producing sound (Table 1). Large males produce greater
sound power than smaller males, and moist calling sites increase the total output
(Fig. 5). The radiation pattern of sound from the calling burrow is hemispherical in
shape (Fig. 7), and the total power can be estimated from a single measurement of
sound pressure above the burrow (Fig. 6).

CHAPTER II
OPPOSITION AND MATERNAL INVESTMENT
It’s only the giving that makes you what you are.
(Ian Anderson)
Introduction
Females of some insects may lay as many as 600,000 eggs during their
lifetime although the norm is usually a few hundred (Hinton 1981). These nutrient-
filled eggs are a major investment in offspring, and parental investment, especially
the differences in investment between the sexes, is a predominant force in selection
on reproductive strategies (Trivers 1972), life history tactics, and resource allocation
(Boggs 1981). In most instances, investment by males ends at copulation or
fertilization (see Thornhill [1976] on paternal investment in insects), and females
generally do not invest after oviposition. Unlike paternal investment, which is
frequently difficult to assess, maternal investment can often be measured simply by
determining the mass of eggs produced.
In crickets oviposition and oviposition sites arc diverse and eggs are
extremely variable in size, number laid, and whether or not they are diapausing.
While a great deal of attention has been given to nuptial feeding and paternal
investment in katydids (Gwynne 1981, 1983), crickets (Sakaluk 1984), and
cockroaches (Mullins and Keil 1980), few studies have involved orthopteran egg
deposition and measurement of maternal investment in eggs. This study reports the
oviposition of southern and tawny mole crickets. I followed the reproductive output
of females caged under field conditions and studied the influence of size, season,
36

37
and aging on female investment in eggs and female fitness as measured by the
number of hatchlings produced. Egg laying in these species was earlier described by
Hayslip (1943), but his study concerned mole cricket life histories and lacked
long-term monitoring of individual females. Walker and Nation (1982) studied
fall mating, egg maturation, and sperm storage of individuals but did not examine
maternal investment in offspring.
Materials and Methods
All studies of oviposition were done in Gainesville, Florida, and involved
females that were collected after they were attracted to the conspecific male calling
song: either an artificial song (Walker 1982) or the natural song of a male calling
from a soil-filled bucket surrounded by a trapping device (Forrest 1983). Females
were placed in buckets of soil (19—liter) and were fed 10 emJ of ground, dry dog
food each week, enough to ensure that food was not a limiting factor in egg
production. Dog food was chosen because it had proven to be an adequate food in
maintaining other crickets in the laboratory. The buckets were kept outdoors under
two conditions. 1) The buckets were on the ground and screen lids prevented the
crickets’ escape. 2) Buckets were buried flush with the soil surface, surrounded by a
doughnut-shaped pitfall, and covered with a cylindrical, hardware cloth cage.
Females had to fly to leave the buckets and in so doing they hit the cage, fell, and
were trapped. Trapped females were then placed in new buckets and the old
buckets were checked for eggs. If females could not leave because of screen lids, or
if they did not fly from a bucket, their buckets were examined for oviposition and
mortality about every 21 days (13 to 24 days depending on temperature).
Examination of buckets consisted of carefully scraping thin layers of soil (0.5-
1 cm) from the bucket using a spatulalike piece of sheet metal. This allowed the egg
cells to be excavated without damaging the eggs. The depth and the number of eggs

38
of each egg cell were noted and the eggs were placed on moist soil in numbered,
aluminum containers. To estimate investment in each clutch, a sample of eggs was
dried (at 30-37°C) and weighed to the nearest 1.0 mg using a model AC 100 Mettler
balance. Remaining eggs were kept in the aluminum containers at room
temperature and monitored daily until hatched.
Oviposition Season
Females of each species (n= 11 vicinus. 9 acletus) were collected before
oviposition during the 1982 fall flight season and placed singly in buckets with a
male (condition 1 above). During the following spring, surviving females and spring
females collected before oviposition were kept (condition 2) throughout the flight
and reproductive seasons. Because females were found to lay no eggs in the fall,
data from spring 1982 were included in analysis of reproductive season.
Oviposition Cycles and Flight
During the springs of 1982 and 1983, females were kept in buckets (condition
2) and their flights monitored to determine the relationship between oviposition
and flight activity. The duration between hatching dates of successive egg clutches
from individual females was used as an estimate of time between egg clutches.
Effects of Size and Age on Fecundity
Females varying in size were kept singly in buckets (condition 2) with a male
and monitored for oviposition throughout their reproductive lifetime. Only females
collected prior to any reproductive activities or that were known virgins were used.
The wet weight (to the nearest 1.0 mg) of females was measured only once prior to
their use in the experiment with a model AC100 Mettler balance. Pronotal length
measured to the nearest 0.1 mm with vernier calipers was used as an indication of
female size. In most cases, pronotal length was used because it has a direct

39
relationship to female weight (r^=0.72 aclctus. 0.66 vicinus. p < 0.001 see Chapter
III), is less variable than weight, and is easily measured in the field.
Insect eggs of many speeies absorb water during development, thus
increasing both volume and weight (Hinton 1981). Because of differences in water
content among eggs, dry weights were used as a measure of investment. Investment
in individual eggs (mean dry weight per egg of samples), investment in clutches
(mean dry weight of egg times number of eggs), and number of offspring produced
(percent hatch times number of eggs) were measured for successive clutches of each
female.
Results
Mortality. Longevity, and Seasonal Qviposition
Of 9 acletus and 11 vicinus monitored during the fall, only 3 (33%) and 4
(36%) survived the winter. Since mortality was concentrated in colder months, this
high mortality was probably due to buckets being placed above ground, increasing
their exposure to cold temperatures during the winter. There was no apparent
influence of size on mortality. No eggs were laid by any of the fall females and
when females were found while examining buckets for eggs they were motionless or
extremely sluggish. Of all females monitored during 1982 and 1983, 54% (n = 24) of
vicinus and 41% (n = 22) of acletus females died before reproducing (including one
vicinus and one aclctus that were killed during examination of their buckets).
Longevity of females in buckets was highly variable and ranged from 16 to 278 days
(mean 91) for acletus and from 7 to 254 days (mean 89) for vicinus females.
Egg—laying season was similar for the two species during the springs of 1982
and 1983 (Fig. 8). Oviposition started in late April and stopped in late July or early
August, with most egg clutches produced in May and June. The proportion of

Oviposition season in mole crickets (a) aclctus and (b) S. vicinus.
Graphs show the monthly proportion of egg clutches laid. For each
month, bar on the left represents data for spring 1982 and the bar on
right data for spring 1983. Note greater proportion of unsuccessful
clutches (0-50% hatch) late in the season.

o
PROPORTION OF CLUTCHES LAID
ppo
o —*■ Ko 03
1 1 1 J i l i i I
O'
>
T)
ID
S. vicinus
0-50% hatch
» 50% hatch
APR MAY JUN JUL AUG
PROPORTION OF CLUTCHES LAID
poop
o -1 fo CO ^
1 1 I 1 I I I I I
Q)
â–¡
Co
CO
o
CD
I

42
Table 2. Correlation matrix3 of paired comparisons for S. acletus (top half of table)
and S. vicinus (bottom half, underlined).
Clutch
no.
% Hatch
Egg
wt
No. of eggs
per clutch
Clutch
no.
-0.40
-0.72
-0.32
% Hatch
-0.61
0.38
0.32
Egg
wt
-0.66
0.49
0.30
No. of eggs
per clutch
-0.61
0.65
0.57
3 Spearman’s rank correlation (rho). Ail correlations are significant (p < 0.01).

43
unsuccessful clutches (<50% hatch) increased as the reproductive season progressed
(Fig. 8). This was probably the effect of decreased hatching in later clutches of
females (Table 2) rather than an influence by season alone.
Oviposition Cycles and Flight
The average interval between clutches, as estimated by the hatching dates of
successive clutches, was 10 days for acletus (n = 38, 95% CL 8-11 days) and 9 days
for vicinus (n = 32, 95% CL 7-12 days).
Females rarely flew between egg clutches. In only 1 of 68 instances did a
vicinus female (n = 9) take flight between the production of egg clutches. Of all
females producing eggs, seven vicinus never left buckets, three flew prior to egg
production but not after, and one flew after egg production but not before. Female
acletus (n = 13) were more likely to take flight between clutches (13 of 43
occasions). For acletus females producing eggs, six never flew from their buckets,
two flew after they began laying eggs, and five flew both before and after they
started ovipositing.
Reproduction and Investment bv Individuals
Females produced variable numbers of eggs and clutches; some died before
ever reproducing, while others produced more than 450 eggs and as many as 10
clutches. Part of the variation in fecundity was due to size differences among the
females. Excluding females that produced fewer than three clutches, there was a
significant positive relationship between pronotal length of acletus females and the
total number of offspring produced (sum of the number of eggs per clutch times
percent hatch of the clutch, Fig. 9, p < 0.05) and between pronotal length and
average number of eggs per clutch (Fig. 10, p < 0.01). These relationships were not
significant for vicinus females. There was no relationship between female size and
the number of clutches or the size of eggs produced for either species.

Figure 9. Relationship of size and total number of offspring produced for 12
S. acletus females eagefl outdoors. Line is calculated using the
regression equation = 0.33, p < 0.05).

Total
Offspring

Figure 10. Mean number of eggs per clutch for 12 acletus females varying in
size caged outdoors. Line is calculated using the regression
equation (r-1" = 0.50, p < 0.01).

47
Heart Number
of
Eggs/Clutch

Figure 11. Plot of ranked egg weight for successive clutches of female mole
crickets. Egg weight is the mean dry weight per egg of a sample
from each clutch. Clutches with the same mean weight were given
averaged ranks. Points connected by lines represent points at the
same position on the graph. The number of females producing a
specific number of clutches can be found by counting the points for
each clutch number. Note that females invest less (smaller weight =
higher rank) in later clutches (higher clutch numbers), (a) S; acletus.
(b) Si vicinus.

CLUTCH CLUTCH
RANKED EGG WEIGHT
Ol a> -i 00 «3
niH*
I ! + •
0)
•*t :
i
•t
HANKED EGG WEIGHT
-*fOtd^CJiOl-NJCO<0
f.
tt
•+ *i
• t
O

50
Average investment per clutch was from 10 to 24 percent of female body
weight for aclctus and ranged from 5 to 16 percent of female body for vicintis
females. Total lifetime investment was greater than 100 percent of female body
weight for 6 of 12 acletus females and for 1 of 8 vicinus females. Since dry weight of
eggs was measured and female body weight was measured while the crickets were
alive (i.c., they do not represent dry weight of females), the investments in eggs are
actually substantially larger.
in both species, investment per egg and per clutch decreased as females
produced successive clutches. There was a significant inverse correlation between
weight of eggs and clutch number (Fig. 11) as well as an inverse correlation between
the number of eggs per clutch and clutch number (Table 2). Also, the percent hatch
for clutches and mass per egg were significantly and negatively correlated with
clutch number (Table 2). Later clutches of a female had fewer eggs, smaller eggs,
and lower percent hatch.
Discussion
My observations on oviposition did not differ from Hayslip’s (1943) Plant
City, Florida, observations with regard to depth and average number of eggs found
for egg cells of acletus or vicinus. Oviposition season was also similar for vicinus:
however, acletus in Plant City extended oviposition into September. This may be
the result of warmer temperatures and a longer growing season farther south (ca.
190 km), conditions that result in a bivoltinc life cycle for acletus in South Florida
(Walker et al. 1983). I did not find fall oviposition in buckets by females of cither
species, but flush samples taken during November in Gainesville have contained
hatchling aclctus suggesting that fall oviposition may occur, although young

51
juveniles apparently do not survive the winter (Stackhouse W., E. L. Matheny and
W. G. Hudson unpublished data).
Even though oviposition seasons of acletus and vicinus are similar, flight
seasons are not. Most of the flight season of vicinus is over by the start of
oviposition, while acletus flights are concurrent with oviposition (compare Fig. 2 of
Walker et al. [1983] with Fig. 8). This agrees with differences in flight from buckets
in the two species. That acletus females are more likely to fly between clutches may
have been one factor contributing to the more rapid spread of this species after its
introduction into the United States (Walker and Nickle 1981).
Clutch cycles run between 7 and 12 days as evidenced by the average time
between hatching of successive egg clutches of females. This agrees with earlier
work that estimated a 9 day period for a female to mature an egg clutch based upon
the measurement of primary oocytes of females (Forrest 1981). That flying females
have small oocytes suggests that they recently matured or laid a clutch and are
starting a new clutch cycle (Forrest 1981). Ngo and Beck (1982), working on
dispersal flights of acletus. noticed a significant 10—day peak in recaptures of
marked and released females. Oviposition cycles may have accounted for this peak.
They also found 12—day cyelcs in overall flight activity and attributed it to female
oviposition cycles. This assumes that females are synchronous in their egg laying. I
found no such synchrony. Because acletus females in my tests flew between only
30% of the egg clutches produced, it seems unlikely that peaks in flight activity are
due to oviposition cycles. Such peaks are more likely caused by environmental
factors making flights favorable or synchronizing maturation.
Larger eggs generally produce higher quality, more vigorous progeny that
may be more mobile, may suffer less mortality during early stages, may develop
faster, or may require fewer instars (Capincra 1979). But given finite resources,

52
producing larger eggs may not always increase the fitness of the female and a trade¬
off should exist between number of eggs produced and their size (Smith and
Frctwell 1974, Parker and Begon 1986). Producing small eggs is apparently
disadvantageous for mole cricket females; because egg size and hatching success
were positively correlated in both species. Producing fewer eggs may also hamper
escape from the egg cell if siblings must cooperate while digging to the surface.
Large clutches of large eggs then should maximize a female’s fitness. However, as
females aged they produced smaller clutches with smaller eggs. If clutch size is
important and resources are limited as females get older, they may divert some of
their resources away from egg size, at the cost of hatching success, to increase the
clutch size. That later clutches are smaller with smaller eggs could have also been
the effect of longer lifespans than would normally occur under natural conditions.
Organisms rarely survive their full potential life span and theories on the evolution
of senescence suggest that selection should act to increase reproductive output early
in life at its expense later (Hamilton 1966). Females kept in buckets were protected
from hazards, and, thus, their becoming senescent may have influenced the size of
their later clutches. Another orthopteroid, the stick insect Diapheromera velii.
shows a similar decline in the rate of egg production and a decrease in investment in
eggs later in life (Sivinski 1977).
Gwynne (1983) related sexual differences in mating behaviors to differences
in parental investment in Tettigoniidae and other Orthoptcra. As sexual selection
theory predicted (Trivers 1972), when males invest more they are more selective of
mates than females, whereas females are more selective and males more
competitive when maternal investment is greater than paternal investment.
Investment in progeny by female mole crickets is far greater than that for males.
Males provide no paternal investment except the possible energy a female receives

53
from consuming the small spermatophore (<1 mm diam, estimated «0.1% of male
body weight) a male passes to the female during mating. On the other hand, a
female’s average investment per clutch may be as much as 24% of her body weight.
Behavioral differences between the sexes support predictions based on differences
in parental investment. Male mole crickets display acoustically, are aggressive, and
often fight near calling burrows, whereas females are discriminating in their choice
of mates (Forrest 1983).
In a model of optimum egg size and clutch size in insects, Parker and Begon
(1986) showed that if larval success was determined solely by the size of the egg,
then selection would act such that females all produced eggs that were the same
size. A consequence of this is that large females can produce larger clutches than
smaller females. Data from mole crickets fit the model. There was a positive
correlation between egg size (investment) and hatching success of clutches for mole
crickets. Mean clutch-specific egg size did not differ between large and small
females nor did the rate of decline of investment in eggs or egg clutches. Also,
larger females produced larger clutches.
Although a direct influence of size on fecundity is common in insects, few
studies have been conducted under natural conditions and few data are available for
Orthoptcra. I found a significant relationship between female size and mean clutch
size (Fig. 10) and total offspring produced (Fig. 9) for acletus but not vicinus
females. That only one of eight vicinus females invested more than her body weight
in lifetime egg output compared with she of twelve acletus females suggests that this
difference may be due to the inability of vicinus. primarily a herbivore, to assimilate
the dog food provided as food. Female vicinus would then have to rely on juvenile
reserves for egg production, and although a larger female may carry greater reserves
these would probably not be enough to produce more than a few clutches. Dog

54
food, however, was able to keep the females alive for an average of 90 days and in
some cases for more than 250 days, indicating that this diet was reasonably adequate
for the crickets. Female vicinus might simply consume more of the poor quality
food, and, thus, expend more energy to compensate for its lack of nutritional value
(see Slansky [1982] for a review of insect nutritional ecology).
Large aclctus females can produce almost 3 times as many offspring and
more than 1.5 times as many eggs per clutch as small females. Gwynne (1984)
found that female size and nuptial feeding by males directly affected fecundity in the
bushcricket, Requena verticalis. Females feeding on seven spermatophylaxes more
than doubled egg production, and the size of eggs produced also increased. There is
also a relationship between the number of ovariolcs (and presumably reproductive
output) and size in bctwecn-spccics comparisons of melanopline grasshoppers
(Bellinger and Pienkowski 1985).
Female investment and fecundity and their relationship with size tactics and
developmental strategics in insects have received almost no attention, though such
selective forces shape population size structure and influence population dynamics.
Assuming a direct relationship between fecundity and size, Masaki (1967) explained
the latitudinal size variation in the Emma field cricket. Telcogrvllus emma. on the
basis of a balance between the opposing selection pressures of increasing fecundity
with increasing size and decreasing size with shortened growing season of more
northern latitudes. I have used similar explanations for within-population size
variation and changes in the seasonal distributions of sizes of both male and female
mole crickets (see Chapter III). These types of data should improve models of
population dynamics and help explain the life-history strategies in insects from an
individual, evolutionary perspective.

CHAPTER III
SIZE TACTICS AND DEVELOPMENTAL STRATEGIES
From early days of infancy,
through trembling years of youth
Long murky middle age
and final hours long in the tooth.
(Ian Anderson)
Introduction
Size is one of the most important attributes of any organism, and within any
species there is often a great deal of variation in size among individuals. An
animal’s size will profoundly affect the prey it can eat, what predators it has, its
physiology and energetics, and its reproductive success (Peters 1983). Thus an
organism’s size and life history are intimately related. However, research on life
history strategies has virtually ignored the relationship between size and
development, and has focused primarily on tactics of diapause, migration and
dispersal (Blakley 1981; Denno and Dingle 1981).
Animals with complex life cycles are interesting with regard to size tactics
because certain life stages are specifically evolved for growth and increasing body
size, while other stages are adapted for dispersal and/or reproduction. Amphibians
and insects are perhaps the best known groups with complex life cycles. Of the two,
insects offer a simpler system in which to study size tactics because growth occurs
only in juvenile and non-reproductive stages (see Wilbur and Collins 1973; Smith-
Gill and Berven 1979; Werner 1986 for a discussion of amphibian metamorphosis).
Unlike amphibians, there is usually no growth once an insect becomes adult (i.e., no
increase in size although abdominal stretching may occur especially in females), and
55

56
adult size is determined by what happens during juvenile stages. Individuals can
make decisions about their size by using certain cues to determine when to molt to
adult.
Costs and Benefits
Regarding decisions, I am not implying that the animals make a conscious
decision but rather these arc evolved responses to cues the individual has available.
The decisions an individual makes about changes in size carry with them costs and
benefits. Although difficult to assess, these costs and benefits are ultimately
measured in terms of fitness. Accordingly, selection should act such that individuals
evaluate the costs and benefits and make those decisions that will maximize fitness.
For insects the decision an individual makes about its size can be simplified to a
decision of whether or not to become an adult. Figure 12 shows some of the costs
and benefits associated with a yes or no decision to mature, that is to molt to pupa
or adult. For instance, at a point in the development of an individual the decision
to continue as a juvenile (no-decision Fig. 12) has associated with it costs due to
delayed reproduction with the risk of dying before maturing and reproducing.
There is also the cost of limited mobility since juveniles cannot fly. Benefits to an
individual remaining a juvenile are due to an increase in fitness because of a longer
period of growth and thus a further increase in size, assuming there is a direct effect
of adult size on fitness (see below).
Graphically the cost/benefit curves might look like Figure 13. Early in its
development an individual might benefit a great deal because of small increases in
size while the costs of the same size change increase only slightly. Later on small
changes in size increase fitness (benefits) very little while costs for the same amount
of change become increasingly more expensive. The individual is expected to adjust
its development such that it maximizes the difference between benefits and costs

Figure 12. Costs and benefits to an individual associated with yes or no
decisions of whether or not to become an adult.

58
BENEFITS
COSTS
BECOME ADULT?
NO YES
attain larger size longer/earlier reproduction
flight
delayed reproduction size restricted
no flight

Figure 13. Hypothetical cost/bencfit curves for changes in an individual’s size.
Selection is expected to act such that individuals weigh costs and
benefits and become adult when the difference between the curves is
maximized (dashed line).

Cost
or
Benefit
*
Size

61
(i.e. the individual is expected to make the decision to molt to adult at the size
indicated by the vertical line). Cues such as population density, availability and
suitability of food and mates, size of competitors, sex ratio, length of the remaining
growing season, and "expected" temperature (particularly important for
poikilotherms) should be used in making the decision of when to molt.
Although some authors have focused on an optimal body size within a
species (see Roff 1981), it is the variation in sizes among individuals and the
deviation from the optimal that needs to be investigated. Previous work on size
variation in insects dealt with variation between populations. Primarily latitudinal or
altitudinal differences among the populations were used to explain the variation
(Masaki 1967, 1978; Dearn 1977), and Roff (1980,1983) has proposed models that
use season length, = generation length, which varies with latitude and altitude, as a
tool in predicting insect size and life histories. Proximate causes of size variation in
insect populations include temperature gradients (Sweeney and Vannote 1978),
larval diet (Slansky 1982; Andersen and Nilssen 1983; Palmer 1984) and population
density (Peters and Barbosa 1977). Borgia (1979) discussed variation in male size
and the decisions males should make in weighing costs and benefits of changes in
size. Borgia was primarily interested in male size and its importance in a male’s
ability to control specific resources, but such tactics and decisions should be
applicable whenever size influences an individual’s fitness.
Size and fitness in insects
For poikilotherms, reproductive success or output is almost always
influenced by size (Peters 1983). It is well known that for many insects there are
positive relationships between female size and the number of eggs, size of eggs, and
size of clutches produced (Hinton 1981). Only recently have there been attempts to
correlate male reproductive success with size. In species where males compete for

62
females (female defense polygyny) or for resources used by females (resource
defense polygyny) large males often have a reproductive advantage over smaller
males that may have to adopt alternative mating strategies to acquire mates. Where
female choice is involved, females often choose males on the basis of cues
correlated with male size (Thornhill and Alcock 1983 and refs.) Thus it is well
documented among insects that large size confers some advantage in fitness for
both sexes. For this reason insects are appropriate for investigating the occurrences
of size tactics and developmental strategies.
Size Tactics in Mole Crickets
Costs and benefits associated with size are almost impossible to measure
directly since they may differ from one individual to the next and may change at any
time during an individual’s life. However, developmental strategies should be
reflected in adult sizes, and certain predictions about changes in size distribution for
a species can often be tested. To examine whether individuals weigh costs and
benefits associated with changes in size I tested predictions about seasonal
differences and within-season changes in size distributions for two species of mole
cricket.
Size is an important component in the sexual success of an individual. For
males, the intensity of the calling song is correlated with male size-larger males
produce louder calls (Forrest 1980,1983 and Chapter I). Flying females selectively
respond to and land at louder calls, and all things being equal, larger males will
attract and mate with more females than smaller males (Forrest 1983). Larger
females also have a reproductive advantage over smaller females. Large females
may produce as many as 3 times as many offspring and more than 1.5 times as many
eggs per clutch than smaller females (see Chapter If).

63
Predictions
Since reproduction occurs only during a brief period the following spring,
during the fall the cost to an individual due to delayed reproduction associated with
remaining a juvenile is zero (see Fig. 12). One would predict that individuals that
are smaller than average would overwinter as juveniles to increase their size, while
those that are larger than average would gain relatively little by increasing in size,
but more by becoming adult, dispersing to favorable overwintering sites, and
preparing for reproduction early the following spring. There may be a cost of
delayed maturation due to limited movement. For instance, if an individual found
itself in an unfavorable site where the likelihood of surviving the winter is very low
(i.e., the costs are very high), that individual should mature and disperse to a more
favorable site. Ffowevcr, this cost would be borne by large individuals as well as
small, while the additional cost of maturing at a small size is borne only by small
individuals. Therefore Í expected a greater proportion of larger than average
individuals in fall flights compared to spring flights.
The second prediction involves an expected change in the size distribution
during the spring reproductive season. Because the egg laying (reproductive)
season is brief, individuals that remain juveniles incur greater and greater costs as
the season progresses. Because the chance of surviving to the following
reproductive season (the next spring) is very small, the costs of delaying
reproduction increase dramatically and soon outweigh the benefits of increasing in
size. Í predicted that as the spring season progressed smaller and smaller
individuals should mature.
Materials and Methods
Quarter-monthly samples (up to 50 individuals for 1981, 25 for 1982) of adult
male and female mole crickets, acletus and vicinus. were obtained from a routine

64
sound trapping station (Walker 1982) on the Agronomy Farm of the University of
Florida. Samples were taken during the spring and fall flight seasons of both years.
Since fall adults are of the same generation as adults of the following spring,
measurements spanned three generations, and samples from spring 1981 and fall
1982 represented only part of the 80-81 and 82-83 generations respectively.
To prevent desiccation, adults were placed in 13 dram snap cap vials
containing moist soil but no food. Measurements of pronotal length (to nearest
0.1mm measured along the midline using vernier calipers) and mass (to nearest 1.0
mg using a model AC100 Mettlcr balance) were taken for each individual within 24
hr of capture.
Results
There was a significant positive relationship between mass and pronotal length (Fig.
14). A linear model is shown because this model explained as much of the variation
in the data as did the more usual model of mass equals the cube of a linear
dimension. The regressions differed between species, whereas within a species the
sexes had similar slopes that did not differ significantly. In both species the
intercept was smaller for males than females (significant for acletus but not vicinus).
thus for a given pronotal length males weighed less even though males on average
are the larger sex (Fig. 14).
Figure 15 shows the size distributions for each sex during spring and fall
seasons (combined for both years). Distributions were approximately normal and
male and female sizes were broadly overlapping. On average males were larger
than females (Fig. 15, Table 3). For acletus the distributions changed seasonally.
Fall adults were larger than spring adults (Fig. 15a & b). The distributions for
vicinus females suggest the opposite trend (though not significant Table 3), but the
exaggeration of the fall proportions is the result of a small sample of fall vicinus

Figure 14. Linear relationship between pronotal length and mass for male and
female S. acletus (circles'! and vicimis (squares). Lines are plotted
using regression equations and the endpoints represent the extreme
values of pronotal length for each group. All regressions arc
significant p<.001. N=1222 male and 1431 female acletus: 284 male
and 714 female vicinus.

Mass [mg]
66

67
Table 3. Mean pronotal length3 (mm), standard deviation, and number of adult
mole crickets trapped during spring and fall flight of 1981 and 1982.
SPECIES
sex
SPRING
1981
1982
FALL
1981
1982
S. aclctus
males
8.8, .7a
8.8, .7a
9.5, .6b
9.3, .5b
(n=594)
(369)
(32)
(227)
females
7.9, .53
7.9, .53
8.4, .4C
8.2, .4b
(709)
(371)
(102)
(249)
S. vicinus
males
8.4, ,5b
8.2, .5a
8.3, .4ab
(162)
(119)
(0)
(3)
females
7.9, .4b
7.8, ,4a
8.0, .6ab
7.8, .4ab
(379)
(273)
(3)
(59)
“Means in the same row followed by different letters are significantly different
(p<.05).

Frequency distributions (expressed as a proportion of season total)
of adult mole crickets captured during 1981 and 1982 flights at
Gainesville, Florida.

Pronolal Length (mm)
OV
VO
S acietus

70
(n=62). Only three vicinus males were captured during the two fall seasons and
were not included in the figure.
Mean pronotal lengths were significantly larger for fall aclctus than for
spring individuals, whereas vicinus did not differ in average pronotal lengths
between the two seasons. Pronotal lengths were similar for seasons compared
between years (Table 3). Note also that fall individuals typically showed less
variation in size than those in the spring.
Mean pronotal lengths of quarter-monthly samples decreased during the
spring flight season and reached a minimum toward the end of the reproductive
season (Fig. 16). During the spring there was a significant relationship between
pronotal length and the week of the season, but this relationship was not significant
for fall individuals. This relationship explained less than 10 percent of the variation
in the data when individual measurements were used in the regression analysis (Fig.
16). A regression using sample means (for samples with more than 10 individuals)
and date of sample explained 37-89% of the variation in the mean pronotal length
(Fig. 16). Data for vicinus males are not shown because few samples contained
more than 10 males (1981 n=4, 1982 n=5). The relationship was significant for
individual measurements of male vicinus for spring of 1982 (p<.05) and both years
combined (p=.05); however no significant relationships were found between the
nine sample means and date of capture.
Discussion
The data on seasonal and distributional shifts in mole cricket sizes support
the predictions and suggest that individuals weigh costs and benefits associated with
changes in size (Fig. 12). There was one major difference between the two species
that may indicate different overwintering strategics. There was a significant
difference between fall and spring adult sizes for aclctus but not vicinus. According

Plot of mean pronotal length of quarter-monthly samples (where
N>10) for adult mole crickets captured during 1981 and 1982 flights.
For all means shown standard errors are about 0.1mm. Trend lines
plotted from regression equations for individual measurements for
1981 and 1982 combined (solid) or separately in the case of vicinus
(dashed). Spring declines in sample means were significant--
A) 1981:r2=.3S, p<.03. 1982: .89, <.001.1981+82: .70, <.001.
B) 19Sl:r2=.89, p<.001. 1982: .72, <.001.1981+82: .79, <.001.
C) 1981:r2=.45, p<-03. 1982: .62, <.004. 1981+82- NS.

FEMALES
Pronotal Length (mm)
Pronotal Length (mm) Pronotal Length (mm)
-j
K
S. acletus

73
to the predictions made, individuals that are smaller than average in the fall should
wait until the following spring to mature. That adult vicinus arc less variable in size
than acletus (Fig. 15) may have made the detection of seasonal differences more
difficult. It is interesting that a greater percentage (85%) of the vicinus population
overwinter as adults than in the acletus population (25%). Forage quantity and
quality are greatly reduced during the winter months and, since vicinus is mainly
herbivorous, this may limit the potential for growth during that part of the year.
Benefits due to overwintering as a juvenile would be lessened and it may be more
important to locate overwintering sites that are favorable for survival and allow
early reproduction the following spring.
I found that as the reproductive season progressed there was a decline in the
pronotal lengths of individuals for successive samples. That is, a significant
relationship was found between pronotal length and capture date. Although the
model did not explain much of the variation in measurements of individuals the
overall trend in the data (sample means Fig. 16) was highly significant. This might
be expected since all sizes of individuals should be maturing throughout the season,
but the proportion of small individuals increases dramatically toward the end of the
season. Small individuals have probably found themselves in areas where growing
conditions are poor and they mature at the smaller size in order to makes the ’best
of a bad situation’ (Dawkins 1980).
Rogers (1942), working with a number of tipulid species, noticed a decline
(as much as 20%) in the size of late season specimens and attributed the decline to
lack of food and moisture. Similar seasonal changes were also found in the
milkweed leaf beetle, Lahidomera clivicollis (Palmer 1984). There was a significant
gradual decline in male and female sizes during the growing season caused by a
decline in the amount of available food. Food quality was not discussed and may be

even more important. Slansky (1982) pointed to the growing interest in insect
nutritional ecology and discussed the decisions individuals must make relative to
nutrition and their final adult size.
74
For mole crickets, the similarity from year to year in the time at which the
smallest sample means occur suggests that photoperiod may be one proximate cue
used in the molting decision. However, cold weather during the spring of 1981
caused a delay in the vicinus flights (Walker et al. 1983) and probably delayed
maturation and caused the shift seen in the seasonal decline in size for 1981
(dashed lines Fig. 16c). Since acletus flights occur later in the spring, cold weather
had no affect on delaying maturation or the decline in size during the spring
months. Other proximate causes for the observed decline, such as food and
temperature, seem unlikely. Food quantity and quality as well as temperature are
increasing during the months of May, June, and July.
Insect size variation, though environmentally induced, may still be under the
influence of natural selection. The mechanisms can be simple. It is well known that
in many insects different numbers of molts precede adulthood. For instance, mole
crickets may have from 6 to 10 molts before becoming an adult. Molting is brought
about by changes in hormone titers of the individual. These hormone levcis, while
under genetic control, are known to be influenced by many environmental cues.
Therefore even though variation in size can be caused environmentally, the
decisions to molt have a genetic basis that can be acted on by natural selection.
One pervasive point in most studies of development is that an individual
must reach a ’critical size’ before becoming adult (Blakley and Goodner 1978,
Blakley 1981, Palmer 1984). Starvation of juveniles before they attain this size
results in death. On the other hand, some cockroaches apparently determine their
ultimate size at a specific instar and adjust the number of instars to reach this size at

75
adulthood (Tanaka 1981). Slansky (1982) suggested that this minimum size is
adjusted by selection and the balance between costs of prolonged development and
the costs of reduced size and fitness.
The understanding of size variation as it relates to developmental strategies
should broaden our understanding of an organism’s life history. An individual’s size
is the direct result of its development. Size has been implicated as the factor
affecting a number of alternative reproductive behaviors in insects (Thornhill and
Alcock 1983) and is a major determinant in the sex change of sequential
hermaphrodites (Charnov 1979). Size tactics may even be one generation removed.
For instance females that provision broods and female parasites may adjust the
amount of investment in their offspring, thus influencing their offspring’s adult size
and reproductive behaviors (Alcock 1979). Once size tactics and the developmental
strategies of an organism are explained from an individual’s standpoint, new light
will be shed onto other aspects of that organism’s life history.

CONCLUSIONS
Male mole crickets produce sound. Like most crickets these acoustic signals
function to attract females for mating purposes. Even though males use their
burrows as a baffle and they call from within an acoustic horn, the production of
sound is an inefficient process. Males are less than 0.2 percent efficient in
producing from 2 to 22 uWatts of acoustic power. The power produced is
dependent on male size and the moisture of the soil surrounding the calling burrow.
The sound field is hemispherical, and thus, has no directional characteristics. By
preferentially landing at louder males, females may use the calling songs to locate
large mates or moist soil for oviposition.
Oviposition occurs during the spring, primarily in May and June. The
investment in eggs, the numbers of eggs, and the percent hatch of eggs decline as
females age. Large females produce more hatchlings and have larger egg clutches
than smaller females.
Because size is such an important aspect of the reproductive success of both
sexes, males and females should make decisions about when to mature that will
maximize fitness. Each decision about whether or not to mature has certain costs
and benefits that must be evaluated. During the fall, individuals that are smaller
than average overwinter as juveniles in order to increase in size before spring
reproduction. As the reproductive season progresses the cost of delaying
maturation increases and smaller individuals mature.
76

77
It is important to note that the observations are made from the standpoint of
the individual. As such, they can be used to predict aspects of mole cricket life
history and will be useful in the models concerning the population dynamics of these
pest species.

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1
BIOGRAPHICAL SKETCH
Timothy G. Forrest was born Friday, the 131*1 of April, 1956, in St. John’s,
Newfoundland, Canada. He received his Bachelor of Science degree in 1978 from
Florida Southern College where he majored in biology and mathematics. He
received his Master of Science degree in entomology from the University of Florida
during the spring of 1981. Since that time he has been working toward his
doctorate. In January of 1982 he married Susan Lynn Mashke. They became
parents on June 25,1986 when their son, Justin Broc, was born.
Along the coast road, by the headland
the early lights of winter glow
I’ll pour a cup to you....
Raise it up- say Cheerio.
(Ian Anderson)
84

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
(LhódlL
Thomas J. Walkér, Chairman
Professor of Entomology and Nematology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
quality, as a dissertation for the degree of Doctor of Philosophy.
James E. Lloyd
Professor of Entomology and Nematology
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope and
qualify, as a dissertation for the degree of Doctor of Philosophy.
Martha L. Crump (J
Associate Professor of Zoology
This dissertation was submitted to the Graduate Faculty of the College of
Agriculture and to the Graduate School and was accepted as partial fulfillment of
the requirements for the degree of Doctor of Philosophy.
December 1986
Dean, College of Agriculture
Dean, Graduate School

UNIVERSITY OF F|||jj||||
ill! 08553 3692




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BIOACOUSTICS, MATERNAL INVESTMENT, AND DEVELOPMENTAL STRATEGIES IN THE MOLE CRICKETS, SCAPTERISCUS ACLETUS AND VICINUS By TIMOTHY G. FORREST A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1986

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I believe in fires at midnight. . . Silken mist outside the windowFrogs and newts slip in the dark Too much hurry ruins a'body I'll sit easy, fan the spark (Ian Anderson) To Sue, thank you for everything.

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ACKNOWLEDGMENTS Now is the solstice of the year. Winter is the glad song that you hear. Ring out, ring Solstice Bells. (Ian Anderson) I am most appreciative of the guidance that Tom Walker and Jim Lloyd have given me. Their high standards of scholarship and science have been truly inspiring. I thank them both for what I have received in terms of knowledge, opportunity, and friendship. My work will always be a product of their teachings. I wish to thank Marty Crump for adding a new dimension to my interest in animal sound production. Her knowledge of anuran biology has been most rewarding and has made me look at insects in a different light. All three gave helpful suggestions that much improved this dissertation. Henry Bennet— Clark, Dave Green, Les Bernstein, and Ginny Richards have all been helpful in transmitting their knowledge of sound and acoustics, without which I would never have completed Chapter I. Ken Prestwich provided unpublished data and his help with the energetics of sound production was invaluable. Frank Slansky was extremely helpful with comments on Chapters II and III that much improved their content. Bill Oldacre provided the schematics for the portable multiplexing unit and was always willing to give his time. John Allen furnished a computer program that made the mentally impossible visually possible. Sue Wineriter gave many hours of work on many drafts of many figures. Her expertise in art and illustration is much appreciated. I would also like to thank my friends Will Hudson and Steve Wing for making this time and degree an enjoyable and enlightening one.

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I also wish to acknowledge my parents for their help when help was needed most. And finally I thank my family. Sue, I thank for her devotion and dedication that have given me confidence in myself and the drive to finish. Her many hours of assistance in the field will always be remembered. Justin, I thank you for making Sue and me very happy. IV

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TABLE OF CONTENTS ACKNOWLEDGMENTS ill LIST OF FIGURES vi ABSTRACT vii INTRODUCTION 1 CHAPTERS I BIOACOUSTICS 3 Introduction ....... 3 Materials and Methods ..... 21 Results ....... 27 Discussion ....... 32 II OVIPOSmON AND MATERNAL INVESTMENT . 36 Introduction ....... 36 Materials and Methods ..... 37 Results ....... 39 Discussion ....... 50 III SIZE TACTICS AND DEVELOPMENTAL STRATEGIES 55 Introduction ....... 55 Size Tactics in Mole Crickets .... 62 Materials and Methods ..... 63 Results ....... 64 Discussion ....... 70 CONCLUSIONS 76 REFERENCES CITED 78 BIOGRAPHICAL SKETCH 84

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LIST OF FIGURES Figure 1. Pressure variation in a 2 Hz sine wave. ... 6 2. Phase relationship between velocity and displacement of a pendulum. .... 10 3. Acoustic interference and the summation of sinusoids. ..... 14 4. Sound produced by a vibrating disc. ... 19 5. Relationship between total power output, male size and soil moisture in Scapteriscus . . . 26 6. Total power output as a function of sound pressure above the calling burrows of Scapteriscus acletus . ...... 29 7. Sound field of Scapteriscus acletus . ... 31 8. Scapteriscus egg laying seasons. .... 41 9. Relationship between female size and total offspring produced in acletus . .... 45 10. Relationship between female size and average number of eggs per clutch in acletus . . . 47 11. Effect of female age on mass of eggs produced in Scapteriscus . ....... 49 12. Costs and benefits associated with maturing. . . 58 13. Hypothetical cost/benefit curve associated with changes in an individual's size. ... 60 14. Relationship between adult pronotal length and mass in Scapteriscus . ..... 66 15. Seasonal size distributions in mole crickets. . . 69 16. Seasonal change in size distribution in mole crickets.. ...... 72

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy BIOACOUSTICS, MATERNAL INVESTMENT, AND DEVELOPMENTAL STRATEGIES IN THE MOLE CRICKETS, SCAPTERISCUS ACLETUS AND VICINUS By Timothy G. Forrest December 1986 Chairman: Thomas J. Walker Major Department: Entomology and Nematology Three aspects of the reproductive ecology of the southern and tawny mole crickets, Scapteriscus acletus and vicinus . were investigated. Twenty-two sound fields of calling males were measured under field conditions. Total power output ranged from 1.9 to 22.4 uWatt and was positively correlated with male size and soil moisture. Efficiency of sound production was only 0.02 to 0.16 percent. The shape of the sound field approximates a hemisphere. Lifetime oviposition by caged, individual female mole crickets was monitored outdoors. Oviposition is highly seasonal, and females that mature in the fall wait until the following spring to oviposit. Most eggs are laid in May and June and percent hatch of clutches decreases later in the season. Dry weight of eggs, number of eggs per clutch, and percent hatch decrease with successive clutches from individual females. Average investment in eggs per clutch ranged from 10 to 24

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percent of female body weight for acletus and from 5 to 16 percent for vicinus females. Lifetime investment ( dry weight of eggs) was greater than 100 percent of female body weight (wet weight) for 6 of 12 acletus females, but only 1 of 8 vicinus females had a lifetime investment in eggs equaling its body weight. In acletus . but not vicinus , large females produced more offspring and the average size of their egg clutches was larger than those of smaller females. Insect size tactics and developmental strategies are discussed in relation to decisions individuals make about when to mature. Such decisions carry with them costs and benefits in terms of an individual's fitness. Whenever size affects reproductive success, selection should act such that individuals evaluate the costs and benefits due to changes in size and mature when the cost/benefit ratio is maximized. Predictions about seasonal changes in adult sizes within a natural population of mole crickets are tested. The changes in seasonal distributions of adult sizes of mole crickets support the predictions and suggest that individuals make decisions about when to mature based on costs and benefits associated with changes in size. vin

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INTRODUCTION The southern and tawny mole crickets, Scapteriscus acletus and vicinus . were accidentally introduced into the United States around the turn of the century (Walker and Nickle 1981). Because they lack a specialized predator/parasite complex their populations have gone unchecked, their numbers have increased, and they are one of the major pests of turf grasses and pasturelands in the southeast. Their economic importance has prompted large amounts of funding for research aimed at controlling mole crickets and lessening the damage caused by them. Only recently has there been progress toward this goal. The primary stumbling block has been an insufficient knowledge of the basic biology of these pest species. During the past decade numerous researchers have contributed to our understanding of the animals and their elusive subterranean lifestyle. Mole cricket biolog y. Scapteriscus spp. are usually associated with habitats that are temporary and early successional. In the southeastern United States both acletus and vicinus are common in pastures, cultivated fields, lawns, and other turf. All adults have long hindwings and are capable of flight. Dispersal flights occur during the spring and fall, consist mostly of females, and are important to the crickets' ecology in that they allow individuals to colonize newly opened areas or to leave unsuitable or deteriorating ones (Ulagaraj 1975, Walker et al. 1983). Flying crickets land in response to their species— specific calling song (Ulagaraj 1975, Ulagaraj and Walker 1973, 1975). Adults are easily collected at traps broadcasting such songs. Both males and females respond to the calls and preferentially land

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near males whose calls are louder than those of neighboring males (Forrest 1980, 1983). The two species differ markedly in their food habits with acletus being primarily carnivorous on soil arthropods, earthworms, etc., whereas vicinus is mainly an herbivore, feeding on roots as well as grass blades (Matheny 1981). In Gainesville both species are univoltine; however, they differ in that about 85 percent of the vicinus population overwinter as adults compared with only 25 percent of the acletus population (Hayslip 1943). The rest of the populations overwinter as late — instar juveniles that mature the following spring when eggs are laid. Egg clutches are laid during the spring in underground ovoid chambers (4 by 3 cm) constructed by females. Once the eggs are laid, the chamber is sealed and no parental guarding of eggs occurs. Eggs within a clutch hatch synchronously (within a 24-h period), and after a few days the young tunnel to the surface. The topics that follow are part of the ongoing effort to understand mole cricket populations and biology. The subjects add to the understanding of the reproductive ecology of mole crickets and should be useful in monitoring changes in the population and in designing models concerning mole cricket population dynamics. Chapter I covers the bioacoustics of mole crickets and is an investigation in the efficiency of acoustic communication by the two crickets. Chapter II deals with female oviposition and the maternal investment by females and how these are influenced by season, female size and female age. The third chapter is a study of how individuals evaluate costs and benefits associated with size and how these relate to the decisions of whether or not to mature.

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CHAPTER I BIOACOUSTICS Let me bring you songs from the wood: To make you feel much better than you could know(Ian Anderson) Introduction Since man's beginnings the sounds of animals have frightened, amused, and intrigued him. With the advent of the tape recorder and audiospectrograph, he could accurately quantify the sounds of animals, and in the last two decades the interest and research in the areas of animal communication and bioacoustics have increased dramatically. A new generation of digital recording and measuring devices has made endeavors into bioacoustics even more reliable and from an empirical standpoint more profitable. It is from the world of insects that such research has really profited. Here there is a wealth of species with a wide variety of sound producing and sound receiving mechanisms, many hundreds of which have evolved independently, each one having been shaped by natural selection for efficient communication purposes. A thorough background of the physical nature of vibration and sound will be given and this understanding will be the basis for a discussion of mole cricket bioacoustics and sound production. Vibration and Sound In simple terms sound is a disturbance or vibration in some elastic medium. This disturbance need not be uniform or repetitive. Such a disturbance transmits energy to the particles of the medium; the particles move from their resting position,

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collide with neighboring particles and transmit the energy to them. These neighbors transmit the energy to other neighbors and so on. Because of the elasticity of the medium, the particles tend to return to their resting position after being displaced. Thus the energy is transferred in waves away from the source of the disturbance. Waves can be of two types depending on the axis the particles move relative to the movement of the wavefront. When the particle motion is perpendicular to the movement of the wave it is said to be transverse , whereas particle motion parallel to the motion of the wave is called lon gitudinal . Sound waves in air are longitudinal; waves on the surface of water are transverse. The characteristics of a sound will depend upon the type of disturbance as well as the physical characteristics of the medium. In most instances the medium is air, although sound can be transmitted through liquids and solids and many insects use these media for communication. For simplicity I will restrict my discussion to sound in air, although the principles generally apply to all sound. Pressure, power and intensity . A disturbance in air causes a change in the density of molecules and in the pressure of the gas. These fluctuations in the pressure of a sound wave are easily detected and measured with instruments, such as microphone, that are sensitive to such pressure changes. Because of their ease of measurement, changes in instantaneous pressures are commonly used to describe sound. Perhaps the most well known sounds are sinusoids in which the pressure fluctuates in a sinusoidal manner and can be described by the equation p(t)=A*sin((2*pi*f*t)+theta) 1 where pressure (p) is a function of time (t), A is the amplitude of the pressure change, pi is the constant 3.14159..., f is the frequency of the sine wave and theta is the phase ang le. Since the instantaneous pressure changes sinusoidally with respect to both time and space, p(x), the pressure at some distance (x) from the source, can

PAGE 13

Figure 1. Pressure variation in a 2 Hz sine wave. Pressure, p, is a sinusoidal function of time, t, and the wave has an amplitude of A. Positive values of p(t) are compressions and negative values are rarefactions.

PAGE 14

PRESSURE pCt) -A

PAGE 15

also be described by substituting +x/c for t in the above equation, where c is the velocity of sound in the medium. In air, c is about 330 meters per second. Note that the plus and minus signs in the space formula are used to describe sound travelling in opposite directions from the source. Figure 1 shows the instantaneous pressure changes for a 2 Hz (cycles per second) wave with initial phase angle theta=0. At t=0 and x=0 the pressure is 0, although the pressure may have been any value between -A and A depending on the initial phase angle. With an initial phase angle of 90° the pressure at t=0 is equal to A. When pressure, p(t), is positive, the particles of the medium are compressed. This part of the sound wave is called the compression . When the pressure is negative, a vacuum occurs and this part of the wave is the rarefaction . The sinusoid in Fig. 1 is repetitive every 1/2 second because the sin(0)=sin(2*pi); i.e. the period is 1/2 second and is equal to the reciprocal of the frequency for any sinusoid. The waveleng th (lambda) of the sinusoid is the distance the wave travels in one period and equals period*c or c/f. The 2 Hz wave has a period of 1/2 second and a wavelength of 115 meters. Insects typically use much higher frequencies with shorter wavelengths. For instance the calling song of the southern mole cricket, Scapteriscus acletus . has a carrier frequency of 2.7 kHz with a wavelength of 12 cm. In dealing with sound one often wishes to know the power or intensity (I) of the sound. Intensity differs from the scalar quantity of pressure, in that intensity is a vector quantity having both magnitude and direction and its units are given in energy per time per unit area or Watts per square meter (W/mr) whereas units of pressure are in force times area (Newton-meter 2 : Nm 2 ). Sound power, the total power produced by a source, is measured in the same units as electrical power, the Watt. Since pressure is an easily measured aspect of sound it is important to understand the relationship between the scalar pressure and the vector intensity.

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Intensity is the product of the instantaneous pressure (p) and particle velocity (u) I=p(t)*u(t)orp*u. 2 This equation is analogous to the power equation used in electrical systems where power is voltage times current. Particle velocity describes the fluid movement of the medium. Near the source particle velocity and pressure are 90° out of phase. This relationship is particularly important and has relevance to measurement of pressure, and therefore intensity. To illustrate this phase difference consider a pendulum (Fig. 2). At its resting position the pendulum hangs vertically and there is no movement, no kinetic energy and no potential energy. However, suppose the pendulum is displaced some distance, x, from the resting position. If it is held at this position, the pendulum has obtained some potential energy but has no kinetic energy. Releasing the pendulum will cause it to swing toward its resting position and there is an exchange between potential and kinetic energy. When the pendulum has reached its resting position its velocity is maximum, its displacement a minimum. The pendulum then swings past the resting position, and because of gravity the kinetic energy begins to change back to potential energy. At the end of the swing the pendulum reaches its maximum displacement (now -x) and its velocity has slowed to a minimum of zero. It can be seen that the phases of displacement (=pressure) and velocity (=particle velocity) are shifted by 90°. The phase relationship changes as the sound moves away from the source and can be described by Tangent theta=lambda/r. 3 Lambda is the wavelength of sound, r is the distance from the source, and theta is the phase angle (in radians) between particle velocity and pressure (Bennet— Clark 1971 and refs). It can be seen that the phase shift approaches 90° as r becomes zero and the phase difference between particle velocity and pressure decreases to zero as

PAGE 18

10 DISPLACEMENT -X VELOCITY -X TIME

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11 the distance r increases. Though the boundaries are not well defined, the near field is where pressure and particle velocity are not in phase, whereas in the far field particle velocity and pressure are in phase. For practical purposes the far field begins at distances greater than about 1 wavelength of the lowest frequency sound produced by the source. In the far field the sound field is said to be purely active (Gade 1982). This is useful because in the active field particle velocity can be calculated from pressure using the acoustical equivalent to Ohm's law, current equals the ratio of voltage to resistance: u=p/p c, 4 where p c is the characteristic impedance of the medium, about 40 dyne*sec/c]Tr for air (Gade 1982). Substituting p/p c for particle velocity in the intensity equation (2) it can be seen that intensity is proportional to the square of the pressure I=p 2 / Po c 5 and can thus be calculated using measurements of pressure-provided these measurements are taken in the far field. Decibels. Pressure, power and intensity levels are often given in decibel units. These units are logarithmic expressions of the ratio of any two sound pressures or powers. Normally the levels are described relative to some standard 1 9 reference. For instance, the commonly used reference for power level is 10 W, that for intensity is 10" 12 W/m 2 and that for pressure is 2x10"^ Nm 2 = 2xl0" 5 Pa = .0002 dyne/cm 2 = .0002 ubar. All of these standards represent decibels (dB). Sound intensity, I in dB, can be calculated using the equation 1=10 log W/W r 6 1 7 7 where W is the intensity and W f is the reference or 10 W/m . An intensity ratio of 2 (i.e. W/W r = 2) is 3 dB relative to the reference. A power level, PWL, can be

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12 1 n calculated using the same equation except that the reference is 10 W. Often the terms power and intensity arc interchanged although each has a specific meaning. Similarly sound pressure level, SPL in dB, can be calculated by SPL= 20 log P/P r . 7 In this equation P is the pressure and P r is the reference. A pressure ratio of 2 equals 6 dB relative to the reference. Note from the equations that for any given level in dB the power ratio and intensity ratio, W/W r , equals the square of the pressure ratio, P/P f . Since dB corresponds to both references, a sound pressure level can be converted into an intensity level directly, provided, of course, the pressure is measured in the far field. Acoustic interference. Consider two sound waves travelling in opposite directions. One logical question to ask is: What happens when the two waves meet? The resultant pressures are simply the sum of the pressures of the two waves. This is graphically shown for a pair of equal— amplitude sinusoids in Figure 3. In Fig. 3a the waves are approaching each other from opposite directions. In 3b the waves are positioned in space so that a portion of the waves occupying the same space are in phase (i.e. the compressions of one wave occurs with the compression of the other) and the pressures add to produce pressure that is greater than the pressure of each individual wave. This is called constructive interference . In 3c the pressures sum to zero since the phases of the individual waves are opposite and they exactly cancel. This condition is destructive interference . As each wave continues on its way they are unchanged even though there were great pressure changes when the two waves met. When two sounds interfere destructively the energy of the sound is not lost. When the pressure decreases, the particle velocity increases and

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Figure 3. Acoustic interference and the summation of sinusoids. a. Two equal— amplitude sine waves of the same frequency travelling in opposite directions toward each other. b. During this instant in time the two occupy the same space and the pressure of the resultant sinusoid (solid line) is the sum of the two individual waves. The resultant has an amplitude greater than either of the individual waves; this condition is termed constructive interference. c. At this instant the individual waves are opposite in phase and their pressures sum to zero (solid line). This is destructive interference.

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14 PRESSURE — 1— b

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15 there is a conservation of energy. The sum of any two sinusoids that have the same frequency is another sinusoid of the same frequency but with a different amplitude. Propagation and the inverse square law. In an ideal environment sound waves radiate at the same rate in all directions from the source, and thus, the wavefront is spherical in nature. Because the same acoustic energy must occupy a greater surface area as the distance from the source increases, sound intensity decreases with distance from the source, since intensity is measured in Watts per unit area. The surface area of a sphere increases proportionately with the square of its radius (4*pi*r' 6 ), and thus the intensity of a sound is inversely proportional to the square of the distance from the source--the inverse square law. Suppose the intensity is measured at some distance from the source. The intensity at twice that distance will be one-fourth or about 6 dB less [10*log(0.25)j. If the power ratio is 0.25, the pressure ratio is 0.5. For the same doubling of distance the pressure will also decrease by 6 dB [20*log(0.5)]. Pressure, the scalar quantity, is proportional to the distance from the source. For every doubling of distance the pressure is halved and it is this proportionality that will be used to calculate sound fields of crickets. Insect Bioacoustics The study of insect bioacoustics, the special problems that insects have in using acoustic signals to communicate, and the biophysical aspects of sound production began more than 15 years ago and have been reviewed by Bennet-Clark 1971, 1975, Michelsen and Nocke 1974, Michelsen 1983 and Eisner and Popov 1978. The interest in the field has continued to grow primarily because the animals are easily maintained, they are easily manipulated for experimental purposes, and they offer a host of sound producing mechanisms to study.

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16 Many insect groups use acoustic signals to communicate: ants do it, cicadas do it, lacewings do it, and moths and beetles do it. The complexity of the communication varies from very simple sounds of hitting a part of the body on the substrate to sounds that are extremely complex with distinct signals for special contexts. For instance crickets have different types of calls classified as calling, courtship, or aggressive (Alexander 1961). The different life stages of passalid beetles use acoustic signals to communicate, adult to young and young to adult. These beetles have a repertoire of as many as seven different sounds used in 13 different behavioral contexts (Schuster 1975). Some species of insects may use acoustic signals to communicate to other species of animals-for example, many of the tiger moths that use sounds to confuse and deceive bat predators (Fullard et al. 1979). However, insects are very inefficient at producing sound because the efficiency of sound production is dependent on the relationship between the size of the oscillator or sound source, the frequency of the sound produced by the oscillator and the immediate surroundings of the oscillator. To be an efficient radiator, the diameter of the sound source must be about the size of, or larger than, the wavelength of sound produced by the source. Because insects are small, and thus their sound producing organs are small, they must produce a sound with very small wavelengths to be efficient. That is, they must use high frequency sounds. However, there is a disadvantage. High frequencies are attenuated by air at a faster rate than low frequencies (Wiley and Richards 1978, Michelsen 1978). Therefore, given the same power output for a high and low frequency signal, the intensity at the same distance from the source will be greater for the low frequency than the high frequency sound. Because of this, low frequency sounds are better for long distance

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17 communication and insects must make a trade-off between the effective distance of communication and the efficiency of signal production. Insect muscles cannot contract at the high rate needed to produce frequencies in the kHz range. Therefore, insects usually use some mechanism for frequency multiplication. Most often this is accomplished by stridulation (Eisner 1983). During stridulation one part of the insect body is rubbed against another. One part usually has a row of teeth, and during each stroke a number of teeth are struck, each adding to the sound. The frequency of the muscle contraction is multiplied by the number of teeth hit. One of the most familiar groups of singing insects are the Orthoptera. This group contains the grasshoppers, which produce song in at least three different ways (Otte 1970). It includes cockroaches, some of which produce a hissing sound by the expulsion of air from the spiracles (Nelson and Fraser 1980). But above all, the crickets and katydids are the most noted and familiar singers of the group. Few people have not heard a cricket. Sound production in the Ensifera . The Ensifera are an ancient insect group and include the Tettigoniidae (katydids), the Gryllidae (crickets), the Gryllotalpidae (mole crickets) and the Haglidae. The forewings are similarly modified for sound production in the four families. While calling the wings are raised above the abdomen and moved back and forth one on top of the other. Located on the underside of the Cu2 vein of each wing is a row of teeth called the pars stridens or file. On the posterior margin of each wing is the plectrum or scraper. Stridulation occurs during the closing stroke of the wings when the plectrum is pulled against the grain across the teeth of the file. In katydids the left wing is usually over the right, in crickets the right is usually over the left, while in haglids and mole crickets each configuration occurs with equal frequency (Masaki ct al. in press, Forrest in press).

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Figure 4. Sound produced by a vibrating disc. a. As the disc moves to one side it compresses the particles of air and at the same time produces a vacuum on the other side of the disc. The shading of the circles represents the degree of displacement of the particles (ie. black circles arc compressions and no circles are rarefactions). b. Sound output and phase relationships of a dipole source. c. Sound field of the dipole source. If the vibrating disc is small relative to the wavelength of sound produced, the outputs from each side of the disc will destructively interfere along the edges of the disc. The result is a dumbbell shaped sound field.

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19 & o ")F // \\ )) B • • . . . .vsa

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20 In some species the wings have become specialized for their tasks and there is an asymmetry between the left and right wings. The file and scraper are closely associated with specific membranous areas of the wings. The cell or cells are caused to vibrate at the frequency of the tooth— impact rate of the scraper on the file. The cells are resonant near the impact frequency, and the vibrations of the cells produce the sound. The vibration of the wing membrane can be likened to the movement of a piston or disc. As the disc moves back and forth in air, it produces a compression and a rarefaction simultaneously on opposite sides of the disc. That is, a sound wave is generated from each side of the disc, each being equal in amplitude and frequency, but opposite in phase (Fig. 4). When the diameter of this doublet or dipole source is small relative to the wavelength of sound produced, the sounds radiating from the two sides of the disc destructively interfere along the edges of the disc (Fig. 4b). The result is a dumbbell shaped sound field with a maximum pressure perpendicular to the disc surface and a minimum pressure along its edges (Fig. 4c). Of course the particle velocity at the edges is high and the intensity is equal in all directions. The membranes of crickets are less than one-tenth the wavelength of sound produced, and they are inefficient primarily because of their size. Energetically, producing sound may cost a male ten times as much as resting (Prestwich and Walker 1981). However, because calling contributes importantly to a male's reproductive success, selection should be strong on males to increase signaling efficiency and effectiveness. Crickets have evolved a number of behaviors that use baffles to increase calling efficiency (see Forrest 1982 and refs.). The study of these baffles and their effect on sound production is one important aspect of cricket mating systems and reproductive biology.

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21 One such baffle system is used by the southern and tawny mole crickets, Scapteriscus acletus and vicinus . Both species construct a calling burrow shortly after sunset, prior to the female flight period. The burrow is shaped by the cricket and expands exponentially from the throat to the opening at the soil surface (Nickerson et al. 1979). Because the males call from within a burrow, their sound fields are very suitable for measuring. The position of the male in the burrow is constant, as is the position of the burrow opening. The calling songs are continuous trills that change less than 1 dB in sound pressure during the 1 to 1.5 hours of calling (Forrest 1981, 1983), and an observer can move around outside the burrow with little disturbance to the calling male. If the animal is disturbed, he runs down into the burrow system but returns in a short period of time to the same calling position and resumes advertising for females. I measured 22 sound fields of Scapteriscus males (17 acletus , 5 vicinus ) under field conditions to determine the power output of the crickets, the efficiency of sound production and the directional properties of the sound field. Materials and Methods Sound pressure level readings were taken using nine Bruel & Kjaer, model 4125 condenser microphones and type 2642 preamplifier. The typical frequency response for these microphones is flat (+1 dB) from 200 Hz to 10 kHz for a free field response and an angle of incidence of 0°. Directional characteristics show less than 1 dB loss for +30° deviations from a 0° incidence angle for signal frequencies at 2, 4, and 8 kHz (Bruel & Kjaer specifications). The output of each microphone could be adjusted independently, and each microphone was calibrated using a model 4230 B&K pistonphone calibrator. Initially, each microphone was calibrated after each sound field was measured.

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22 Later the calibration was done every two weeks. The stability of the microphones was such that later they were calibrated only every month. Never was the level of any of the microphones more than 0.5 dB from the calibration level of 94 dB. The outputs of the microphones were multiplexed to a single input of a model 2219 Bruel & Kjaer sound level meter that measured the sound pressure re 0.0002 dyne/cnr (dB A). The entire system was battery operated and field portable. Cables from the microphones were long enough so that the observer and equipment were more than a meter from the sound source and nearest microphone. This avoided reflection of sound waves off the observer's body, and interference in the pressure readings was reduced to a minimum. The nine microphones were equally spaced around a semicircle of tubular aluminum (every 22.5°, +2°) such that the diaphragm of each microphone was 25 cm from the center of the semicircle and the angle of incidence was zero (+2°). An angle of incidence of 0° is recommended when the direction of the sound source is known (Bruel 1983). At 25 cm the microphones were always more than 1 wavelength of the carrier frequency of the calling song of the mole crickets ( acletus 2.7 Khz or 12 cm wavelength, vicinus 3.3 Khz or 10 cm wavelength). Thus sound pressure measurements were taken in the far field (Michelsen 1978). By rotating the semicircle in 22.5° steps along one of the axes, sound pressure levels could be taken at every 22.5° latitude and longitude of a hemisphere surrounding the sound source located on the ground in the center of the semicircle. A stand anchored one end of the hemisphere to the ground and a handle that moved the semicircle locked every 22.5°. Axis of rotation was always with the longitudinal axis of the cricket so that on every 22.5° rotation the two microphones at the ends of the semicircle remained at the same anterior and posterior position relative to the cricket burrow. This arrangement provided a check for changes in

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23 the crickets' output and added reliability to the measurements. Altogether, pressure measurements were taken at 65 different positions surrounding the opening of the cricket's burrow with the anterior and posterior positions each having 9 separate measurements. Sound fields with more than +0.5 dB variation in these 9 measurements were not used in analysis. These measurements represent the most detailed measurements of cricket sound fields. All sound fields were measured outdoors at the Horticultural Farm of the University of Florida. Sound fields of crickets were measured while they called in the field or while they called from soil— filled, 12-liter buckets buried so that the bucket lip and the soil in the bucket were at ground level. Only crickets calling on level ground were measured and all obstacles that might cause interference or reflections were removed from the area. To determine the influence of male size on acoustic output, the males were captured and the lengths of their pronota along the midline were measured to the nearest 0.1 mm using vernier calipers. The mass of each male was measured to the nearest 1.0 mg with a model AC100 Mettler balance. A soil sample (2 cm dia X 4.5 cm deep) was taken within 5 cm of the burrow opening. This sample was dried and moisture content determined. Soil temperature at a depth of 2.5 cm was also taken. Calculation of sound fields and power output . Each of the 65 sound pressure levels measured at 25 cm was converted to a radial distance from the cricket at which the pressure would be 70 dB. For instance a pressure that read 73 dB at 25 cm would be 70 dB at about 35 cm. The points were converted to polar coordinates and the areas of all triangles defined by adjacent triads of points were summed to obtain the surface area of the sound field. A test of this procedure revealed that it underestimates the surface areas of 24-200 cm dia hemispheres by only 2-3%.

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24 Table 1. Power input, output and efficiency of calling male Scapteriscus . PNL MASS TEMP PULSE 3 SOIL POWER PWL POWER b EFF. RATE MOIST OUT IN mm mg °C p/sec uW dB mW % Scapteriscus acletus

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Figure 5. Relationship between total power output and male size and total power and soil moisture in Scapteriscus . 1 9 a. Total power (dB re 10 W) as a function of male size (pronotal length mm). Power (in Watts) of males with more than one sound field measurement were averaged for regression analysis. Vertical lines at these points show the range in power for individual males. Regression equation is y = 2.4x + 46.4 ( acletus only p<0.05, r -0.45). Open circles are acletus males, filled circles are vicinus males. b. Total power as a function of soil moisture (percent of dry weight of soil sample). Connected points are for sound fields by the same male at different soil moistures. Percent soil moisture was log transformed for regression analysis y = 6.21og(x) + 61.5 ( acletus only p<0.05, r 2 =0.40).

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26 PWL (dB) Pronotal Length (mm) 75 70 . PWL (dB) 55 50 I • * — * — — i i i i 0.0 5.0 10.0 15.0 20.0 25.0 Soil Moisture

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27 Since the measurements were taken in the far field, the 70 dB isobar represents a 0.01 mW/m 2 intensity level and the total power, in mW, is given by the intensity level times the surface area. Results The acoustic power produced by calling male mole crickets ranged from 1.9 to 22.4 «W (62.8 73.5 dB re 10~ 12 W) for the 17 acletus sound fields and from 2.3 to 9.6 «W (63.5 69.8 dB) for the 5 vicinus sound fields (Table 1). Part of this variation in the power output was due to the size of the male and the moisture in the soil surrounding the male's burrow. There was a significant relationship between male size and power level (p<0.05, r -0.45; Fig. 5a, Note that regression was done only on sound fields from acletus males and that the acoustic powers for individuals for which more than one sound field was measured were averaged and used for computing the regression). There was also a significant relationship between the soil moisture and power level ( acletus only p<0.05, r -0.40; Fig. 5b, Percent soil moisture was log transformed for regression analysis). The data from individual males also showed the same trend. That is, their power output increased with increasing soil moisture (5 out of 7 acletus and 1 of 1 vicinus ). However, a check to see if soil moisture and male size were correlated revealed no significant relationship and large males did not produce more power simply because they called from moister soil. The power output of a male can be estimated by a single measurement of the sound pressure level directly above the burrow. There was a direct relationship between the sound pressure directly above the burrow and the power level of the sound field produced (p<0.05, r -0.62; Fig. 6). Using soil temperature, the pulse rate of each cricket (wing stroke rate) was estimated (Ulagaraj 1976). Since the power used by the cricket is directly related to

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Figure 6. Total power putpui (dB re 10" 12 W) as a function of sound pressure (dB re 2xl0" 3 Nm z ) directly above the calling burrow. Sound pressures are measured at 25 cm from the burrow opening. The regression is significant (p<0.05, r 2 =.62) y = 0.58x + 26.0.

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29 PWL (dB) 60 65 70 75 Sound PressurQ (dB) 80

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Figure 7. Sound field of Scapteriscus aclctus . Sound field is base on an average of 17 radial distances at each of 65 radii. Standard errors around the 65 points are always less than 4 cm. Anterior is designated by ant, posterior by pst, right by rit and left by 1ft. a. View from above the sound field. b. Looking from the crickets' left. c. A perspective view.

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31 "' c\ Ax •' y^TV

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32 the wing stroke rate while calling (Prcstwich and Walker 1981), I estimated the percent efficiency of sound production for each sound fi°ld. Subtracting an estimated resting metabolism jf a jout 370 wliter C^/g h (Frestwich unpublished, personal communication 1986) from the metabolic rates while calling revealed that the mole crickets were very inefficient. Percent efficiency ranged from 0.02 to 0.16 percent (Table 1). The data for estimated power used while calling are similar to actual measurements of calling metabolism of male acletus (Prestwich, personal communication 1986). Because the individual sound fields were variable, an average sound field based on the average radial distances along each of the 65 radii was computed for the 17 sound fields from acletus males. The standard errors of the 65 means were about 2.5 and always less than 4 cm. The shape of the sound field most closely resembled a hemisphere and the mean distances did not differ significantly from the radius of a 26 cm hemisphere. At ground level (X-Y plane, Fig. 7) the transverse axis of the sound field was 48.4 cm and the longitudinal axis was 53.7 cm across. Height of the sound field was 26.6 cm. Unlike the sound fields of other crickets, the burrow opening acts as a point source and radiates equally in all directions. The total power of the averaged sound field was 4.33 «W or 66.4 dF Using 26 cm as the radius for a hemisperical sound field, the power should be 4.25 uW or 66.3 dB. Discussion The acoustic power produced by male Scapteriscus is less than onehundredth of the power produced by the French mole cricket. Gryllotalpa vmeae. Bennet— Clark (1970) calculated a mean power output of 1.2 mW. Assuming that this was produced by a total muscle power of 3.5 mW, Bennet— Clark calculated an efficiency of 28-35 percent. However, the power used was probably underestimated since he estimated power used from data on grasshopper flight muscles. Using data

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33 from Prestwich and Walker (1981) Gryllotalpa vineae should use about 70 mW of power for an average 3.3 g cricket calling with a pulse rate of 66 pulses/sec. The efficiency then becomes 1.7 percent, a little less efficient than a typical loudspeaker. This is still more than ten times more efficient than Scapteriscus . Since female Scapteriscus preferentially land at louder males (Forrest 1980, 1983), males are expected to call at their maximum output, but because Scapteriscus are one— sixth to one— third the size of Gryllotalpa it seems likely that they are unable to produce as much acoustical power because of their much smaller muscle mass. Note that the power produced by Scapteriscus acletus was directly related to the size of the male (Fig. 5a). Soil moisture also influenced output probably because it enables the animal to pack the sandy soil better, and thus, it absorbs less of the acoustic energy. Bennet— Clark (1970) also noted that moist soil increased output in Gryllotalpa vineae . The efficiency of sound production is somewhat lower in Scapteriscus spp. than that reported for other animals that use acoustic signals to communicate. Hylid frogs are about 1 to 3 percent efficient (Prestwich and Bruger, unpublished). The frogs are probably more efficient because the calls are produced by vibration of a vocal sac that acts much like a pulsating sphere. Such a source does not have the acoustic short circuit a dipole source has, and if the vocal sacs of the frogs are the proper dimensions relative to the wavelength of their calling songs they may have resonant properties that would also increase the efficiency. Male bladder cicadas, Cystosoma saundersii , produce about 0.35 mW of acoustic power and are about 0.82 percent efficient. Like the frogs, the abdomen of the cicada acts as a baffle and resonating system (MacNally and Young 1981). Other crickets have only slightly higher calling efficiencies than those of Scapteriscus . Anurogryllus arboreus are

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34 about 0.2 percent efficient and Oecanthus celerinictus are from 0.3 to 1.3 percent efficient (Prestwich unpublished). The sound fields of individual males were often irregular although they were similar in overall shape to the average distribution pattern (Fig. 7). Since a new burrow is constructed each evening, the openings may be very different from cricket to cricket and from night to night for the same cricket and this probably causes the irregularities. The distribution pattern of sound from a mole cricket burrow differs considerably from the directional patterns of other crickets. The burrow acts as an infinite baffle around the vibrating membranes raised in the throat of the horn (Bennct— Clark 1970, Nickerson et al. 1979). The burrow opening then produces a point source of sound and the waves radiate equally in all directions. The result is a spherical shaped distribution pattern. The sound fields of other crickets usually have some directional properties. Anurogryllus arboreus (Paul and Walker 1979) and Gryllus campestris (Nocke 1971) have directional characteristics due to the destructive interference along the edges of the wings. Probably the most noticeable sound fields for their directional quality are those of tree crickets (Williams 1945). The sound pressure may be more than 20 dB less at the sides of the cricket compared to anterior and posterior pressures, and males often turn while calling seemingly to direct sound in different directions (Toms 1984, Forrest unpublished). Male Gryllotalpa vineae also produce a directional pattern because the two burrow openings act like dual speakers, and there is a characteristic radiation pattern dependent upon the relationship of distance between the two openings and the wavelength of the sound produced. This radiation pattern of Gryllotalpa vineae is such that it should increase the probability that a female will intercept the sound fields (see Bennet— Clark 1975 and Forrest 1982 for a discussion of selection on directional sound fields).

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35 In summary, the power output of male Scapteriscus is relatively small compared to other species of acoustically signaling animals, and male mole crickets are very inefficient at producing sound (Table 1). Large males produce greater sound power than smaller males, and moist calling sites increase the total output (Fig. 5). The radiation pattern of sound from the calling burrow is hemispherical in shape (Fig. 7), and the total power can be estimated from a single measurement of sound pressure above the burrow (Fig. 6).

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CHAPTER II OVIPOSITION AND MATERNAL INVESTMENT It's only the giving that makes you what you are. (Ian Anderson) Introduction Females of some insects may lay as many as 600,000 eggs during their lifetime although the norm is usually a few hundred (Hinton 1981). These nutrientfilled eggs are a major investment in offspring, and parental investment, especially the differences in investment between the sexes, is a predominant force in selection on reproductive strategies (Trivers 1972), life history tactics, and resource allocation (Boggs 1981). In most instances, investment by males ends at copulation or fertilization (see Thornhill [1976] on paternal investment in insects), and females generally do not invest after oviposition. Unlike paternal investment, which is frequently difficult to assess, maternal investment can often be measured simply by determining the mass of eggs produced. In crickets oviposition and oviposition sites are diverse and eggs are extremely variable in size, number laid, and whether or not they are diapausing. While a great deal of attention has been given to nuptial feeding and paternal investment in katydids (Gwynne 1981, 1983), crickets (Sakaluk 1984), and cockroaches (Mullins and Keil 1980), few studies have involved orthopteran egg deposition and measurement of maternal investment in eggs. This study reports the oviposition of southern and tawny mole crickets. I followed the reproductive output of females caged under field conditions and studied the influence of size, season, 36

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37 and aging on female investment in eggs and female fitness as measured by the number of hatchlings produced. Egg laying in these species was earlier described by Hayslip (1943), but his study concerned mole cricket life histories and lacked long— term monitoring of individual females. Walker and Nation (1982) studied fall mating, egg maturation, and sperm storage of individuals but did not examine maternal investment in offspring. Materials and Methods All studies of oviposition were done in Gainesville, Florida, and involved females that were collected after they were attracted to the conspecific male calling song: either an artificial song (Walker 1982) or the natural song of a male calling from a soil-filled bucket surrounded by a trapping device (Forrest 1983). Females were placed in buckets of soil (19— liter) and were fed 10 cm J of ground, dry dog food each week, enough to ensure that food was not a limiting factor in egg production. Dog food was chosen because it had proven to be an adequate food in maintaining other crickets in the laboratory. The buckets were kept outdoors under two conditions. 1) The buckets were on the ground and screen lids prevented the crickets' escape. 2) Buckets were buried flush with the soil surface, surrounded by a doughnut-shaped pitfall, and covered with a cylindrical, hardware cloth cage. Females had to fly to leave the buckets and in so doing they hit the cage, fell, and were trapped. Trapped females were then placed in new buckets and the old buckets were checked for eggs. If females could not leave because of screen lids, or if they did not fly from a bucket, their buckets were examined for oviposition and mortality about every 21 days (13 to 24 days depending on temperature). Examination of buckets consisted of carefully scraping thin layers of soil (0.51 cm) from the bucket using a spatulalike piece of sheet metal. This allowed the egg cells to be excavated without damaging the eggs. The depth and the number of eggs

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38 of each egg cell were noted and the eggs were placed on moist soil in numbered, aluminum containers. To estimate investment in each clutch, a sample of eggs was dried (at 30-37°C) and weighed to the nearest 1.0 mg using a model AC 100 Mettler balance. Remaining eggs were kept in the aluminum containers at room temperature and monitored daily until hatched. Oviposition Season Females of each species (n= 11 vicinus , 9 acletus ) were collected before oviposition during the 1982 fall flight season and placed singly in buckets with a male (condition 1 above). During the following spring, surviving females and spring females collected before oviposition were kept (condition 2) throughout the flight and reproductive seasons. Because females were found to lay no eggs in the fall, data from spring 1982 were included in analysis of reproductive season. Oviposition Cycles and Flig ht During the springs of 1982 and 1983, females were kept in buckets (condition 2) and their flights monitored to determine the relationship between oviposition and flight activity. The duration between hatching dates of successive egg clutches from individual females was used as an estimate of time between egg clutches. Effects of Size and Age on Fecundity Females varying in size were kept singly in buckets (condition 2) with a male and monitored for oviposition throughout their reproductive lifetime. Only females collected prior to any reproductive activities or that were known virgins were used. The wet weight (to the nearest 1.0 mg) of females was measured only once prior to their use in the experiment with a model AC100 Mettler balance. Pronotal length measured to the nearest 0.1 mm with vernier calipers was used as an indication of female size. In most cases, pronotal length was used because it has a direct

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39 relationship to female weight (r 2 =0.72 acletus . 0.66 vicinus . p < 0.001 see Chapter III), is less variable than weight, and is easily measured in the field. Insect eggs of many species absorb water during development, thus increasing both volume and weight (Hinton 1981). Because of differences in water content among eggs, dry weights were used as a measure of investment. Investment in individual eggs (mean dry weight per egg of samples), investment in clutches (mean dry weight of egg times number of eggs), and number of offspring produced (percent hatch times number of eggs) were measured for successive clutches of each female. Results Mortality. Longevity, and Seasonal Oviposition Of 9 acletus and 11 vicinus monitored during the fall, only 3 (33%) and 4 (36%) survived the winter. Since mortality was concentrated in colder months, this high mortality was probably due to buckets being placed above ground, increasing their exposure to cold temperatures during the winter. There was no apparent influence of size on mortality. No eggs were laid by any of the fall females and when females were found while examining buckets for eggs they were motionless or extremely sluggish. Of all females monitored during 1982 and 1983, 54% (n = 24) of vicinus and 41% (n = 22) of acletus females died before reproducing (including one vicinus and one acletus that were killed during examination of their buckets). Longevity of females in buckets was highly variable and ranged from 16 to 278 days (mean 91) for acletus and from 7 to 254 days (mean 89) for vicinus females. Egg— laying season was similar for the two species during the springs of 1982 and 1983 (Fig. 8). Oviposition started in late April and stopped in late July or early August, with most egg clutches produced in May and June. The proportion of

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Figure 8. Oviposition season in mole crickets (a) S. acletus and (b) S. vicinus . Graphs show the monthly proportion of egg clutches laid. For each month, bar on the left represents data for spring 1982 and the bar on right data for spring 1983. Note greater proportion of unsuccessful clutches (0-50% hatch) late in the season.

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41 0.4 w 0.3 UJ I o IZ> _l O „ 0.2 O 0.1 1 S. acletus H 0-50% hatch >50% hatch ^i nn APR MAY JUN JUL AUG 0.4 0.3 O u_ 0.2 O QC O 0.1 S. vicinus H 0-50% hatch >50% hatch J APR MAY JUN JUL AUG

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42 Table 2. Correlation matrix 3 of paired comparisons for S. acletus (top half of table) and S. vicinus (bottom half, underlined). Clutch % Hatch Egg No. of eggs no. wt per clutch Clutch -0.40 -0.72 -0.32 no. % Hatch -061 0.38 0.32 wt -066. 049 0.30 No. of eggs per clutch -061 061 052 a Spearman's rank correlation (rho). All correlations are significant (p < 0.01).

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43 unsuccessful clutches (<50% hatch) increased as the reproductive season progressed (Fig. 8). This was probably the effect of decreased hatching in later clutches of females (Table 2) rather than an influence by season alone. Oviposition Cycles and Flig ht The average interval between clutches, as estimated by the hatching dates of successive clutches, was 10 days for acletus (n = 38, 95% CL 8-11 days) and 9 days for vicinus (n = 32, 95% CL 7-12 days). Females rarely flew between egg clutches. In only 1 of 68 instances did a vicinus female (n = 9) take flight between the production of egg clutches. Of all females producing eggs, seven vicinus never left buckets, three flew prior to egg production but not after, and one flew after egg production but not before. Female acletus (n = 13) were more likely to take flight between clutches (13 of 43 occasions). For acletus females producing eggs, six never flew from their buckets, two flew after they began laying eggs, and five flew both before and after they started ovipositing. Reproduction and Investment by Individuals Females produced variable numbers of eggs and clutches; some died before ever reproducing, while others produced more than 450 eggs and as many as 10 clutches. Part of the variation in fecundity was due to size differences among the females. Excluding females that produced fewer than three clutches, there was a significant positive relationship between pronotal length of acletus females and the total number of offspring produced (sum of the number of eggs per clutch times percent hatch of the clutch, Fig. 9, p < 0.05) and between pronotal length and average number of eggs per clutch (Fig. 10, p < 0.01). These relationships were not significant for vicinus females. There was no relationship between female size and the number of clutches or the size of eggs produced for either species.

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Figure 9. Relationship of size and total number of offspring produced for 12 S. acletus females caged outdoors. Line is calculated using the regression equation (r^ = 0.33, p < 0.05).

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45 Total Offspring 280 240 200 160 120 80 40

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Figure 10. Mean number of eggs per clutch for 12 S. acletus females varying in size caged outdoors. Line is calculated using the regression equation (r 2 = 0.50, p < 0.01).

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47 Mean Number of Eggs/Clutch 7.4 7.8 8.2 8.6 Pronotal Length (mm)

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Figure 11. Plot of ranked egg weight for successive clutches of female mole crickets. Egg weight is the mean dry weight per egg of a sample from each clutch. Clutches with the same mean weight were given averaged ranks. Points connected by lines represent points at the same position on the graph. The number of females producing a specific number of clutches can be found by counting the points for each clutch number. Note that females invest less (smaller weight = higher rank) in later clutches (higher clutch numbers), (a) ^ acletus , (b) Si vicinus .

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49 9-

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50 Average investment per clutch was from 10 to 24 percent of female body weight for aclctus and ranged from 5 to 16 percent of female body for vicinus females. Total lifetime investment was greater than 100 percent of female body weight for 6 of 12 acletus females and for 1 of 8 vicinus females. Since dry weight of eggs was measured and female body weight was measured while the crickets were alive (i.e., they do not represent dry weight of females), the investments in eggs are actually substantially larger. In both species, investment per egg and per clutch decreased as females produced successive clutches. There was a significant inverse correlation between weight of eggs and clutch number (Fig. 11) as well as an inverse correlation between the number of eggs per clutch and clutch number (Table 2). Also, the percent hatch for clutches and mass per egg were significantly and negatively correlated with clutch number (Table 2). Later clutches of a female had fewer eggs, smaller eggs, and lower percent hatch. Discussion My observations on oviposition did not differ from Hayslip's (1943) Plant City, Florida, observations with regard to depth and average number of eggs found for egg cells of acletus or vicinus . Oviposition season was also similar for vicinus : however, acletus in Plant City extended oviposition into September. This may be the result of warmer temperatures and a longer growing season farther south (ca. 190 km), conditions that result in a bivoltine life cycle for acletus in South Florida (Walker et al. 1983). I did not find fall oviposition in buckets by females of either species, but flush samples taken during November in Gainesville have contained hatchling acletus suggesting that fall oviposition may occur, although young

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51 juveniles apparently do not survive the winter (Stackhouse W., E. L. Matheny and W. G. Hudson unpublished data). Even though oviposition seasons of acletus and vicinns are similar, flight seasons are not. Most of the flight season of vicinus is over by the start of oviposition, while acletus flights are concurrent with oviposition (compare Fig. 2 of Walker et al. [1983] with Fig. 8). This agrees with differences in flight from buckets in the two species. That acletus females are more likely to fly between clutches may have been one factor contributing to the more rapid spread of this species after its introduction into the United States (Walker and Nickle 1981). Clutch cycles run between 7 and 12 days as evidenced by the average time between hatching of successive egg clutches of females. This agrees with earlier work that estimated a 9 day period for a female to mature an egg clutch based upon the measurement of primary oocytes of females (Forrest 1981). That flying females have small oocytes suggests that they recently matured or laid a clutch and are starting a new clutch cycle (Forrest 1981). Ngo and Beck (1982), working on dispersal flights of acletus . noticed a significant 10— day peak in recaptures of marked and released females. Oviposition cycles may have accounted for this peak. They also found 12— day cycles in overall flight activity and attributed it to female oviposition cycles. This assumes that females are synchronous in their egg laying. I found no such synchrony. Because acletus females in my tests flew between only 30% of the egg clutches produced, it seems unlikely that peaks in flight activity are due to oviposition cycles. Such peaks are more likely caused by environmental factors making flights favorable or synchronizing maturation. Larger eggs generally produce higher quality, more vigorous progeny that may be more mobile, may suffer less mortality during early stages, may develop faster, or may require fewer instars (Capinera 1979). But given finite resources,

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52 producing larger eggs may not always increase the fitness of the female and a tradeoff should exist between number of eggs produced and their size (Smith and Fretwell 1974, Parker and Begon 1986). Producing small eggs is apparently disadvantageous for mole cricket females; because egg size and hatching success were positively correlated in both species. Producing fewer eggs may also hamper escape from the egg cell if siblings must cooperate while digging to the surface. Large clutches of large eggs then should maximize a female's fitness. However, as females aged they produced smaller clutches with smaller eggs. If clutch size is important and resources are limited as females get older, they may divert some of their resources away from egg size, at the cost of hatching success, to increase the clutch size. That later clutches are smaller with smaller eggs could have also been the effect of longer lifespans than would normally occur under natural conditions. Organisms rarely survive their full potential life span and theories on the evolution of senescence suggest that selection should act to increase reproductive output early in life at its expense later (Hamilton 1966). Females kept in buckets were protected from hazards, and, thus, their becoming senescent may have influenced the size of their later clutches. Another orthopteroid, the stick insect Diapheromera velii , shows a similar decline in the rate of egg production and a decrease in investment in eggs later in life (Sivinski 1977). Gwynne (1983) related sexual differences in mating behaviors to differences in parental investment in Tettigoniidae and other Orthoptera. As sexual selection theory predicted (Trivers 1972), when males invest more they are more selective of mates than females, whereas females are more selective and males more competitive when maternal investment is greater than paternal investment. Investment in progeny by female mole crickets is far greater than that for males. Males provide no paternal investment except the possible energy a female receives

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53 from consuming the small spermatophore (<1 mm diam, estimated «0.1% of male body weight) a male passes to the female during mating. On the other hand, a female's average investment per clutch may be as much as 24% of her body weight. Behavioral differences between the sexes support predictions based on differences in parental investment. Male mole crickets display acoustically, are aggressive, and often fight near calling burrows, whereas females are discriminating in their choice of mates (Forrest 1983). In a model of optimum egg size and clutch size in insects, Parker and Begon (1986) showed that if larval success was determined solely by the size of the egg, then selection would act such that females all produced eggs that were the same size. A consequence of this is that large females can produce larger clutches than smaller females. Data from mole crickets fit the model. There was a positive correlation between egg size (investment) and hatching success of clutches for mole crickets. Mean clutch-specific egg size did not differ between large and small females nor did the rate of decline of investment in eggs or egg clutches. Also, larger females produced larger clutches. Although a direct influence of size on fecundity is common in insects, few studies have been conducted under natural conditions and few data are available for Orthoptera. I found a significant relationship between female size and mean clutch size (Fig. 10) and total offspring produced (Fig. 9) for acletus but not vicinus females. That only one of eight vicinus females invested more than her body weight in lifetime egg output compared with six of twelve acletus females suggests that this difference may be due to the inability of vicinus. primarily a herbivore, to assimilate the dog food provided as food. Female vicinus would then have to rely on juvenile reserves for egg production, and although a larger female may carry greater reserves these would probably not be enough to produce more than a few clutches. Dog

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54 food, however, was able to keep the females alive for an average of 90 days and in some cases for more than 250 days, indicating that this diet was reasonably adequate for the crickets. Female vicinus might simply consume more of the poor quality food, and, thus, expend more energy to compensate for its lack of nutritional value (see Slansky [1982] for a review of insect nutritional ecology). Large acletus females can produce almost 3 times as many offspring and more than 1.5 times as many eggs per clutch as small females. Gwynne (1984) found that female size and nuptial feeding by males directly affected fecundity in the bushcricket, Requena verticalis . Females feeding on seven spermatophylaxes more than doubled egg production, and the size of eggs produced also increased. There is also a relationship between the number of ovariolcs (and presumably reproductive output) and size in between-specics comparisons of melanopline grasshoppers (Bellinger and Pienkowski 1985). Female investment and fecundity and their relationship with size tactics and developmental strategies in insects have received almost no attention, though such selective forces shape population size structure and influence population dynamics. Assuming a direct relationship between fecundity and size, Masaki (1967) explained the latitudinal size variation in the Emma field cricket, Teleogryllus emma . on the basis of a balance between the opposing selection pressures of increasing fecundity with increasing size and decreasing size with shortened growing season of more northern latitudes. I have used similar explanations for within-population size variation and changes in the seasonal distributions of sizes of both male and female mole crickets (see Chapter III). These types of data should improve models of population dynamics and help explain the life-history strategies in insects from an individual, evolutionary perspective.

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CHAPTER III SIZE TACTICS AND DEVELOPMENTAL STRATEGIES From early days of infancy, through trembling years of youth Long murky middle age and final hours long in the tooth. (Ian Anderson) Introduction Size is one of the most important attributes of any organism, and within any species there is often a great deal of variation in size among individuals. An animal's size will profoundly affect the prey it can eat, what predators it has, its physiology and energetics, and its reproductive success (Peters 1983). Thus an organism's size and life history are intimately related. However, research on life history strategies has virtually ignored the relationship between size and development, and has focused primarily on tactics of diapause, migration and dispersal (Blakley 1981; Denno and Dingle 1981). Animals with complex life cycles are interesting with regard to size tactics because certain life stages are specifically evolved for growth and increasing body size, while other stages are adapted for dispersal and/or reproduction. Amphibians and insects are perhaps the best known groups with complex life cycles. Of the two, insects offer a simpler system in which to study size tactics because growth occurs only in juvenile and non-reproductive stages (see Wilbur and Collins 1973; SmithGill and Berven 1979; Werner 1986 for a discussion of amphibian metamorphosis). Unlike amphibians, there is usually no growth once an insect becomes adult (i.e., no increase in size although abdominal stretching may occur especially in females), and 55

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56 adult size is determined by what happens during juvenile stages. Individuals can make decisions about their size by using certain cues to determine when to molt to adult. Costs and Benefits Regarding decisions, I am not implying that the animals make a conscious decision but rather these are evolved responses to cues the individual has available. The decisions an individual makes about changes in size carry with them costs and benefits. Although difficult to assess, these costs and benefits are ultimately measured in terms of fitness. Accordingly, selection should act such that individuals evaluate the costs and benefits and make those decisions that will maximize fitness. For insects the decision an individual makes about its size can be simplified to a decision of whether or not to become an adult. Figure 12 shows some of the costs and benefits associated with a yes or no decision to mature, that is to molt to pupa or adult. For instance, at a point in the development of an individual the decision to continue as a juvenile (no-decision Fig. 12) has associated with it costs due to delayed reproduction with the risk of dying before maturing and reproducing. There is also the cost of limited mobility since juveniles cannot fly. Benefits to an individual remaining a juvenile are due to an increase in fitness because of a longer period of growth and thus a further increase in size, assuming there is a direct effect of adult size on fitness (see below). Graphically the cost/benefit curves might look like Figure 13. Early in its development an individual might benefit a great deal because of small increases in size while the costs of the same size change increase only slightly. Later on small changes in size increase fitness (benefits) very little while costs for the same amount of change become increasingly more expensive. The individual is expected to adjust its development such that it maximizes the difference between benefits and costs

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Figure 12. Costs and benefits to an individual associated with yes or no decisions of whether or not to become an adult.

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58 BECOME ADULT? NO YES BENEFITS attain larger size longer/earlier reproduction flight COSTS delayed reproduction size restricted no flight

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Figure 13. Hypothetical cost/benefit curves for changes in an individual's size. Selection is expected to act such that individuals weigh costs and benefits and become adult when the difference between the curves is maximized (dashed line).

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60 Cost or Benefit A Size

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61 (i.e. the individual is expected to make the decision to molt to adult at the size indicated by the vertical line). Cues such as population density, availability and suitability of food and mates, size of competitors, sex ratio, length of the remaining growing season, and "expected" temperature (particularly important for poikilotherms) should be used in making the decision of when to molt. Although some authors have focused on an optimal body size within a species (see Roff 1981), it is the variation in sizes among individuals and the deviation from the optimal that needs to be investigated. Previous work on size variation in insects dealt with variation between populations. Primarily latitudinal or altitudinal differences among the populations were used to explain the variation (Masaki 1967, 1978; Dearn 1977), and Roff (1980,1983) has proposed models that use season length, = generation length, which varies with latitude and altitude, as a tool in predicting insect size and life histories. Proximate causes of size variation in insect populations include temperature gradients (Sweeney and Vannote 1978), larval diet (Slansky 1982; Andersen and Nilssen 1983; Palmer 1984) and population density (Peters and Barbosa 1977). Borgia (1979) discussed variation in male size and the decisions males should make in weighing costs and benefits of changes in size. Borgia was primarily interested in male size and its importance in a male's ability to control specific resources, but such tactics and decisions should be applicable whenever size influences an individual's fitness. Size and fitness in insects For poikilotherms, reproductive success or output is almost always influenced by size (Peters 1983). It is well known that for many insects there are positive relationships between female size and the number of eggs, size of eggs, and size of clutches produced (Hinton 1981). Only recently have there been attempts to correlate male reproductive success with size. In species where males compete for

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62 females (female defense polygyny) or for resources used by females (resource defense polygyny) large males often have a reproductive advantage over smaller males that may have to adopt alternative mating strategies to acquire mates. Where female choice is involved, females often choose males on the basis of cues correlated with male size (Thornhill and Alcock 1983 and refs.) Thus it is well documented among insects that large size confers some advantage in fitness for both sexes. For this reason insects are appropriate for investigating the occurrences of size tactics and developmental strategies. Size Tactics in Mole Crickets Costs and benefits associated with size are almost impossible to measure directly since they may differ from one individual to the next and may change at any time during an individual's life. However, developmental strategies should be reflected in adult sizes, and certain predictions about changes in size distribution for a species can often be tested. To examine whether individuals weigh costs and benefits associated with changes in size I tested predictions about seasonal differences and within-season changes in size distributions for two species of mole cricket. Size is an important component in the sexual success of an individual. For males, the intensity of the calling song is correlated with male size-larger males produce louder calls (Forrest 1980,1983 and Chapter I). Flying females selectively respond to and land at louder calls, and all things being equal, larger males will attract and mate with more females than smaller males (Forrest 1983). Larger females also have a reproductive advantage over smaller females. Large females may produce as many as 3 times as many offspring and more than 1.5 times as many eggs per clutch than smaller females (see Chapter II).

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63 Predictions Since reproduction occurs only during a brief period the following spring, during the fall the cost to an individual due to delayed reproduction associated with remaining a juvenile is zero (see Fig. 12). One would predict that individuals that are smaller than average would overwinter as juveniles to increase their size, while those that are larger than average would gain relatively little by increasing in size, but more by becoming adult, dispersing to favorable overwintering sites, and preparing for reproduction early the following spring. There may be a cost of delayed maturation due to limited movement. For instance, if an individual found itself in an unfavorable site where the likelihood of surviving the winter is very low (i.e., the costs are very high), that individual should mature and disperse to a more favorable site. However, this cost would be borne by large individuals as well as small, while the additional cost of maturing at a small size is borne only by small individuals. Therefore I expected a greater proportion of larger than average individuals in fall flights compared to spring flights. The second prediction involves an expected change in the size distribution during the spring reproductive season. Because the egg laying (reproductive) season is brief, individuals that remain juveniles incur greater and greater costs as the season progresses. Because the chance of surviving to the following reproductive season (the next spring) is very small, the costs of delaying reproduction increase dramatically and soon outweigh the benefits of increasing in size. I predicted that as the spring season progressed smaller and smaller individuals should mature. Materials and Methods Quarter-monthly samples (up to 50 individuals for 1981, 25 for 1982) of adult male and female mole crickets, acletus and vicinus . were obtained from a routine

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64 sound trapping station (Walker 1982) on the Agronomy Farm of the University of Florida. Samples were taken during the spring and fall flight seasons of both years. Since fall adults are of the same generation as adults of the following spring, measurements spanned three generations, and samples from spring 1981 and fall 1982 represented only part of the 80-81 and 82-83 generations respectively. To prevent desiccation, adults were placed in 13 dram snap cap vials containing moist soil but no food. Measurements of pronotal length (to nearest 0.1mm measured along the midline using vernier calipers) and mass (to nearest 1.0 mg using a model AC100 Mcttler balance) were taken for each individual within 24 hr of capture. Results There was a significant positive relationship between mass and pronotal length (Fig. 14). A linear model is shown because this model explained as much of the variation in the data as did the more usual model of mass equals the cube of a linear dimension. The regressions differed between species, whereas within a species the sexes had similar slopes that did not differ significantly. In both species the intercept was smaller for males than females (significant for acletus but not yjcjnus), thus for a given pronotal length males weighed less even though males on average are the larger sex (Fig. 14). Figure 15 shows the size distributions for each sex during spring and fall seasons (combined for both years). Distributions were approximately normal and male and female sizes were broadly overlapping. On average males were larger than females (Fig. 15, Table 3). For acletus the distributions changed seasonally. Fall adults were larger than spring adults (Fig. 15a & b). The distributions for vicinus females suggest the opposite trend (though not significant Table 3), but the exaggeration of the fall proportions is the result of a small sample of fall vicinus

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Figure 14. Linear relationship between pronotal length and mass for male and female S. aclctus (circles) and vicintis (squares). Lines arc plotted using regression equations and the endpoints represent the extreme values of pronotal length for each group. All regressions are significant p<.001. N=1222 male and 1431 female aclctus ; 284 male and 714 female vicinus.

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1400r 66 1200 1000 r-, 800 O) E 600 400 200 acletus
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67 Table 3. Mean pronotal length 3 (mm), standard deviation, and number of adult mole crickets trapped during spring and fall flight of 1981 and 1982. SPECIES sex

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Figure 15. Frequency distributions (expressed as a proportion of season total) of adult mole crickets captured during 19S1 and 1982 flights at Gainesville, Florida.

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69 S acletus JJ spring n =1080 C] l»ll n=351 6.5 7.0 7.5 80 8.5 90 95 100 105 11.0 11.5 Pronotal Length (mm) 6,5 7 7.5 8.0 8.5 9.0 9 5 10.0 10.5 11.0 11.5 Pronotal Length (mm) S vKtnus $>J spring n»652 (all n-62 6 5 7 7 5 8 8 5 9 9 5 10 10.5 11,0 11.5 Pronotal Length (mm)

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70 (n=62). Only three vicinus males were captured during the two fall seasons and were not included in the figure. Mean pronotal lengths were significantly larger for fall acletus than for spring individuals, whereas vicinus did not differ in average pronotal lengths between the two seasons. Pronotal lengths were similar for seasons compared between years (Table 3). Note also that fall individuals typically showed less variation in size than those in the spring. Mean pronotal lengths of quarter-monthly samples decreased during the spring flight season and reached a minimum toward the end of the reproductive season (Fig. 16). During the spring there was a significant relationship between pronotal length and the week of the season, but this relationship was not significant for fall individuals. This relationship explained less than 10 percent of the variation in the data when individual measurements were used in the regression analysis (Fig. 16). A regression using sample means (for samples with more than 10 individuals) and date of sample explained 37-89% of the variation in the mean pronotal length (Fig. 16). Data for vicinus males are not shown because few samples contained more than 10 males (1981 n=4, 1982 n=5). The relationship was significant for individual measurements of male vicinus for spring of 1982 (p<.05) and both years combined (p=.05); however no significant relationships were found between the nine sample means and date of capture. Discussion The data on seasonal and distributional shifts in mole cricket sizes support the predictions and suggest that individuals weigh costs and benefits associated with changes in size (Fig. 12). There was one major difference between the two species that may indicate different overwintering strategies. There was a significant difference between fall and spring adult sizes for acletus but not vicinus . According

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Figure 16. Plot of mean pronotal length of quarter-monthly samples (where N>10) for adult mole crickets captured during 1981 and 1982 flights. For all means shown standard errors are about 0.1mm. Trend lines plotted from regression equations for individual measurements for 1981 and 1982 combined (solid) or separately in the case of vicinus (dashed). Spring declines in sample means were significant-A) 1981:r 2 =.38, p<.03. 1982: .89, <.001. 1981+82: .70, <.001. B) 1981:r 2 =.89, p<.001. 1982: .72, <.001. 1981+82: .79, <.001. C) 1981:r 2 =.45, p<.03. 1982: .62, <.004. 1981+82NS.

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S. acletus 72 9.6 9.4 E >§ 9.2 g> 9.0 8.6 8.4 8.2 • • • • • MALES o 81 • 82 Sep ' Oct ' Nov * Dec Fall Feb ' Mar ' Apr ' May ' Jun ' Jul Spring 8.6 E 8.4 E £ 8 " 2 O) c 0) 8.0 _i o A7.6 FEMALES A 81 a 82 Sep I Oct I Nov I Dec! ^H ^ I M " a 7 I ^ I m"^ I J^ I ~i I Fall Spring S. vicinus 8.4 E >§ 8.2 c |> 8.0 (D _) CO o o 7 6 rx 7.4 A A A A A FEMALES A 81 a 82 Sep I Oct I Nov I Dec! H Feb I Mar I Apr I May I Jun ' Jul I Fall Spring

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73 to the predictions made, individuals that are smaller than average in the fall should wait until the following spring to mature. That adult vicinus are less variable in size than acletus (Fig. 15) may have made the detection of seasonal differences more difficult. It is interesting that a greater percentage (85%) of the vicinus population overwinter as adults than in the acletus population (25%). Forage quantity and quality are greatly reduced during the winter months and, since vicinus is mainly herbivorous, this may limit the potential for growth during that part of the year. Benefits due to overwintering as a juvenile would be lessened and it may be more important to locate overwintering sites that are favorable for survival and allow early reproduction the following spring. I found that as the reproductive season progressed there was a decline in the pronotal lengths of individuals for successive samples. That is, a significant relationship was found between pronotal length and capture date. Although the model did not explain much of the variation in measurements of individuals the overall trend in the data (sample means Fig. 16) was highly significant. This might be expected since all sizes of individuals should be maturing throughout the season, but the proportion of small individuals increases dramatically toward the end of the season. Small individuals have probably found themselves in areas where growing conditions are poor and they mature at the smaller size in order to makes the 'best of a bad situation' (Dawkins 1980). Rogers (1942), working with a number of tipulid species, noticed a decline (as much as 20%) in the size of late season specimens and attributed the decline to lack of food and moisture. Similar seasonal changes were also found in the milkweed leaf beetle, Labidomera clivicollis (Palmer 1984). There was a significant gradual decline in male and female sizes during the growing season caused by a decline in the amount of available food. Food quality was not discussed and may be

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74 even more important. Slansky (1982) pointed to the growing interest in insect nutritional ecology and discussed the decisions individuals must make relative to nutrition and their final adult size. For mole crickets, the similarity from year to year in the time at which the smallest sample means occur suggests that photoperiod may be one proximate cue used in the molting decision. However, cold weather during the spring of 1981 caused a delay in the vicinus flights (Walker et al. 1983) and probably delayed maturation and caused the shift seen in the seasonal decline in size for 1981 (dashed lines Fig. 16c). Since aclctus flights occur later in the spring, cold weather had no affect on delaying maturation or the decline in size during the spring months. Other proximate causes for the observed decline, such as food and temperature, seem unlikely. Food quantity and quality as well as temperature arc increasing during the months of May, June, and July. Insect size variation, though environmentally induced, may still be under the influence of natural selection. The mechanisms can be simple. It is well known that in many insects different, numbers of molts precede adulthood. For instance, mole crickets may have from 6 to 10 molts before becoming an adult. Molting is brought about by changes in hormone titers of the individual. These hormone levels, while under genetic control, are known to be influenced by many environmental cues. Therefore even though variation in size can be caused environmentally, the decisions to molt have a genetic basis that can be acted on by natural selection. One pervasive point in most studies of development is that an individual must reach a 'critical size' before becoming adult (Blakley and Goodner 1978, Blakley 1981, Palmer 1984). Starvation of juveniles before they attain this size results in death. On the other hand, some cockroaches apparently determine their ultimate size at a specific instar and adjust the number of instars to reach this size at

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75 adulthood (Tanaka 1981). Slansky (1982) suggested that this minimum size is adjusted by selection and the balance between costs of prolonged development and the costs of reduced size and fitness. The understanding of size variation as it relates to developmental strategies should broaden our understanding of an organism's life history. An individual's size is the direct result of its development. Size has been implicated as the factor affecting a number of alternative reproductive behaviors in insects (Thornhill and Alcock 1983) and is a major determinant in the sex change of sequential hermaphrodites (Charnov 1979). Size tactics may even be one generation removed. For instance females that provision broods and female parasites may adjust the amount of investment in their offspring, thus influencing their offspring's adult size and reproductive behaviors (Alcock 1979). Once size tactics and the developmental strategies of an organism are explained from an individual's standpoint, new light will be shed onto other aspects of that organism's life history.

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CONCLUSIONS Male mole crickets produce sound. Like most crickets these acoustic signals function to attract females for mating purposes. Even though males use their burrows as a baffle and they call from within an acoustic horn, the production of sound is an inefficient process. Males are less than 0.2 percent efficient in producing from 2 to 22 w Watts of acoustic power. The power produced is dependent on male size and the moisture of the soil surrounding the calling burrow. The sound field is hemispherical, and thus, has no directional characteristics. By preferentially landing at louder males, females may use the calling songs to locate large mates or moist soil for oviposition. Oviposition occurs during the spring, primarily in May and June. The investment in eggs, the numbers of eggs, and the percent hatch of eggs decline as females age. Large females produce more hatchlings and have larger egg clutches than smaller females. Because size is such an important aspect of the reproductive success of both sexes, males and females should make decisions about when to mature that will maximize fitness. Each decision about whether or not to mature has certain costs and benefits that must be evaluated. During the fall, individuals that are smaller than average overwinter as juveniles in order to increase in size before spring reproduction. As the reproductive season progresses the cost of delaying maturation increases and smaller individuals mature. 76

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77 It is important to note that the observations are made from the standpoint of the individual. As such, they can be used to predict aspects of mole cricket life history and will be useful in the models concerning the population dynamics of these pest species.

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REFERENCES CITED Alcock, J. 1979. The evolution of intraspecific diversity in male reproductive strategies in some bees and wasps. Pages 381-402 in Blum, M.S. and N.A. Blum, eds. Sexual selection and reproductive competition in insects. Academic Press, New York. Alexander, R.D. 1961. Aggressiveness, territoriality, and sexual behavior in field crickets (Orthoptera: Gryllidae). Behavior 17:130-221. Andersen, J. and A.C. Nilssen. 1983. Intrapopulation size variation of free-living and tree-boring Coleoptera. Can. Entomol. 115:1453-1464. Bellinger, R.G. and R.L. Pienkowski. 1985. Interspecific variation in ovariole number in Melanopline grasshoppers (Orthoptera: Acrididae) Ann. Entomol. Soc. Am. 78:127-130. Bennet— Clark, H.C. 1970. The mechanism and efficiency of sound production in mole crickets. J. Exp. Biol. 52:619-652. Bennet-Clark, H.C. 1971. Acoustics of insect song. Nature 234:255-259. Bennet— Clark, H.C. 1975. Sound production in insects. Sci. Prog. (Oxford) 62:263283. Blakley, N. 1981. Life history significance of size-triggered metamorphosis in milkweed bugs ( Oncopeltus ). Ecology 62:57-64. Blakley, N. and S.R. Goodner. 1978. Size-dependent timing of metamorphosis in milkweed bugs ( Oncopeltus ) and its life history implications. Biol. Bull. 155:499-510. Boggs, C.L. 1981. Nutritional and life-history determinants of resource allocation in holometabolous insects. Am. Nat. 117:692-709. Borgia, G. 1979. Sexual selection and the evolution of mating systems. Pages 19-80 in Blum, M.S. and N.A. Blum, eds. Sexual selection and reproductive competition in insects. Academic Press, New York. Bruel, P.V. 1983. Sound level meters--the Atlantic divide. Bruel & Kjaer Technical Review 4:3-23. Capinera, J.L. 1979. Qualitative variation in plants and insects: effect of propagule size on ecological plasticity. Am. Nat. 114:350-361. 78

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79 Charnov, E.L. 1979. Natural selection and sex change in pandalid shrimp: test of a life-history theory. Am. Nat. 1 13:715-734. Dawkins, R. 1980. Good strategy or evolutionary stable strategy? Pages 331-367 in Barlow, G.W. and J. Silverberg, eds. Sociobiology: beyond nature/nurture? Westview Press, Boulder, Colo. Dearn, J.M. 1977. Variable life history characteristics along an altitudinal gradient in three species of Australian grasshopper. Oecologia 28:67-85. Denno, R.F. and H. Dingle, eds. 1981. Insect life history patterns: habitat and geographical variation. SpringerVerlag, New York. Eisner, N. 1983. Insect stridulation and its neurophysiological basis. Pages 69-92 jn Lewis, B. ed. Bioacoustics a comparative approach. Academic, London. Eisner, N. and A.V. Popov. 1978. Neuroethology of acoustic communication. Adv. Insect Physiol. 13:229-355. Forrest, T.G. 1980. Phonotaxis in mole crickets: its reproductive significance. Fla. Entomol. 63:45-53. Forrest, T.G. 1981. Acoustic behavior, phonotaxis, and mate choice in two species of mole crickets (Gryllotalpidae: Scapteriscus V M.S. thesis, Univ. of Florida, Gainesville. Forrest, T.G. 1982. Acoustic communication and baffling behaviors of crickets. Fla. Entomol. 65:33-44. Forrest, T.G. 1983. Calling songs and mate choice in mole crickets. Pages 185-204 jn Gwynne, D.T. and G. K. Morris, eds. Orthopteran mating systems: sexual competition in a diverse group of insects. Westview Press, Boulder, Colo. Forrest, T.G. (in press). Sinistrality in the southern and tawny mole crickets (Gryllotalpidae: Scapteriscus ). Fla. Entomol. Fullard, J.H., M.B. Fenton and J.A. Simmons. 1979. Jamming bat echolocation: the clicks of arctiid moths. Can. J. Zool. 57:647-649. Gade, S. 1982. Sound intensity (Part I. theory). Bruel & Kjacr Technical Review 3:3-39. Gwynne, D.T. 1981. Sexual difference theory: Mormon crickets show role reversal in mate choice. Science 213:779-780. Gwynne, D.T. 1983. Male nutritional investment and the evolution of sexual differences in Tettigoniidae and other Orthoptera. Pages 337-366 in Gwynne, D.T. and G. K. Morris, eds. Orthopteran mating systems: sexual competition in a diverse group of insects. Westview Press, Boulder, Colo.

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80 Gwynne, D.T. 1984. Courtship feeding increases female reproductive success in bushcrickets. Nature (London) 307:361-363. Hamilton, W.D. 1966. The moulding of senescence by natural selection. J. Thcor. Biol. 12:12-45. Hayslip, N.C. 1943. Notes on biological studies of mole crickets at Plant City, Florida. Fla. Entomol. 26:33-46. Hinton, H.E. 1981. Biology of insect eggs. Pergamon, Oxford. MacNally, R. and D. Young. 1981. Song energetics of the bladder cicada, Cystosoma saundersii . J. Exp. Biol. 90:185-196. Masaki, S. 1967. Geographical variation and climatic adaptation in a field cricket (Orthoptera: Gryllidae). Evolution 21:725-741. Masaki, S. 1978. Climatic adaptation and species status in the lawn ground cricket. II body size. Oecologia 35:343-356. Masaki, S., M. Kataoka, K. Shirato and M. Nakagahara. (in press). Evolutionary differentiation of right and left tegmina in crickets. Bull. Ital. Entomol. Soc. Matheny, E.L. 1981. Contrasting feeding habits of pest mole cricket species. J. Econ. Entomol. 74:444-445. Michelsen, A. 1978. Sound reception in different environments. Pages 345-373 in Ali, M.A. ed. Sensory ecology. Plenum, New York. Michelsen, A. 1983. Biophysical basis of sound communication. Pages 3-38 in Lewis, B. ed. Bioacoustics a comparative approach. Academic, London. Michelsen, A. and H. Nocke. 1974. Biophysical aspects of sound production in insects. Adv. Insect Physiol. 10:247-296. Mullins, D.E. and C.B. Keil. 1980. Paternal investment of urates in cockroaches. Nature 283:567-569. Nelson, M.C. and J. Fraser. 1980. Sound production in the cockroach, Gromphadorhina portentosa : evidence for communication by hissing. Behav. Ecol. Sociobiol. 6:305-3 14. Ngo, D. and H.W. Beck. 1982. Mark-release of sound-attracted mole crickets: flight behavior and implications for control. Fla. Entomol. 65:531-538. Nickerson, J.C., D.E. Snyder and C.C. Oliver. 1979. Acoustical burrows constructed by mole crickets. Ann. Entomol. Soc. Am. 72:438-440. Nocke, H. 1971. Biophysik der Schallerzeugung durch die Vorderfliigel der Grillen. Z. Vergl. Physiol. 74:272-314.

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BIOGRAPHICAL SKETCH Timothy G. Forrest was born Friday, the 13 tn of April, 1956, in St. John's, Newfoundland, Canada. He received his Bachelor of Science degree in 1978 from Florida Southern College where he majored in biology and mathematics. He received his Master of Science degree in entomology from the University of Florida during the spring of 1981. Since that time he has been working toward his doctorate. In January of 1982 he married Susan Lynn Mashke. They became parents on June 25, 1986 when their son, Justin Broc, was born. Along the coast road, by the headland the early lights of winter glow I'll pour a cup to you. . . . Raise it upsay Cheerio. (Ian Anderson) 84

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. (I Thomas J. Walker, Chairman Professor of Entomology and Nematology I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. James E. Lloyd Professor of Entomology and Nematology I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Martha L. Crump Associate Professor of Zoology This dissertation was submitted to the Graduate Faculty of the College of Agriculture and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. December 1986 Dean, College of Agriculture Dean, Graduate School

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UNIVERSITY OF FLORIDA 3 1262 08553 3692