THE ROLE OF THE COCOONS OF ORB-WEAVING SPIDERS
CRAIG S. HIEBER
A DISSERTATION SUBMITTED 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 1984
It is a pleasure to acknowledge the many friends and colleagues who aided me in this work. My major professor, John F. Anderson, has been a source of stimulation and encouragment. His comments on Mr work were always critical but fair, and he always emphasized the importance of doing good, solid research. Most importantly, he encouraged me to be as independent as possible while doing my research. To John I extend my sincerest thanks. The other members of my dissertation committee, Lincoln P. Brower, Jonathon Reiskind, and Reese I. Sailer, and the additional colleague, David H. Evans, who sat at my defense, all provided ready assistance during the research phase and many helpful editorial suggestions, from which I benefitted. I also recognize the advice or equipment provided by Drs. L. Berner, P. Feinsinger, M.H. Greenstone, F.J. Maturo, and B.K. McNab.
I was also fortunate to have the cooperation of a number of
people in positively identifying my egg predators. Thanks for this service go to H. Townes (ichneumonids), American Entomological Institute; Dr. M.E. Schauff (eulophids), Systematic Entomology Lab, USDA; and Dr. L. Stange (mantispids), Division of Plant Industries, FDACS. Further thanks go to Mr. and Mrs. H.M. Chitty, who generously allowed me access to the Stardust Ranch for many of my experiments. I also acknowledge the Department of Zoology, Sigma
Xi, The Research Society, and the Florida Entomological Society for their generous financial support during ay graduate program.
Numerous other post doctoral associates and graduate students in the Zoology Department have listened to qy ideas and provided helpful suggestions. Among these I would especially like to thank J.B. Anderson, W.W. Henneman, J.R. Lucas, S.B. and B.J. Malcolm, P.G. May, M.S. Obin, K. Prestwich, N.E. Stamp, and N.T. Wheelwright. I also extend qy thanks to A.D. Austin and K.E. Redborg for providing me with relevant manuscripts concerning my research.
I would like to separately thank qy office mate and friend James A. Cohen (UVPD) for the countless stimulating discussions concerning my work and biology in general, the lovebug collaboration, and, in particular, for his influence on ayr writing. Jim, the "big chair" will always be open!
1 would also like to separately thank Sherry M. Hieber for
putting up with the late nights, the wild talk, and the oftentimes crazed behavior of this researcher. I could not have done this without her support and love.
Finally, I would like to thank Wy parents, Bernie and Doris
Hieber, who have always told me I could be anything I wanted to be.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ................................................. ii
LIST OF TABLES ............................................... ... vi
LIST OF FIGURES .................................................. ix
ABSTRACT ......................................................... xi
CHAPTER I INTRODUCTION ............................ .......... 1
CHAPTER II THE EXPERIMENTAL ANIMALS, THEIR
COCOONS, AND THEIR HABITATS .......... o ............ *5
Mecynogea lemniscata ............. ............. 5
Argiope aurantia ......... o ....................... 8
CHAPTER III THE ''INSULATION'' LAYER IN THE
COCOONS OF ARGIOPE AURANTIA ....................... 12
Introduction.... ........................ *.... 12
Materials and Methods ........................... 13
Results ......................................... 15
Discussion ...................................... 20
CHAPTER IV THE ROLE OF SPIDER EGGS AND
COCOONS IN RESISTING WATER LOSS .. ...... ..... 25
Introduction ........................ .......... 25
Materials and Methods ............................ 26
Results ....................................... .29
Discussion ....................................... 36
CHAPTER V THE ROLE OF THE COCOON IN
LIMITING EXCESS WATER AND
FUNGAL ATTACK ................... o ................. 42
Introduction ............ o .......... o ............. 42
Materials and Methods ............................ 43
Results .......................................... 47
Discussion ....................................... 54
CHAPTER VI THE ROLE OF THE COCOON IN
LIMITING EGG AND SPIDERLING
PREDATORS .......................................... 62
Introduction ..................................... 62
Materials and Methods ............................ 64
Results .......................................... 71
Discussion .............................. ........ 89
CHAPTER VII COCOON SPACING AND THE TIMING OF
PRODUCTION AS METHODS TO AVOID
EGG AND SPIDERLING PREDATORS ............ ......... 100
Introduction .................................... 100
Materials and Methods ...................... *..*.102
Results .......................... # .............. 104
Discussion ...................................... 115
CHAPTER VIII GENERAL DISCUSSION AND
CONCLUSIONS ....................................... 125
General Discussion ............ ........ o ... o....125
Conclusions ..... o .................. ........... 143
LITERATURE CITED ....................... o ........................ 146
BIOGRAPHICAL SKETCH ...... o ......... o .............
LIST OF TABLES
Table 4-1. The distributions of sphere sizes found on the chorionic surfaces of Mecynogea lemniscata and Argiope aurantia eggs ................................................... 31
Table 4-2. Mean percentages of eggs successfully hatched at the four experimental humidities for intact Mecynogea lemniscata and Argiope aurantia egg masses with and without cocoons, and for reduced A. aurantia egg masses without cocoons ......................................................... 32
Table 4-3. Mean percentages for molting success and spiderling survival at the four experimental humidities for Mecynogea lemniscata egg and spiderling masses with and without cocoons ...... ........................... .. .... 34
Table 4-4. Mean percentages for hatching and molting success and spiderling survival in the field for the spiders Mecynogea lemniscata and Argiope aurantia ................................. 35
Table 4-5. Mean percentages for molting success and spiderling survival at the four experimental humidities for Argiope aurantia egg and spiderling masses with and without cocoons ...................... 37
Table 5-1. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the eggs of Mecynogea lemniscata ......................... .52
Table 5-2. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the spiderlings of Mecyncgea lemniscata ............................. 53
Table 5-3. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the eggs of Argiope aurantia .................................... 55
Table 5-4. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the spiderlings of Argiope aurantia ................................ 56
Table 6-1. Numbers of Mecynogea lemniscata cocoons attacked by various predators for the years 1981 to 1983.......................................... 72
Table 6-2. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the eggs of Mecynogea lemniscata ....................................... 74
Table 6-3. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the spiderlings of Mecynogea lemniscata ............................. 76
Table 6-4. Numbers of Argiope aurantia cocoons suspended in the vegetation which were successfully attacked by various predators for the years 1981 to 1983 ...................... #............... ..... 77
Table 6-5. The mean percentage of eggs which survive in Argiope aurantia cocoons attacked solely by various predators .................................... 81
Table 6-6. Means for the variables cocoon diameter, cocoon length, egg mass diameter, egg mass length, distance to the egg mass, and ovipositor length ........................................... 84
Table 6-7. The mean numbers of suspension line deltas and oviposition holes in the upper, middle, and lower thirds of the covers of Argiope aurantia cocoons .............................. 85
Table 6-8. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the eggs of Argiope aurantia ............................................. 87
Table 6-9. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the spiderlings of Argiope aurantia ................................. 88
Table 7-1. Densities of Mecynogea lemniscata cocoons, and the percentages of cocoons attacked for the years 1981 to 1983 ............................ 105
Table 7-2. Mean numbers of Mecynogea lemniscata cocoons per string, and the mean number of eggs in each cocoon in the string for the years 1981 to 1983 ............................................. 109
Table 7-3. A comparison of the contents of attacked Mecynogea lemniscata cocoons .......................... 110
Table 7-4. The number and distribution in space and time of all Mecynogea lemniscata web-sites with cocoons, and of sites with cocoons in the proper stage for attack in the experimental plot for 1983 ....................... 112
Table 8-1. The mean sphere density and median clutch size for individual spiders from different families ... .......................................... 131
LIST OF FIGURES
Figure 2-1. A string of Mecynogea lemniscata cocoons.................................... o o ........6
Figure 2-2. The cocoon of Argiope aurantia ....................... 9
Figure 3-1. Cooling rates in still air in the laboratory for whole and modified Argiope aurantia cocoons. ............. ............... o........... 16
Figure 3-2. Cooling rates outdoors for whole and modified Argiope aurantia cocoons in an artificial closed habitat over a 50 min period during an approximate 2.50 C change in ambient temperature....................... ..... ..... 18
Figure 3-3. Heating rates in still air outdoors for whole and modified Argiope aurantia cocoons under radiant loads of 2 and 10 min duration .................. 19
Figure 3-4. The relationship of internal cocoon temperature to habitat position for two Argiope aurantia cocoons. ............................ 21
Figure 4-1. The layer of spherical mucoid granules on the chorionic surface of a Mecynogea lemniscata egg and an Argiope aurantia egg .............................................. 30
Figure 5-1. The rate of water loading in the laboratory for the covers of Mecynogea lemniscata cocoons submerged in distilled water .................................................... ....... 50
Figure 6-1. A schematic view of an Argiope aurantia cocoon showing the four measurements taken to determine the position of the egg mass in relation to the cocoon cover ................................. 66
Figure 6-2. The relationship between the diameter of Argiope aurantia cocoons and the number of spiderlings (eggs) therein ........................................................ 68
Figure 6-3. The covers of the cocoons of Mecynogea lemniscata and Argiope aurantia comparing the differences in the tightness of the weaving ........................................ 92
Figure 6-4. The ovipositor of the ichneumonid Tromatobia ovivora rufopectus showing recessed pits along its length and concentrated at the tip ....................................... 95
Figure 7-1. The relationship between proportional survival and cocoon density for Mecynogea lemniscata cocoons in four different sites ........................................... 107
Figure 7-2. The relationship between proportional survival and cocoon density for Mecynogea lemniscata cocoons within one site ................................................ 108
Figure 7-3. The number of Meycynogea lemiscata cocoons in a three-cocoon string available to a Tetrastichus wasp or her emerging progeny assuming an 11-day period for attack, or a more conservative 5-day period ............................ 119
Figure 7-4. The spatial distribution of all the Mecynogea lemniscata web-sites in the experimental plot for 1983 ................................. 121
Figure 8-1. The relationship between mean sphere density and clutch size for individual spiders from different families ....................................................... 133
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
THE ROLE OF THE COCOONS OF ORB-WEAVING SPIDERS
Craig S. Hieber
Chairman: John F. Anderson
Major Department: Zoology
The cocoons of spiders are believed to function in protecting the enclosed eggs and spiderlings from a number of biotic and abiotic factors. It has also been suggested that the wide diversity in cocoon architecture is the result of a "coevolutionary arms-race" between spiders and predators of their offspring. However, few studies have provided evidence to support these claims. This study provides evidence for some of these suggested functions by comparing the abilities of the cocoons of Mecnogea lemniscata and Argiope aurantia to control or limit temperature extremes, dessication, and attack by fungi and predators.
Results indicate that the cocoon of M. lemniscata limits
dessication and fungal attack on the spiderling stage. In contrast, the cocoon of A. aurantia does not. Both cocoons probably play a
limited role in controlling temperature extremes from short-term radiant loads.
The cocoons of both species play a significant role in
protecting the eggs and spiderlings from generalist and specialist predators. The association between the various layers making up these cocoons, and the specific methods used to gain entrance by their predators strongly supports the notion of a co-evolved system. However, the multiple functions of many cocoon components suggest the architectural details have probably resulted from diffuse evolutionary pressure from a number of simultaneously operating factors, instead of as counter-adaptations to specific predators.
In general, these results support many of the previous
suggestions concerning cocoon function. Although specific functions are related to specific components within the cocoon, there can be great interspecific variation with relation to cocoon size and the relative sizes or thicknesses of the component parts of the cocoon. In addition, factors such as the size of the egg mass, the ultrastructure of the eggs, the habitat used for oviposition, the position of the cocoon within the habitat, and the reproductive behavior of the spider may also influence the ability of any part of the cocoon, or the whole structure, to limit the effects of any or all of the suggested factors affecting egg and spiderling survival.
All spiders place their eggs in some form of silken cocoon (Turnbull 1973). For some, the cocoon is a relatively simple structure consisting of a few threads (e.g., the Pholcidae) and is used to move the egg mass about the web. In others, the cocoon provides a more complete container for carrying the eggs while hunting (e.g., the Lycosidae) or an additional level of protection for spiders which actively guard their eggs (e.g., the Oxyopidae). For many web-building spiders (e.g., the Araneidae), the cocoon is an elaborate structure composed of a variety of layers and is suspended in the habitat by different devices (McCook 1890, Scheffer 1905, Kaston 1948, Turnbull 1973). In many araneids where active care is non-existent, the cocoon represents the sum total of maternal care given to the eggs and spiderlings by the female.
Many web-building spiders deposit large clutches of eggs
(Kaston 1948, Foelix 1982). Within the cocoon, the eggs undergo embryonic development, hatch to the deutova stage, and molt to the spiderling stage. The spiderlings can spend as long as 6 to 10 months in the cocoon overwintering until emergence in the spring (Anderson 1978). This long time interval, the large clutch sizes, and the absence of active maternal care have led to many suggestions concerning the function of cocoons. These include providing physical support for the egg mass (Christenson and Wenzl 1980),
protecting the enclosed eggs and spiderlings from thermal extremes (McCook 1890, Kaston 1948, Turnbull 1973, Gertsch 1979), dessication (Foelix 1982), fungal attack (Christenson and Wenzl 1980), drowning (Reichert 1981), UV light (Yoshikura 1969), and predator and parasite attack (Moore 1977, Austin and Anderson 1978, Robinson 1980, Austin, In press). Few studies, however, have examined the cocoons' effectiveness in controlling these factors.
I present here the results of several studies bearing on the functional roles that the cocoons of Mecynogea lemniscata (Walckenaer) and Argiope aurantia Lucas (Araneidae) play in protecting eggs and spiderlings from a number of biotic and abiotic factors. I also present results on the significance of cocoon timing and spacing, and how they interact with cocoon architecture to reduce predation. Both of these spiders use cocoons with dense covers (as opposed to the flocculent silk cocoon used by other araneids). However, they inhabit different environments, construct cocoons of different size and complexity, produce differing numbers of clutches, reproduce at different times of the year, and are attacked by different parasites. These differences are discussed in detail in Chapter II.
In Chapter III, I examine the role that cocoons, and in
particular the flocculent silk layer within many cocoons, play in controlling temperature extremes. Many authors have suggested that the flocculent silk layer acts as insulation (McCook 1890, Kaston 1948, Turnbull 1973, Gertsch 1979). Published reports on this function have been contradictory. Schaefer (1976) reported that
the cocoons of Floronia bucculenta (Clerck) (Linyphiidae) protect the eggs from daily changes in ambient temperature. In contrast, Austin and Anderson (1978) concluded that the flocculent silk cocoon of Nephila edulis (Koch) (Araneidae) does not do so.
Chapter IV presents a comparative analysis of the capabilities of the cocoons of A. aurantia and M. lemniscata to control water loss. Although this is commonly assumed to be one of their major functions (e.g. Bristowe 1941, Foelix 1962), the data supporting this view are contradictory. Schaefer (1976) showed that the cocoon of F. bucculenta effectively controlled water loss from diapausing eggs. However, Austin and Anderson (1978) demonstrated that the flocculent silk cocoon of N. edulis had no effect on egg hatching success. Casual observations by McEwon (1963) suggest that the cocoon of an Australian Nephila species limits spiderling desiccation.
Closely related to the problem of desiccation is the problem of excess water which, if allowed to enter the cocoon, could dissolve materials from the surface of the eggs and increase their susceptibility to disease (Austin and Anderson 1978). Water entering the cocoon could also carry pathogens (Christenson and Wenz! 1980) or could drown the eggs or developing spiderlings. Although Reichert (1981) strongly suggests that the cocoons of Agelenopsis aperta (Gertsch) (Agelenidae) protect eggs from flooding, only Schaefer (1976) and Austin (1984) have demonstrated such a role for a cocoon or brood nest, respectively. In Chapter V, I compare the abilities of the cocoons of M. lemniscata and
A. aurantia to control the entrance of excess water and to limit fungal attack.
In Chapter VI, I examine the architecture of the cocoons of M. lemniscata and A. aurantia and its relation to the control of predator and parasite attack. Although considered one of the primary roles of cocoons (Austin and Anderson 1978, Robinson 1980, Austin, In press), this function has not been examined for any spider.
Many parasites locate their hosts in a stepwise manner using a variety of chemical and physical cues (Salt 1935, Vinson 1975, 1976). A number of biotic and abiotic factors have been suggested as disrupting these cues, thereby affecting the success of the parasites (Hassell 1971, Hassell and May 1973, Vinson 1976, Morrison and Strong 1980, Stiling and Strong 1982). However, host behaviors such as the phenology of emergence and their effects on the temporal and spatial distribution of the host have not often been considered as factors affecting parasite success. In Chapter VII, I examine the seasonal timing of reproduction, the length of the reproductive season, the timing of cocoon production, and the resulting spatial and temporal distribution of cocoons as methods used in conjunction with cocoons to interrupt the foraging behavior of egg predators.
Chapter VII sum nrizes my major results and discusses their
relevance to the architecture of cocoons and to the diversity of reproductive tactics displayed by orb-weaving spiders.
THE EXPERIMENTAL ANIMALS, THEIR COCOONS, AND THEIR HABITATS
Mecynogea lemniscata (Walkenaer), the Basilica Spider
In northern Florida (at my study sites), M. lemniscata emerges in late March. Males mature in May and are found on the webs of females from early June to early July. The females mature in late May. Oviposition starts in mid-June and extends to mid-August with a peak in mid-July. Females disappear from their webs by late August.
The eggs of M. lemniscata take 16 days to eclose to the
deutoval stage at 25 C and 70% RH. The deutoval stage lasts 4 days until the molt to the first instar spiderling stage (Hieber 1984). The spiderlings remain in the cocoon through the late summer, fall, and winter, and emerge in mid- to late March the following year after spending approximately 290 days in the cocoon (Anderson 1978).
The webs of M. lemniscata are placed in the shrub layer of deciduous forests, approximately 1-2 m above the ground. The cocoons are deposited at the web site, suspended above the domed orb-web from a single support line (Fig. 2-1a). The cocoons are produced sequentially, and the strings may contain 1-10 cocoons (X =
3.1 cocoons, SD = 1.6, n = 38). The individual cocoons are small (3-4 mm dia.) (Fig. 2-1b), and contain 8-30 eggs (R = 13.5 eggs, SD = 6.3, n = 35) (Hieber 1984). The cocoon covering is olive-green in
Fig. 2-1. A string of Mecynogea lemniscata cocoons suspended in the vegetation (A), showing the relative size of the cocoons (B), and with the external layer of silk peeled away to show the individual cocoons underneath (C). The external layer of siLk is composed of old orb-webs applied to the cocoon string by the female spider. This layer also contains detritus and prey remains.
color, extremely hard, and tightly woven. internally, there is a thin flocculent silk layer between the loose, non-agglutinated mass of eggs and the cocoon cover. The eggs are not enclosed in a membranous silk bag. The cocoons in a string are periodically covered with the damaged orb-web when it is replaced by the spider, giving the cocoon string an additional layer of loose dirty grey silk, detritus, and prey remains (Fig. 2-1c).
The eggs in M. lemniscata cocoons are attacked by two principal predators, the neuropteran Mantispa viridis Walker (Mantispidae) and the hymenopteran Tetrastichus sp. (Eulophidae) [near T. banksii Howard; see Hieber (1984)A. T"1ne eggs and spiderlings may also occasionally be attacked by ants, particularly if the support line breaks and the cocoons contact nearby vegetation or fall to the ground.
Mecynogea lemniscata is found in the shrub layer of southern deciduous forests (Levi 1980). In Florida, this spider prefers mesic hammocks, but can occasionally be found in more open habitats such as upland woods. In its preferred habitat, the humidity remains relatively constant (75-85% RH) during June and July (the egg laying and molting period) due to the overhead vegetation. From August on, the habitat dries and the RH may fall to 50% between rains. During the fall and winter, RH may fall as low as 30-40%. The temperature in this habitat is somewhat buffered due to the overhead vegetation which blocks the sun and it remains relatively constant (25-290 C) during the summer. From fall on, the temperature in the habitat follows the ambient temperature closely.
Argiope aurantia Lucas,
the Black and Yellow Garden Spider
In northern Florida (at my study sites), male A. aurantia mature in July and are seen on the webs of females from approximately mid-July to mid-August (see Levi 1968). Females mature in late July to August (see Levi 1968). Oviposition starts in early August and continues to October, with the greatest number of clutches laid in mid-August to mid-September. Females disappear from most study sites by late October.
Development to the deutoval stage (eclosion) takes
approximately 20-25 days in the laboratory at 250 C and 70-100% RH (see also Anderson 1978, Riddle and Markezich 1981). The deutoval stage lasts approximately 6 days before the molt to the pigmented first instar spiderlings. These spiderlings remain in the cocoon until late April or early May of the following year when they emerge.
Argiope aurantia builds its webs primarily in old field
vegetation, or in the vegetation at field edges. Its cocoons are large brown spheres, 1-2 cm in diameter. Females deposit their cocoons singly, away from the web site, suspended in the vegetation
0.5 to 4.0 m above the ground by a cloud of fine support lines originating from "minute conical or pyramidal deltas" on the cocoon surface (McCook 1890; pg. 75) (Fig. 2-2a). The cocoons are multi-layered (Fig. 2-2b). The outer covering is usually composed of a thin, stiff, parchmentlike material with a glazing applied to it (almost like fiberglass), although it may occasionally be made of
Fig. 2-2. The cocoon of Argiope aurantia suspended in the vegetation (A), and in cut away view (B) showing the position of the egg mass (em), silk cap (sc), silken cone and cord (scd), and flocculent silk layer (fsl) within the cocoon cover (cv).
a soft, thick, feltlike material. The agglutinated egg mass of 800-2000 eggs (R = 978.7 eggs, SD = 419.2, n = 40) is suspended within the cocoon from a thick silk cup, which in turn is attached to a cone and cord of strong silk which fills the stalk of the cocoon. The egg mass is covered by a fine membranous layer of silk, and is separated from the cover by a thick flocculent silk layer (see McCook 1890, pp. 79-80 for a complete description of the internal structure of the cocoon).
The eggs and spiderlings in A. aurantia cocoons are attacked by a number of general and specialized predators including the hymenopterans Tromatobia ovivora rufopectus (Cresson) (Ichneumonidae) (Cresson 1870, Keobele 1887, McCook 1890, Howard 1892, Champlain 1922, Enders 1974, Tolbert 1976), Pimpla aquilonia aquilonia (Cresson) (Ichneumonidae) (Davidson 1896), Chrysocharis banksii and Chrysocharis pikei (Entodontimidae) (McCook 1890), and Pediobius wilderi (Howard) (Entodontimidae) (McCook 1890) [this is probably a hyperparasite attacking T. ovivora rufopectus]; the dipterans Pseudogaurax signata (Loew) (Chloropidae) (Coquillet 1898), Pseudogaurax anchora (Loew) (Kaston and Jenks 1937, Eason et al. 1967), and Megaselia sp. (Phoridae) (Kaston and Jenks 1937); the neuropteran Mantispa viridis Walker (Mantispidae) (Enders 1974, Tolbert 1976); the coleopteran Chauliognathus sp. (Cantharidae) (Enders 1974); salticid spiders (Salticidae) (Enders 1974); and birds (Enders 1974, Tolbert 1976).
Argiope aurantia prefers vegetation along water courses in
Florida (Levi 1968), and is common in roadside hedgerows bordering
drainage ditches. In these ha-,bitats, the relative humidity is usually quite high (70-90% RH) in late August-September (the egg-laying and molting period), although exposure to high levels of insolation may cause periodic fluctuations of the RH over a wide range. By late fall, the RH may drop to 30-40%. The temperatures may be buffered somewhat by the vegetation in the summer, but are usually relatively close to ambient, although they may be quite high (35-400 C) in southern exposures. From fall on, the temperatures in the habitat follow ambient air temperature closely.
THE "INSULATION" LAYER IN THE COCOONS OF ARGIOPE AURANTIA
All spiders place their eggs in some form of silken cocoon
(Turnbull 1973). These structures must provide protection for the eggs, create a proper microclimate for embryonic development, hatching, and subsequent molting, and provide a safe retreat for the spiderlings to overwinter until emergence in the spring. Because of the lengthy development and overwintering period of some spiders (Anderson 1978), many cocoons are exposed to severe temperature extremes. Fluctuations in cocoon temperature have been shown to have a direct effect on the development and survival of spider eggs (Norgaard 1956, Schaefer 1976) and spiderlings (Norgaard 1956). Several authors have observed spider behaviors which presumably modify the thermal environment of the cocoon. These include burying the cocoon (Levi and Levi 1969), shuttling the cocoon between different microclimates (Norgaard 1951, Humphreys 1974), and placing it in protected microclimates. Others have suggested that a layer of silk within some cocoons functions as insulation, modifying the thermal environment experienced by the eggs or spiderlings (McCook 1890, Kaston 1948, Turnbull 1973, Gertsch 1979). Schaefer (1976) reported that the cocoons of Floronia bucculenta (Clerck) (Linyphiidae) are capable of protecting the eggs from daily
fluctuations in ambient temperature. However, Austin and Anderson (1978) found no significant differences in the hatching success of spider egg masses, with or without cocoons, exposed to a range of constant temperatures. They concluded that the flocculent silk cocoons of Nephila edulis (Koch) (Araneidae) did not function to protect the eggs from temperature extremes.
The cocoon of Argiope aurantia Lucas (Araneidae) is a
multilayered structure, with a flocculent layer of silk located between the eggs or spiderlings and the cocoon cover (see Fig. 2-2b). In late August-September, the cocoons are usually placed in the upper strata of the vegetation where they may be exposed to direct sunlight, wind, and the night sky, all of which may alter the internal temperature of the cocoon throughout the development and overwintering period. Here, I examine the the role of the cocoon, and in particular the effectiveness of the flocculent silk layer within A. aurantia cocoons, in buffering temperature extremes. I also examine the role that cocoon positioning in the habitat plays as an adjunct method of temperature regulation.
Materials and Methods
I used three types of cocoons: whole, nonparasitized cocoons containing spiderlings, and two types of modified cocoons, "insulated" and "non-insulated". The latter two types were created by halving whole cocoons 'longitudinally with a razor blade and removing either the eggs alone, or the eggs and the flocculent silk layer. The halves of these modified cocoons were rejoined with
clear nail polish. These whole and modified cocoons were mounted on thermal probes (YSI #513, time constant of 0.02 s), with the tip of the probe in the approximate position of the egg or spiderling mass.
Cocoons were allowed to equilibrate to room temperature (T a 260 C) in a large cardboard enclosure, and then placed in a styrofoam cooler packed with salted ice. The internal temperature of the cocoon in the cooler was recorded every 30 s until it reached the ambient temperature of the cooler (T a = 8-9 0 C). The cooling trials were performed in a dark room to avoid radiant loading from the overhead lights, and with the air conditioning off to minimize the effects of convective cooling from air movement. The walls of the cardboard enclosure and the cooler were monitored with banjo probes (YSI #427, time constant of 1.10 s) taped to the inner surfaces. The temperatures of these surfaces matched the ambient air temperatures within each container. Only data from trials where the ambient temperature in the cooler varied by no more than
0-50C were used for the calculation of the cooling curves.
In the field, the changes in internal temperatures of whole and modified cocoons (using YSI #513 probes) in an artificial closed habitat (1 x I x 1 m cardboard enclosure) were recorded at one min intervals for 50 min over an approximate 2.5 0 C drop in ambient air temperature. On a different day, changes in internal temperatures (YSI #513) of whole and modified cocoons exposed to 10 min radiation loads, and to 2 min loads followed by 8 min of cooling were also recorded. This experiment was conducted between 1100 and 1300 h when the sun was at its zenith. During this time, ambient
temperature was relatively constant (12.9-13.60 C), and there was no wind. All the cocoons used in this experiment were approximately the same size, color, and mass. On a third day, the hourly changes in ambient air temperature (YSI #405 probe, time constant of 0.60 s) and in the internal (YSI #513) and surface temperatures (Ysi #427) of two cocoons in two different habitats (closed canopy under trees, open canopy in an old field) were recorded for 24 h. Simultaneous measurements of the sky and vegetation temperatures or enclosure wall temperatures were made with a Stoll-Hardy HL4 radiometer during all of the field experiments to measure the radiant heat load in the respective test habitats.
There were no significant differences between the slopes of the cooling curves for the modified cocoons (Fig. 3-1). Both types of modified cocoons showed cooling rates of approximately 11 0 C/ min. In contrast, the whole cocoons cooled significantly more slowly than the modified cocoons (F = 356-71, p < 0.001); the presence of spiderlings (total cocoon weights 0.63 to 0.93 g; spiderling weights
0.56 to 0.83 g) decreasing the cooling rate to approximately 2.0 0 C/ min (Fig. 3-1).
Outdoors, in the artificial closed habitat, there were no
significant differences between the rates of cooling for the whole cocoon (total weight 0.809 g; spiderling weight 0.682 g), and the modified cocoons with and without the flocculent silk layer.
0 I 2 3 4 5
Fig. 3-1. Cooling rates in still air in the laboratory for whole and modified Argiope aurantia cocoons. The whole cocoons (squares) contained spiderlings and the flocculent silk layer; the modified cocoons contained only the flocculent silk layer (open circles), or were empty (closed circles). Th cooling curve for the whole cocoons is Y = -0.208X + 2.73 (r = 0.95, n = 6); for te modified cocoons with the flocculent layer Y = -0.824X + 2. 5 (r = 0.96, n = 10); for the empty cocoons Y = -0.810X + 2.66 (r = 0.90, n = 8). During cooling T a= 8-9 C. The data points represent the range for each set of experimental trials.
The internal temperatures of all three cocoons closely followed the change in ambient temperature (Fig. 3-2).
Among the cocoons exposed to radiation loads for 10 min, there was no significant difference between the rates of heating for the modified cocoon with the flocculent silk layer and the cocoon without it (Fig. 3-3). Both cocoons heated rapidly to an equilibrium temperature of 22.5 0 C in approximately 5 min. The rate of heating for the whole cocoons (total weights 0.9735 g and 0.9025 g; spiderling weights 0.7500 g and 0.7400 g, respectively) was significantly different from that for the modified cocoons (F 26.11, p < 0.0001; calculated for the first 4 min). These cocoons heated to approximately 19.0 0 C in 5 min; an internal temperature some 3.5 0 C cooler. Beyond this point, they heated at a slower rate. An extension of this second, flatter heating curve suggests that the whole cocoons would not reach the equilibrium temperature of the modified cocoons until another 15 min had elapsed.
The whole and modified cocoons exposed to the 2 min loads
heated at the same rates as their counterparts in the 10 min group (Fig. 3-3). The rates for the two groups of cocoons were significantly different from one another as well (F = 7.490, p
0.034; calculated on the first 2 min). When the radiation load was removed, the modified cocoons rapidly cooled to ambient air temperature. The whole cocoons also cooled upon removal of the load, but at a much slower rate (Fig. 3-3).
The absolute internal temperature achieved by whole cocoons in the field depended on their location in the habitat. While the
C 0 0
0 10 20 30 40 50
Fig. 3-2. Cooling rates outdoors for whole and modified Argiope aurantia cocoons in an artificial closed habitat (1 x 1 x 1 m cardboard enclosure) over a 50 min period during an approximate
2.50 C change in ambient temperature. The whole cocoon (squares) contained spiderlings and the flocculent silk layer; the modified cocoons contained only the flocculent layer (open circles), or were empty (closed circles). The cooling curve for the whole cocoon is Y = o.067X 1.57 (r = 0.99, n = 1); for t e modified cocoon with the flocculent layer Y = 0.066X 21.61 (r = 0.99, n = 1); for the empty cocoon Y = 0.065X 1.50 (r = 0.98, n = 1). The curve for2 the change in ambient temperature (stars) is Y = 0.060X 1.90 (r = 0.99). The temperatures of the top and sides of the enclosure during this period were 40 C and 50 C, respectively.
0 @ 0 0
22.0 0 o
0 1 2 3 4 5 6 7 8 9 10
Fig. 3-3. Heating rates in still air outdoors for whole and modified Argiope aurantia cocoons exposed to radiant loads of 2 and 10 min duration. The whole cocoons (open squares, 2 min; closed squares, 10 min) contained spiderlings and the flocculent silk layer; the modified cocoons contained only the flocculent layer (stars, 2 min; open circles, 10 min), or were empty (closed circles, 10 min).2 The heating curve for the whole cocoons is Y = 0.50X + 13.44 (r = 0.98, n = 2); for the modified cocoons with the flocculent layer Y = 1.01X + 14.29 (r = 0.93, n = 1); for the empty cocoons Y = 0.96X + 14.47 (r2 = 0.94, n = 1). The ambient air temperature during the experiment ranged from 12.90 C to 13.70 C. The vegetation and sky temperatures during the experiment were +15 C and -20 C, respectively.
cocoon (total weight 0.820 g; spiderling weight 0.724 g) in the sheltered site (under overhanging vegetation) attained highs and lows close to the ambient air temperature, the internal temperature of the cocoon (total weight 0.991 g; spiderling weight 0.842 g) in the exposed site (open field) fluctuated widely, attaining much higher and lower values (Fig. 3-4). The surface temperatures of the cocoons equaled the ambient temperature in the shade (T surf = T a ) and were greater than both ambient and the internal temperature of the cocoons in the sun (T < T > M
a surf c
Under laboratory conditions, the internal temperatures of whole and modified A. aurantia cocoons exposed to sudden drops in ambient temperature (from 260 C to 8-90 C) do not change instantaneously (see Fig. 3-1), indicating that some part of the cocoon acts as insulation. However, under these conditions the cooling rates for cocoons with and without the flocculent silk layer are identical, suggesting that it is the cover of the cocoon, and not the flocculent layer, that creates the dead air space that acts as insulation (Kaufman et al. 1982). This explains why Austin and Anderson (1978) found that the coverless, flocculent cocoons of N. edulis offered no protection to their egg masses; these cocoons have little or no dead air space. The whole cocoons also cooled four times more slowly than the modified cocoons (see Fig. 3-1). Presumably, this additional thermal inertia is due to the mass of the spiderlings, which are approximately 70% water (Anderson 1978).
[=Direct Sunlight MShade or Darkness Habitat/ OPen
4 o Open Habitat
14 / *Closed Habitat
1200 1800 2400 0600 1200
TIME OF DAY (h
Fig. 3-4. The relationship of internal cocoon temperature to habitat position for two Argiope aurantia cocoons. The solid lines
( -) are ambient temperatures, and the dashed lines ( --- ) are internal cocoon temperatures. Over 24 h, the cocoon in the open habitat (old field) was exposed to extended periods of direct sunlight, and faced greater reflected radiation from the vegetation (320 C to -150 C), and colder skyj temperatures (20 C to -39 C) than the cocoon in the closed habitat (old field edge under trees) faced from the vegetation (25 0 C to -10O0 C), or from the sky (250 C to -17 0 C).
The cocoon, in combination with the egg or spiderling mass,
acts as a buffer against rapid temperature changes (Fig. 3-1). Such rapid changes could result from exposure to the night sky immediately after sundown, particularly in the winter (see Fig. 3-4). However, most A. aurantia cocoons are placed within the vegetation. In these protected locations, radiant heat loss to the cold night sky is reduced by the warmer overhead vegetation. Under these conditions, cooling is slower and follows the change in ambient air temperature (see Fig. 3-2). Cold hardiness (Schaefer 1977, Riddle 1981) resulting from physiological solutions such as freezing point depression (Kirchner and Kestler 1969) probably protects the eggs or spiderlings from extremely cold temperatures.
Cocoons in open habitats but under a layer of vegetation, or in closed habitats such as woods may still be exposed to periodic "1sunflecks" as the sun moves across the sky. These short lived, but potentially intense, radiant loads could present a serious thermal challenge to the cocoon. The field experiments indicate that the cocoon and the mass of the eggs or spiderlings provide a significant buffer against short-term loads (see Fig. 3-3), extending the heating time to equilibrium by approximately 15 min. However, the level of protection provided by a cocoon is directly related to the ambient air temperature, the duration of the load, and the elapsed time between consecutive loads. The radiant loading experiments were conducted in the winter, when the ambient air temperature was relatively low (12-l40 C). Even with constant exposure to insolation, low ambient temperatures such as these would result in a
low equilibrium temperature (Heath 1964) that is probably below the danger level for the spiderlings in the cocoon. However, during the early part of the reproductive season, when the ambient temperature is relatively high, radiant loads may represent a more serious problem. Under these conditions, a load of long duration could raise the equilibrium temperature of the cocoon past the safe limit for the eggs or spiderlings. Loads of short duration also pose a problem if they occur in rapid enough succession. Under these conditions, the internal temperature of the cocoon would "step" up with each successive load due to the slow rate of heat loss from the egg or spiderling mass (see Fig. 3-3), finally rising above the lethal limit.
Other behaviors may play a role in insuring that the cocoon is not exposed to lethal thermal loads. Argiope aurantia prefers to oviposit beneath a layer of vegetation or among the leaves of broad-leafed plants. Such sites would limit the number and length of insolation bouts the cocoon encounters, particularly during the late summer when oviposition occurs. The comparison of internal temperatures from cocoons in open and closed habitats supports maternal positioning of the cocoon as an important control measure (see Fig. 3-4). Positioning the cocoon in a closed site rather than in an open site resulted in a 10 0 C difference between the highest internal temperature achieved (because of radiation loading), and a 40 C difference in the lowest temperature achieved (because of exposure to cold night sky temperatures). Although spiderlings can t Tolerate high temperatures (Tolbert 19719), proper site choice could
mean a few degrees difference in internal cocoon temperature. This difference may be important when the ambient temperature approaches the upper critical temperature of the eggs or spiderlings.
The limited ability of the relatively large A. aurantia cocoons to resist temperature fluctuations suggests that cocoons with smaller masses of eggs or spiderlings will have more difficulty in controlling temperature extremes. The non-functional role of the flocculent silk layer as insulation further suggests that cocoons may function to regulate other factors in the environment. Due to the small size of the eggs and spiderlings, and their consequent high surface area to volume ratio, it seems likely that cocoons may play a role in limiting water loss. This role is explored in Chapter IV.
THE ROLE OF SPIDER EGGS AND COCOONS IN RESISTING WATER LOSS
All spiders enclose their eggs in some form of silken cocoon
(Turnbull 1973). Within these structures the eggs undergo embryonic development, hatch to the deutova stage, and molt to the spiderling stage. The spiderlings then spend from 6 to 10 months in the cocoons until emergence in the spring. Dessication may be a problem for these developmental stages. Many insects use cocoon-like structures to limit dessication (Chapman 1967, Chauvin et al. 1979), and this is commonly assumed to be the function of cocoons as well (Bristowe 1941, pg. 421; Foelix 1982, pg. 200). Few data exist, however, to support this view. Schaefer (1976) showed that the parchmentlike cocoon of Floronia bucculenta (Clerck) (Linyphiidae) increased the survival time of post-diapause eggs exposed to a RH of 32% (at 50 C) from 37 to 68 days. Austin (1984) has demonstrated that the silk nests containing eggs of Clubiona robusta L. Koch (Clubionidae) increases humidity levels. The casual observations of McEwon (1963) also suggest that the flocculent cocoon of an Australian Nephila sp. (Araneidae) provides desiccation protection for the spiderlings. Austin and Anderson (1978), however, have showed that the flocculent silk cocoon of the congener N. edulis
(Koch) has no effect on the hatching success of spider egg masses at different controlled humidities.
Some spider eggs are dessication-resistant as well (Schaefer 1976). This resistance may result from a layer of spherical granules on the chorion surface (Austin and Anderson 1978, Grim and Slobodchikoff 1978, 1982, Humphreys 1983). Overwintering spiderlings within the cocoon can also attenuate water loss by reducing their rates of metabolism (Schaefer 1976, Anderson 1978), or by metabolizing lipids. Presumably, the cuticle of spiderlings also impedes water loss (Lees 1947, Davies and Edney 1952).
I investigate here the role of the cocoon in controlling water loss from the egg and spiderling masses of Mecynogea lemniscata (Walckenaer) and Argiope aurantia Lucas (Araneidae) during egg development, deutoval molting, and spiderling overwintering. I also examine the effect of clutch size, and egg morphology (Austin and Anderson 1978, Grim and Slobodchikoff 1978, 1982) on water loss.
Materials and Methods
The Cocoons and Their Habitats
The cocoons of M. lemniscata and A. aurantia differ in size, density of the cover, and internal construction. The preferred habitats of the two spiders also differ, the habitat of M. lemniscata being more consistently humid than that of A. aurantia (see Chapter II).
The cocoons and egg masses of A. aurantia were obtained from spiders collected in the field and maintained at 24-250C and 60-70% RH in 500 ml jars in the laboratory. The M. lemniscata cocoons and egg masses were obtained by daily censusing 100-150 marked web-sites. The cocoons used in the experiments on spiderling survival were collected in the field in late August (for M. lemniscata), and during September-October (for A. aurantia). Only undamaged, nonparasitized cocoons were utilized. The experimental spiderling masses were obtained by removing the cocoon cover and as much of the internal flocculent layer of silk as possible.
Egg Surface Morphology
The surfaces of M. lemniscata and A. aurantia eggs were
photographed with a Hitachi S-415A SEM. They were prepared by attaching them to SEM stubs with double-sided tape, and coating with 75 A of gold. The densities of the spherical granules (spheres) on egg surfaces were measured by placing a grid system (4 x 4 lines) over the electronmicrographs of five eggs of each species and counting all spheres within 2 randomly chosen grid-squares. Since the spheres on both species' eggs are distributed essentially as monolayers (Humphreys 1983), densities were expressed as the mean
number of spheres/ 100 11m Sphere diameters were determined directly by measuring all the spheres within the 10 grid-squares.
Effect on Dessication
The effects of the sphere layer, clutch size, and the cocoon on hatching success, molting success, and spiderling survival were determined by rearing intact egg and spiderling masses without cocoons (and appropriate controls) at 25-260 C in one liter jars controlled at four different RH levels (0%, 33%, 66%, and 100%). The controlled humidities were achieved by using dry anhydrous KOH pellets (0% RH), and saturated salt solutions of 300+ g CaC12/ 100ml H 20 (33% RH), 100 g NaNO2/ 100 ml H20 (66% RH), and 15 g K2 s4/ 100 ml H20 (100% RH) (Winston and Bates 1960, Peterson 1964). The pellets and solutions controlling the humidities were changed every two weeks. Since the egg masses of A. aurantia contain more eggs than those of M. lemniscata (approximately 970 to 20, respectively), artificially reduced egg masses of 15-25 A. aurantia eggs were also reared under these conditions to control for the possible effects of egg mass size on hatching success.
The effect of the cocoon was also tested under field conditions by placing intact egg and spiderling masses (and appropriate controls) in individual screen-covered vials within styrofoam-cup housings. These housings protected the replicates from precipitation, predators, and parasites, and were hung in the shade in locations where females had previously constructed cocoons.
Hatching and molting success was calculated after 30 days. Hatching success was the percentage of total eggs in which the chorion ruptured and a deutova successfully emerged. Molting
success was the number of these deutova which successfully molted to the first instar spiderling stage. Spiderling survival was determined using separate masses, and was calculated after the experiments had run for 90 days.
The Sphere Layers
The spheres associated with the eggs of M. lemniscata and
A. aurantia are shown in Fig. 4-1. For both, the sphere diameter ranged from 0.40 to 7.20 pm. The mean diameter in M. lemniscata eggs was not significantly different from that of A. aurantia eggs (Table 4-1). However, the mean sphere density (spheres/ 100 PM2 ) of M. lemniscata eggs was significantly greater (t = 2.11, df = 20, p < 0.05) than that of A. aurantia eggs. The Effect of Clutch Size and the Sphere Layer
No significant differences in hatching success at 100% RH were found between intact and reduced A. aurantia egg masses without cocoons at each humidity (Table 4-2). Reduction of the egg mass, however, did have a significant effect at 66%, 33%, and 0% RH, indicating that the size of the egg mass of this spider is important to dessication control. There were no significant differences at oo%, 66%, and 33% RH when the hatching success of intact M. lemniscata and A. aurantia egg masses without cocoons was
Fig. 4-1. The layer of spherical mucoid granules on the chorionic surface of a Mecynogea lemniscata egg (A), and an Argiope aurantia egg (B). The scale line in each case equals 15 um. Both eggs were plated with 75 A of gold and observed using secondary electrons.
Table 4-1. The distributions of sphere sizes found on the chorionic surfaces of Mecynogea lemniscata and Argiope aurantia eggs. The mean sphere sizes and mean sphere densities ( 1 SD; n in parentheses) are given below the frequency column for each species.
Diam. (jm) M. lemniscata A. aurantia
0.40 0.79 9 2
0.80 1.19 57 17
1.20 1.59 104 117
1.60 1.99 47 26
2.00 2.39 12 2
2.40 2.79 3 0
2.80 3.19 1 0
3.20 3.59 2 0
3.60 3.99 0 0
4.00 4.39 2 0
4.40 4.79 0 1
4.80 5.19 1 2
5.20 5.59 1 0
5.60 5.99 1 0
6.oo00 6.39 1 0
6.40 6.79 0 0
6.80 7.19 1 1
Mean Sphere X = 1.56 0.79 X = 1.52 0.70
Size (242) (168)
Mean Sphere X = 21.60 5.46 R = 17.00 4.22
Density (10) (10)
Table 4-2. Mean percentages of eggs successfully hatched ( 1 SD; n in parentheses) at the four experimental humidities for intact Mecynogea lemniscata and Argiope aurantia egg masses with and without cocoons, and for reduced A. aurantia egg masses without cocoons. Pair-wise comparisons between treatments are by Mann-Whitney U Tests; pair-wise comparisons between humidities within treatments are by nonparametric multiple comparison tests (Zar 1984).
% Hatching Success
M. lemniscata A. aurantia
Relative No Cocoon/ No Cocoon/ No Cocoon/
Humidity Cocoon Intact Reduced Intact Cocoon
94.9 98.3 96.2 99.7
100% 13.5% 3.1% 4.0% 0.5% 100%
(10) (8) (5) (8) (8)
95.4 97.4 49.2 94.3 90.0
66% 14.6% 4.6% 36.8% ** 11.3% 24.8%
(1O) (10) (8) (8) (7)
96.9 88.7 28.7 87.9 96.6
33% 7.9% 15.5% 32.3% ** 7.0% ** 5.8%
(10) (10) (10) (9) (7)
3.4 3.6 6.7 70.4 88.2
0% 7.3% 8.3% 5.7% 22.8% 17.5%
(10) (10) (O) (8) (8)
KruskallWallis H 29.43 25.80 17.70 21.70 8.29
"p" < 0.001 < 0.001 < 0.001 < 0.001 < 0.05
p < 0.05; ** p < 0.01; *** p < 0.001)
compared at each humidity. There was a significant difference at 0% RH, underscoring t.he advantage of a large egg mass.
A different pattern of hatching success was revealed when
intact M. lemniscata egg masses without cocoons were compared to reduced A. aurantia egg masses without cocoons at each humidity (Table 4-2). The significant advantage previously demonstrated by the whole A. aurantia egg masses at 0% RH disappeared, and hatching success dropped significantly below that of M. lemniscata at 66% and 33% RH, presumably because of differences in the densities of the the sphere layers covering the eggs of each species. There were no significant differences at 100% RH, where dessication is obviously not a problem.
The Effects of the Cocoons
Cocoon removal had no effect on the hatching success of
M. lemniscata at any experimental humidity (Table 4-2). Below 33% RH, there were significant decreases in hatching success for egg masses with and without cocoons. Molting success showed no significant reduction with cocoon removal at any experimental humidity (Table 4-3). Since few eggs hatched at 0% RH, however, the percentage of total eggs which ultimately molted was small. The cocoon did have a significant effect on spiderling survival. At all the experimental humidities survival was greater with a cocoon (Table 4-3). The field results support these findings (Table 4-4); only spiderling survival was significantly affected by removal of the cocoon.
Table 4-3. Mean percentages for molting success and spiderling survival ( 1 SD; n in parentheses) at the four experimental humidities for Mecynogea lemniscata egg and spiderling masses with and without cocoons. Pair-wise comparisons between treatments are by Mann-Whitney U Tests; pair-wise comparisons between humidities within treatments are by multiple range tests (Zar 1984).
% Molting % Spiderling
Humidity Cocoon No Cocoon Cocoon No Cocoon
100% 100% 2.5% 100% *** 25.1%
(8) (8) (13) (6)
66% 100% 100% 29.8% ** 28.8%
(10) (10) (13) (14)
33% 100% 100% 38.1% 2.7%
(10) (10) (11) (14)
0% 100% 100% 13.7% ** 0%
(10) (10) (14) (14)
KruskallWallis H 0.00 0.00 35.44 38.22
p 1.00 1.00 < 0.001 < 0.001
p < 0.05; ** p < 0.01; *** p < 0.001)
Table 4-4. Mean pecentages ( i1 SD; n in parentheses) for hatching and molting success and spiderling survival in the field for the spiders Mecynogea lemniscata (ML) and Argiope aurantia (AA). Conditions in the field (60-80% RH and 24-32uC) approximated the laboratory conditions at 66% RH. The data are presented as percentages, but were arc-sine transformed for the statistical analyses (Sokal and Rohlf 1969).
% Hatching % Molting % Spiderling
Success Success Survival
Spider Cocoon No Cocoon Cocoon No Cocoon Cocoon No Cocoon
99.6 99.3 98.2 50.7
ML 1.9% 2.7% 100% 100% 12.7% 27.0%
(19) (17) (19) (17) (21) (10)
[-- *** ICocoon No Cocoon Cocoon No Cocoon Cocoon No Cocoon
99.8 99.8 93.3 91.8
AA 0.7% 0.4% 100% 100% 0.9% 2.9%
(10) (10) (10) (10) (10) (10)
*** F-test: F = 15.12, df = 29, p = 0.005.
The results for A. aurantia differed from those for
M. leinniscata. In the laboratory, cocoon removal had no significant effect on watching success at 100%, 66%0, or 0%Y RH, although it did significantly affect success at 33% RH (Table 4-2). Cocoon removal had no significant effect on molting success at any experimental humidity. However, at 33% and 0% RH the variances in the non-cocoon cells were mu~ch larger, suggesting that an effect may have been
difficult to detect. Molting success fell off significantly below 66% RH with and without a cocoon (Table 4-5). The absence of a cocoon also made no difference in spiderling survival at either 100% RH or 66% RH (Table 4-5). However, below 100% RH spiderling survival significantly declined with and without cocoons, and below 66% RH all the spiderlings died after 90 days regardless of the treatment. The results from the field experiments support the laboratory results at 100% and 66% RH (Table 4-4). Under field conditions, cocoon removal had no significant effect on egg hatching success, molting success, or spiderling survival.
The results indicate that the enclosed cocoons of M. lemniscata and A. aurantia do not affect water Loss from the egg stage. These findings are paradoxical, since they support the findings of Austin and Anderson (1978) for a flocculent cocoon, but do not agree with Schaefer's (1976) findings for a covered cocoon. This paradox may be explained by the amount of time the eggs of these spiders spend in the cocoon. The developmental times of M. lemniscata
Table 4-5. Mean percentages for Molting Success and Spiderling Survival ( 1 SD; n in parentheses) at the four experimental humidities for Argiope aurantia egg and spiderling masses with and without cocoons. Pair-wise comparisons between treatments are by Mann-Whitney U tests; pair-wise comparisons between humidities within treatments are by nonparametric multiple comparson tests (Zar 1984).
% Molting % Spiderling
Humidity Cocoon No Cocoon Cocoon No Cocoon
100% 100% 100% 1.5% 8.2%
(8) (8) (7) (5)
99.1 61.5 63.5
66% 100% 2.0% 27.5% 19.6%
(7) (8) (6) (8)
33% 4.2% 34.8$ 0% 0%
(7) (9) (8) (8)
0% 19.0% 48.o% 0% 0%
(8) (8) (9) (9)
KruskallWallis H 22.40 11.09 27.18 27.99
itp"i < 0.001 < 0.01 < 0.001 < 0.001
p < 0.05; ** p < 0.01; *** p < 0.001)
and A. aurantia are relatively short (15-20 days and 20-30 days, respectively), and even at low humidities this may not be long enough to demonstrate the water retaining qualities of the cocoon. In contrast, the eggs of F. bucculenta remain within the cocoon in winter diapause for approximately 180 days. Since the eggs are not particularly drought resistant (Schaefer 1976), they probably lose
water during this time. The significant effect of the cocoon on egg survival, that Schaefer demonstrated, can probably be attributed to his use of post-diapause eggs, from which enough water had already been lost to make the effect of cocoon removal apparent.
The significant difference in hatching success between intact
and reduced A. aurantia egg masses without cocoons demonstrates that clutch size can affect hatching success as well. Spider egg masses are usually agglutinated, and as the mass gets larger, its surface area to volume ratio declines as a greater number of eggs are positioned completely within the mass. Presumably, this difference also accounts for the difference in hatching success between intact M. lemniscata and A. aurantia egg masses without cocoons at 05% El (see Table 4-2).
The ability of intact M. lemniscata egg masses without cocoons to maintain hatching success rates equal to those of the much larger A. aurantia egg masses at humidities above 0% RH indicates that some factor other than clutch size is also operating to control water loss (see Table 4-2). A number of authors have suggested that a layer of spherical mucoid granules (Austin and Anderson 1978) on the chorions of spider eggs funct ions to control water loss by reducing
the free surface area through which water can pass (Austin and Anderson 1978, Grim and Slobodchikoff 1978, 1982, Humphreys 1983). The eggs of M. lemniscata have a significantly denser coating of these spheres than do the eggs of A. aurantia (see Table 4-1). Although a direct comparison of the eggs of each of these species with and without spheres is difficult (due to problems in stripping the sphere layer from the eggs and still maintaining viability), the comparison of intact M. lemniscata egg masses with reduced A. aurantia egg masses strongly suggests that the sphere layer does reduce water loss (Table 4-2). However, the drop in hatching success at 0% RH indicates that this barrier has limits in its ability to control water loss at very low humidities.
Molting is a dangerous period of time for deutova with their high surface area to volume ratio (Horner and Starks 1972). At molting, with the breaking and subsequent shedding of the egg chorion, dessication protection shifts back to the level of the cocoon. In spite of this, removal of the cocoons of M. lemniscata and A. aurantia demonstrate no effect on molting success (see Tables 4-3 and 4-5). However, the variances among the no-cocoon treatments of A. aurantia at 33% and 0% RH are high, which may make detection of the cocoon's effect on molting success difficult. Presumably, the main factor affecting molting success for these spiders is the ambient RH at the oviposition site, although the greater molting success of M. lemniscata at low humidities suggests that there may be morphological or physiological differences in the abilities of the deutova of these species to reduce water loss.
The cocoon of A. aurantia also appears to have little effect on spiderling survival. The lack of a cocoon effect on hatching and molting success, and the significant drops in molting success and spiderling survival below 66% RH suggest that other considerations are more important to reducing desiccation. Field observations of damaged A. aurantia cocoons suggest that the large clutch size of this species may protect the eggs from dessication [which may also explain McEwon's (1963) observation for the Nephila sp.1. However, the RH within the habitat or microhabitat used for oviposition is probably most critical for A. aurantia. Indeed, Levi (1968) notes that of the Argiope species in Florida, A. aurantia prefers moist habitats such as pond edges and stream borders, and is one of the first Argiope species to disappear during drought years.
Removal of the cocoon does have a significant effect on the
spiderling survival of M. lemniscata at all experimental humidities (see Table 4-1), and in the field under ambient conditions (see Table 4-3). This spider is the first of the orb-weavers to become active in the spring, emerging in late March. Growth is rapid. Reproduction begins by late June and the adults die by early to mid-August. This early emergence and rapid disappearance is apparently a means of taking advantage of the abundant prey in the habitat, while avoiding competition with other orb-weavers whose populations increase rapidly by August (Anderson 1978). The spiderlings then spend approximately 290 days in the cocoon until they emerge the following March; 2 to 3 months longer than most other orb-weavers (Anderson 1978). The cocoon is apparently part of
a suite of adaptations for controlling water loss [including depressed metabolic rates (Schaefer 1976, Anderson 1978), and lipid-rich eggs (Anderson 1978)], and fits into the foraging/ reproductive strategy of this spider by providing a desiccation-resistant refuge in which the spiderlings spend these additional months overwintering.
A number of the egg and spiderling replicates of M. lemniscata and A. aurantia held at 100% RH in the laboratory were attacked and destroyed by fungus. Although these replicates were removed prior to data analysis, in some cases there was still a significant reduction in survival without cocoons at 100% RH (see Table 4-3 for M. lemniscata spiderlings), a humidity where survival should be unaffected by desiccation. This suggests that the covers of cocoons may be related to resisting fungal attack (Christenson and Wenzl 1980), possibly by limiting the amount of water which can enter the cocoon. Chapter V addresses this possibility, and considers the cocoon as a barrier to fungal attack.
THE ROLE OF THE COCOON IN LIMITING EXCESS WATER AND FUNGAL ATTACK
Although desiccation is a problem for developing spider eggs and overwintering spiders, too much water may also cause adverse effects. Water, entering the cocoon, could dissolve materials from the egg surface and lower the resistance of the eggs to disease (Austin and Anderson 1978). It could also drown the contents of the cocoon (Schaefer 1976, Reichert 1981). Water may also introduce fungal spores that are pathogenic to the eggs or spiderlings. Christenson and Wenzl (1980) suggested that the cocoon of Nephila clavipes (Linneaus) (Araneidae) protects against fungal attack by reducing the chance of spores gaining access to the egg mass. A role in protecting the eggs and spiderlings from fungi seems reasonable, since both spiders (Bijl and Paul 1922, Nellist 1965) and their eggs (Seligy 1971, Christenson and Wenzl 1980) are attacked by fungus.
Both Mecynogea lemniscata and Argiope aurantia utilize a covered cocoon (see Figs. 2-1 and 2-2), in contrast to the flocculent silk type used by many other orb-weavers (McCook 1890, Scheffer 1905, Kaston 1948). Both spiders also utilize habitats where high humidity during the egg laying period may promote fungal attack (see Chapter II). In this chapter, I examine the roles of
the suspension systems, the internal flocculent layers, and the coverings of the cocoons of these spiders in protecting the eggs and spiderlings from excess water, drowning, and fungal attack.
Materials and Methods
The abilities of the cocoon suspension system, the cocoon
cover, and the internal flocculent silk layer to limit the access of water to, or into the cocoon were examined in the laboratory using M. lemniscata or A. aurantia cocoons as appropriate. The temperature and humidity in the laboratory during all the experiments were 25 0 C and 50-60% RH, respectively.
To examine the function of cocoon suspension systems in
shedding water, M. lemniscata cocoon strings were hung between two vertical supports by their silk suspension lines (see Fig. 2-1). The angle of inclination of the suspension lines was set at 10, 20, 30, and 45 0 from the horizontal. At each of these angles, water was applied to the suspension lines with a dropper, and its course down the lines noted.
The ability of cocoons to shed water was examined by applying
1.00 ml of water with a syringe directly to hanging strings of M. lemniscata cocoons. The runoff was collected, and the amount retained by the cocoon string calculated. Both naked strings, and those covered with a layer of silk and detritus (see Fig. 2-1c) were tested in this manner. The abilities of both M. lemni4scata and
A. aurantia cocoons to shed water were also examined in the field during periods of rain.
To determine whether water actually penetrates the cocoon
cover, 10 samples of 2-3 dry M. lemniscata cocoons were submerged in distilled water for time periods of 7, 15, 30, 6o, 120, 24o, and 48o min. Upon removal from the water, they were blotted on Whatman #1 filter-paper discs for 1 min to remove surface water, and weighed on a Mettler AK 160 balance to the nearest 0.0001 gm. The 10 samples were air dried for 24 hrs between each submersion. After the last submersion bout (480 min), the 22 cocoons comprising the 10 samples were opened and examined under a microscope for the presence of water.
The volume of water absorbed by each cocoon sample was
calculated from the increased weight of the sample (1 gm water = 1 ml = 1 cc). This was normalized to ml/ cm 2 by dividing the volume of water by the combined surface area of the cocoons in each sample. A mean value was then calculated from the 10 values for each submersion time.
'"he role of the flocculent silk layer in resisting water was also tested. Here, flocculent silk from A. aurantia. cocoons was used. Uncompressed pieces of this layer were placed on glass slides and drops of distilled water were applied to the silk. The pieces were observed for approximately 10 min, and any wetting of the silk through wicking was noted.
The function of the cocoon suspension system and the cocoon cover in limiting fungal attack and drowning were investigated by modifying these components in the field, and assessing their effect on egg hatching success and spiderling survival. The cocoons used in these studies were either reared in the laboratory, or collected in the field from marked web-sites. Only nonparasitized cocoons were used.
The cocoons were assigned to the four experimental groups
listed below. The experiments using cocoons with eggs were run for 30 days; those using cocoons with spiderlings for 90 days. The cocoons not associated with apparatus were marked by plastic
flagging attached to the vegetation.
Vegetation contact. The effect of cocoon suspension systems in preventing fungal attack was examined by placing M. lemniscata cocoons in contact with the vegetation. This simulated partial collapse of the suspension system, and placed limitations on the cocoons drying normally. Placement was accomplished by cutting the cocoon free, and tying it in place to the vegetation with its support line. Access to the cocoons by terrestrial predators was controlled by applying tack-trap to the branches below the cocoons, and pruning the branches above the cocoon so they did not contact the surrounding vegetation. Argiope aurantia cocoons were not subjected to this treatment.
Ground placement. This manipulation also tested the function of the suspension system, but placed the cocoons in a wetter microhabitat where drying was more difficult and the chance of fungal attack or drowning was greater. Both M. lemniscata and A. aurantia cocoons were placed on the ground in enclosures. For A. aurantia cocoons, these were constructed of hardware cloth (1/411 mesh), and were 10 x 10 x 10 cm. in size. The enclosure for M. lemniscata cocoons was similar in design, but was constructed of aluminum window screen, and was 5 x 5 x 5 cm in size. These enclosures allowed exposure of the cocoon to the elements, protected
them from large predators (although ants could still get in), and prevented them from washing away during heavy rain storms. They were placed below positions where cocoons had previously been collected.
Cover removal. To determine the roles that the covers of
M. lemniscata and A. aurantia cocoons play in controlling fungal attack, approximately 20-30% of their covers were removed. During the cover modification, care was taken not to disturb the flocculent silk layer (see Fig. 2-2b), or damage the eggs or spiderlings. The cocoons of M. lemniscata were modified in place in their respective strings. They were treated normally by the female spiders, which remained at the web-site after the modification. The laboratory produced cocoons of A. aurantia were modified, and attached by alligator clips to the arms of a support system. The support was placed in the vegetation in areas where cocoons where already
hanging. Tack-trap was applied to the support arms to limit the access of terrestrial predators (primarily ants).
Cover removal control. This group functioned as a control for possible contaminating effects of the alligator clips. The clips were attached to the necks of cocoons hanging normally in the habitat, and the cocoons were observed for any detrimental effects.
Control. A number of cocoons were subjected to no structural intervention, and functioned as a control for the handling effects in the vegetation placement, ground placement, and cover removal experiments.
The suspension lines of M. lemniscata cocoons are relatively
poor conductors of water. At all the inclinations tested, dry lines were not easily wetted, and consequently tended to shed water. Suspension lines could be wetted by rubbing them between wet fingers, but even here water moved with difficulty. At the lower angles of inclination (10 and 200), 'water applied to the wetted line formed drops or beads and hung in place. At the higher inclinations (30 and 450), water moved down the line toward the cocoons. However, the suspension lines of M. lemniscata cocoons are made of multiple strands of silk and contain "knots" and incorporated debris. When water moving down the line got to these points it built to a drop and fell from the line.
The cocoons of M. lemniscata also shed water well. Depending on whether the cocoon string is naked, or covered with a layer of old web and detritus (see Fig. 2-1), approximately 0.85 to 0.95 ml of the 1.00 ml of water applied to the cocoon string with a syringe was shed immediately. Most of the remaining water forms a droplet at the end of the last cocoon in the string, or remains in the silk over-layer where it disappears by evaporation. Observations of M. lemniscata and A. aurantia cocoons under rainy conditions in the field yielded results similar to those observed in the laboratory. The suspension lines of both species had beads of water hanging on them and appeared not to conduct water toward the cocoons. Both cocoons also shed drops of water from the bottom of the cocoon or end of the cocoon string.
The ability of cocoons with covers to rapidly shed water
appears to be related to the slow rate at which their covers absorb water (Fig. 5-1). Over a 2-hr period, the covers of averaged sized
M. lemniscata cocoons (0.5 cm, In total surface area), absorb only 0.0014 ml of water. Even after 8 hrs of immersion, the amount of water absorbed has increased to only 0.0020 ml.
The thickness of the covers of M. lemniscata cocoons vary between 0.3 and 0.8 mm. The cover is not solid, rather it is a matrix of fibers with included air spaces, similar to fiberglass insulation. Assuming that approximately 50% of the cover is airspace that can be replaced with water, the cover should be able to hold between 0.0015 and 0.0039 m! of water. The 0.0020 ml of water absorbed by the cocoon after 8 hrs of immersion is consistent
0 4-) C) 4-) P4
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4-)4-' C 4'd ) 4-' ac0) to W 0) Cd
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with these calculations, and suggests that it is the cover alone which absorbs water before the excess is shed by the cocoon. Examination of the cocoons under the microscope supports this. None of the 22 cocoons had water inside them after 4 hrs of submersion.
The flocculent layer of A. aurantia cocoons was completely
non-wettable. Water applied to the silk simply formed beads and sat upon the silk until it disappeared by evaporation.
The incidence of fungal attack on M. lemniscata cocoons containing eggs in the control group was 7.3% (Table 5-1). Chi-square tests between this group, and the Vegetation Contact, Ground Placement, and Cover Removal groups revealed no significant differences in the number of cocoons in the egg stage attacked by fungus. This suggests that neither the cocoon cover, nor the cocoon suspension system function to protect the eggs from fungal attack. There was, however, a significant increase in egg mortality among
those cocoons placed on the ground (X = 12-39, df = 1, P < 0.005). The eggs in these cocoons were all killed, presumably by drowning. They were not covered with fungus.
The natural incidence of fungal attack on M..lemniscata cocoons in the spiderling stage was 1.0% (Table 5-2). Contact with the vegetation did not significantly increase the number of cocoons attacked by fungus in this stage. However, there were significant. increases in the number of cocoons attacked by fungus among those
placed on the ground (X = 9.93, df = 1, P < 0.005). Cocoons with
Table 5-1. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the eggs of Mecynogea leinniscata. Cocoons in the "Other" category contained eggs which were not attacked by fungi, but did not hatch due to inviability or drowning. In many cases the eggs rotted. All cocoons were collected after 30 days.
Experimental Sample Successful Fungal % Cocoons
Modification Size Hatching Attack Attacked Other
Control 171 153 12 7.3% 6
Contact 21 18 3 1)4.3% 0
Placement 19 12 2 14.3% 5
Removal 13 12 1 7.7% 0
Table 5-2. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the spiderlings of Mecynogea lemniscata. Cocoons in the "Other" category contained spiderlings which were not attacked by fungi, but did not survive due to drowning. All cocoons were collected after 90 days, except for those placed on the ground. They were collected after 45 and 90 days.
Experimental Sample Successful Fungal % cocoons Modification Size Survival Attack Attacked Other
Control 239 236 3 1.7% 0
Contact 65 63 2 3.2% 0
Placement (45) 22 17 3 17.6% 2
(90) 12 a 11 1 11.0% 0
Removal 37 32 5 15.6% 0
a A number of cocoons were attacked by ants in this group and
could not be used for the analysis. This explains the low percentage of fungal attack after the longer 90-day period.
modified covers also had significantly higher rates of fungal attack (X2 = 13.03, df = 1, p < 0.005). In addition, spiderling mortality due to other causes, probably submersion and subsequent drowning,
was significantly higher on the ground (X = 13.34, df = 1, p <
The incidence of fungal attack on the cocoons of A. aurantia was very low in both the egg stage (4.9%; Table 5-3), and the spiderling stage (0%; Table 5-4). Chi-square tests between the Control group, and the Ground Placement and Cover Removal groups revealed no significant differences in the number of cocoons attacked by fungus in either the egg stage or the spiderling stage. The two Control groups were not significantly different from each other.
The suspension systems of both M. lemniscata and A. aurantia cocoons are difficult to wet, and conduct water poorly.
Consequently, they act to discourage water from moving down the lines to the cocoons. The covers of both cocoons shed water effectively, and apparently absorb only a small amount. Presumably, the flocculent silk layers of both cocoons are also difficult to wet, and function further to limit the water from entering the cocoon. All of these structures ultimately function to keep the cocoon dry by channeling water away from it. However, the connection between this function, and the prevention of fungal attack is not clear. Modification of the cocoon cover and partial
Table 5-3. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the eggs of Argiope aurantia. Cocoon modification had no effect on egg inviability or drowning. All cocoons were collected after 30 days.
Experimental Sample Successful Fungal % Cocoons Modification Size Hatch Attack Attacked
Control 41l 39 2 4.9%
Placement 12 10 2 20.0%
Removal 13 11 2 15.4%
Table 5-4. The effect of modification of the cocoon suspension system and cover on the incidence of fungal attack on the spiderlings of Argiope aurantia. No spiderlings were killed by drowning in these experiments. All cocoons were collected after 90 days.
Experimental Sample Successful Fungal % Cocoons Modification Size Survival Attack Attacked
Control 4o 4o 0
Placement 22 22 a Q
Removal 20 20 0
a Of the 22 cocoons, 2 had small patches of fungus on the
shell, but the contents were unaffected.
collapse of the suspension system of M. lemniscata cocoons is unrelated to the incidence of fungal attack in the egg stage (see Table 5-1). Even total failure of the suspension system did not result in increased fungal attack, although cocoons placed on the ground did suffer higher egg mortality due to drowning.
The integrity of the suspension system and cocoon cover was
important to M. lemniscata cocoons that contained spiderlings (see Table 5-2). In this stage, total collapse of the suspension system resulted in increased mortality due to drowning. The incidence of fungal attack also increased with modification of the cocoon cover. However, it is unclear whether the spiderlings were attacked by fungus because of damage to the cover, or died for some other reason and were subsequently attacked. Modifying the cocoon cover does increase dessication problems for M. lemniscata spiderlings (see Chapter IV), and this may have been the real cause of the increased spiderling death. However, I have also had fungus attack spiderlings in the laboratory under conditions of high relative humidity (66-100% RH) where dessication should not be a problem.
Christenson and Wenzl (1980) found that 17.5% of the cocoons of N. clavipes were attacked in the egg stage by fungus. They cited the loss of protection from the rain (removal of a leaf canopy above the cocoon), and falling to the ground (support line failure) as reasons why the flocculent silk cocoons of this spider failed. Overall, the lack of any effect on the incidence of fungal attack through manipulation of the covers and suspension systems of M. lemniscata and A. aurantia cocoons is surprising, particularly
since these components presumably perform the same functions as the leaf layer and support system of N. clavipes cocoons. These results may be partially explained by differences in the cocoons of the three species. The cocoons of M. leinniscata and A. aurantia have dense covers, and layers of flocculent silk between the cover and the egg mass. While the covers of both these cocoons are wettable, the flocculent silk layer is not. The cocoon of N. clavipes is composed primarily of a basket-like mesh, with little or no flocculent layer between the eggs and the mesh. The eggs in M. lemniscata, and A. aurantia cocoons with damaged covers are still protected from water (and fungal spores?) by their flocculent silk layer (which was not damaged in this study). The mesh cocoons of N. clavipes, which are also wettable, have no flocculent layer to turn water away from the egg mass once their protective leaf canopy is gone. Other explanations for this difference, such as a layer of spherical granules on the surface of the eggs of these two species which repel water (Austin and Anderson 1978, Grim and Slobodchikoff 1978, 1982, Humphreys 1983), or relatively short developmental periods are not adequate, since all three spiders share these traits.
The differences in the incidence of drowning between
M. lemniscata and A. aurantia cocoons in the egg and spiderling
stages (see Tables 5-2 and 5-4) are probably related to differences in the size of these cocoons, and the thicknesses of their flocculent silk layers. The cocoons of M. lemn-iscata are small, and are worked down into the soil by rain action when placed on the
ground. In this location, the cocoons are in constant contact with soil moisture. Argiope aurantia cocoons are relatively large (see Figs. 2-1 and 2-2), and remain on the surface if they fall to the ground. In this position, the area of the cocoon contacting the soil is minimized, which probably allows the cocoon to dry out between exposure to rain or submersion.
The spiderlings in M. lemniscata cocoons are also packed
together tightly, with no room to move about within the cocoon. In addition, the flocculent silk layer in these cocoons is relatively thin. Fungal infection, once established, can spread more easily to all the spiderlings within the cocoon. The thicker flocculent silk layer in the cocoons of A. aurantia provides protection from water entering the cocoon, even when the cover is damaged. The large size of these cocoons also allows the spiderlings to move about in the cocoon, which may further reduce the probability of infection moving through the mass. Indeed, in cocoons damaged naturally or modified by me, the spiderlings are almost always appressed against the cocoon cover opposite the site of the damage.
The differences between the overall levels of fungal attack on the cocoons of these two spiders are also interesting. The cocoons of M. lemniscata are covered by layers of discarded orb-web that contains detritus composed primarily of prey remains. These remains may provide a high quality medium for fungal spores to germinate on, and from them, extend their hyphae into the cocoons. On occasion, I have observed M. lemniscata cocoons completely enswathed by a fuzzy layer of grey or white fungus. I have also had M. lemniscata
cocoons that were stripped of their outer layer of silk (see Fig. 2-1c) attacked by fungi in the laboratory. In contrast, A. aurantia cocoons, which have a naked cover with an almost lacquered surface finish, are never covered by fungus. In fact, it
is rare to find small patches of fungus on the covers of these cocoons at all, even after they have been lying on the ground for 90 days (see Table 5-4).
Spiders produce a variety of silks and associated materials in a number of different glands located in their abdomens (Witt et al. 1968, Mullen 1969, Foelix 1982). One of these, the aggregate gland, produces the glue-like material found on the catching spiral
of orb-webs. Schildknecht et al. (1972) have demonstrated that this glue-like material contains nitrates (KNO 3), phosphates (KH 2PO 4), and pyrrolidines (C 4H2 NO 2). They suggest that one of the functions of these compounds is to protect the web from bacterial and fungal attack. If spiders are protecting their webs from such attack, it seems reasonable that they would protect their cocoons, which represent a much greater expenditure of energy and time, as well. Given that these compounds can be manufactured in the silk glands associated -with web building, it seems likely that the tubiliform and aciniform glands associated with cocoon production may also able to manufacture similar compounds, so they can be applied to the covers of cocoons.
The preceeding Chapters III, IV, and V have both considered and demonstrated the role of the cocoon, or its various parts, in limiting the effects of temperature extremes, dessication, and
fungal attack on the egg and spiderling stages of M. lemniscata and A. aurantia. However, many predator groups have specialized on the eggs of spiders, and it is probable that cocoons also function to control their attacks. This possibility is considered in Chapter VI.
THE ROLE OF THE COCOON IN LIMITING EGG AND SPIDERLING PREDATORS
A major portion of modern ecology is concerned with the relationship between heterotrophic organisms and their food supplies. Such interactions (e.g., predator vs. prey, host vs. parasite) occur both in ecological time, and in evolutionary time (Brower and Brower 1972, Dawkins and Krebs 1979). As such, natural selection may be presumed to act within interacting populations to optimize both the ability to acquire resources (Schoener 1971, Pyke et al. 1977), and the ability to avoid becoming the acquired resources of similarly-selected organisms of higher trophic levels. Within this context, the strategy of any host organism should be to protect itself, or its offspring, from the effects of predator or parasite attack through processes or
behaviors which limit the types and numbers of attackers that are capable of using it. Ideally, the strategy should result in the total exclusion of all the attackers. In reality, their effect is only partially reduced due to the reciprocal nature of the interaction over evolutionary time.
Spiders deposit their eggs in one or more discreet clutches. These clutches can be extremely large (up to 2000 eggs; Kaston 1948). The individual eggs within the clutch are also
quite large when compared to eggs of other groups such as insects. The eggs are lipid rich and represent a considerable amount of energy (Anderson 1978). In addition, during the reproductive season, clutches may occur in large numbers in the habitat and may be relatively conspicuous. They therefore represent a potential and highly desirable resource for many predators and parasites. Indeed, insects from a number of groups have specialized on spider eggs (Auten 1925, Eason et al. 1967, Evans 1969, Askew 1971, Austin, In press).
All spiders deposit their eggs in some form of silken cocoon
(Turnbull 1973). For many spiders this structure represents the sum total of maternal care bestowed on the eggs by the female spider. The large and diverse number of predators attacking cocoons, and the wide range in complexity of cocoon architecture (McCook 1890, Kaston 1948) have led to speculation that the primary function of the cocoon is to protect the eggs and spiderlings from attack (Austin and Anderson 1978, Robinson 1980). Austin (In press) recently reviewed the literature on cocoon attack by egg predators, and proposed that the wide diversity of cocoon types arose because spiders have attempted to reduce the total number of predators that can use their cocoons by forcing them to "specialize" on a particular cocoon type. The predators have responded in evolutionary time, and the relationships between cocoons and their attackers that we observe today can be attributed to a 11coevolutionary arms-race" (Dawkins and Krebs 1979). While some observations have been reported on parasite difficulty in entering
cocoons (Kaston and Jenks 1937), there is still no evidence that cocoons, or the various layers comprising them, function to turn away or reduce the success of specific parasites or predators.
In this chapter, I investigate the roles that the suspension systems, outer covers, and internal flocculent silk layers of the cocoons of Mecynogea lemniscata Walckenaer and Argiope aurantia Lucas (Araneidae) play in controlling the access of predators to the eggs and spiderlings within. If the suggestions of various researchers, and in particular Austin (In press), are correct, these cocoons should demonstrate defenses which reflect the manner in which their predators attempt to introduce themselves or their
offspring into the cocoon.
Materials and Methods
The numbers and kinds of parasites and predators attacking the cocoons of M. leinniscata and A. aurantia were determined by collecting cocoons in the field and returning them to the laboratory where they were opened and scored for parasites. Scoring was relatively easy since each parasite leaves distinctive evidence in
the cocoon such as the type of hole in the cover, or remains such as pupal cases, shed exuviae, or eggs. The M. lexnniscata cocoons were collected in late August in 1981 to 1983 after the eggs in the last cocoons laid had hatched. A single sample of A. aurantia cocoons was collected in late October in 1981. In 1982 and 1983, a
different 31 in of a 400 m stretch of roadside hedgerow was sampled every two weeks. For each sample, all of the cocoons in the 31 mn section were collected. In 1982, the cocoons were collected from August through November; in 1983, from August through October.
In addition to these data, specific information was also
collected regarding attacks on A. aurantia cocoons by ichneumonid wasps and chloropid flies. For the ichneumonid attacks, these data included two indices relating the distance of the egg mass to the cocoon cover along the X and Y axis. These indices were calculated by first measuring the following four variables: egg mass length, egg mass width, cocoon length, and cocoon width (Fig. 6-1). These values were then used in the following formulas:
Distance to the egg (Cocoon Width Egg Mass Width)
mass from the cocoon cover = ,and
Distance to the egg (Cocoon Length Egg Mass Length)
mass from the cocoon cover =
These two measures were compared to the average ovipositor length of the ichneumonid to predict where the wasp might prefer to attack the cocoon. The predicted preference was checked by examining the shells of attacked cocoons collected in the field for the distribution of ovipositor drill holes. Data on the number of cocoons that the wasp attacked without ovipositing, and the distribution of support line "deltas" (the structure on the cocoon surface the support lines emanate from; McCook 1890) were also
Fig. 6-i. A schematic view of an Argiope aurantia cocoon illustrating the four measurements; cocoon length (cl), cocoon width
(cw), egg mass length (eml), and egg mass width (emw) taken to determine the position of the egg mass in relation to the cocoon cover.
collected. For the chioropid fly attacks, the number of cocoons successfully attacked (producing pupae), the number of cocoons successfully attacked in conjunction with another predator, and the number of cocoons unsuccessfully attacked (eggs laid on the cocoon surface but no pupae produced) were recorded.
The relationship between the size of an A. aurantia cocoon and the number of eggs it contained was established by measuring a number of cocoon dimensions and counting the spiderlings in 40 cocoons. Figure 6-2 shows that there is a strong relationship (r 2 = 0.90) between the diameter of the cocoon and spiderling number. This relationship was used to estimate the original number of eggs in attacked cocoons. The percentage of eggs surviving the attack, a measure of the damage caused by a given predator, was then calculated by the following:
Percentage of Eggs No. of Survivors
Surviving -X 100.
Parasite Attack Estimated No. of Eggs
The relationship of cocoon size to egg number was not calculated for M. lemniscata because all of the eggs in a cocoon are utilized when attacked by its egg predators.
The roles of suspension systems and cocoon covers in
controlling egg predator attacks were investigated by modifying these components in the field and assessing the effects of their change on egg hatching success and spiderling survival. The cocoons
100 0 00
" 000 0@ @0
500 ** **
0 I i I I I
1.3 1.5 1.7 1.9 2.1 2.3 2.5
Cocoon Diameter (cm)
Fig. 6-2. The relationship (Y = 1522.2X 1886.9; n = 40) between the diameter of Argiope aurantia cocoons and the number of spiderlings (eggs) therein. The coefficient of determination (r2) for the relationship is 0.90.
used for these modifications were either produced in the laboratory, or collected in the field. They were assigned to the experimental groups listed below. The cocoons in all the experimental groups were marked either by flagged stakes placed beneath them, or by flagging in the vegetation.
Vegetation contact. The function of cocoon support lines as a deterrent to arboreal predators was examined by repositioning cocoons so that they contacted the vegetation. This was done only for M. lemniscata cocoons and was accomplished by tying cocoon strings to branches with their suspension lines. This mimicked the position of cocoons whose suspension systems had collapsed normally.
Ground placement. The role of the suspension system in keeping cocoons off of the ground and away from terrestrial predators was investigated by placing the cocoons of both species on the ground in open-topped wire corrals. These corrals allowed cocoons to be attacked but prevented them from being washed away in the rain. The corral used for A..aurantia cocoons was constructed of hardware cloth (1/4" mesh) and was 12.5 cm. in diameter and 5 cm in height. The corral for M. lemniscata cocoons was constructed of aluminum window screen and was 5 cm in diameter and 5 cm in height. The corrals were located below sites where cocoons had been collected previously.
Cover removal. The function of the cocoon cover in limiting predator attack was examined by removing approximately 25% of the cccocn cover with a razor blade. Individual cocoons of M. lemniscata were modified in the cocoon string and the string was
left suspended in place at the web site. Female spiders treated these modified cocoons normally. The laboratory reared cocoons of A. aurantia were modified and attached to the arms of a support system using alligator clips. The support systems were placed in the vegetation where normally oviposited cocoons were hanging. They held the cocoons approximately 1 m off of the ground (the average cocoon height as determined by collecting).
Cover removal control. This group was to control for the
possible contaminating effects of the alligator clip used in the apparatus to suspend A. aurantia cocoons. Clips were attached to cocoons hanging normally in the habitat and the cocoons were observed for any effect.
Control. Control cocoons were subj ected to no structural
intervention. These cocoons controlled for handling effects in the ground placement, vegetation placement, and cover removal groups.
Bird damage. These A. aurantia cocoons were subjected to no mechanical modification. They were marked and collected after 150 days in the field to determine whether bird predation increases later in the year after the plants lose their leaves (see also Tolbert 1976). They were compared to the control group. Laboratory Experiments
The role of the flocculent silk layer in the cocoons of A. aurantia as a barrier to predator attack was tested in the laboratory by placing the eggs of Tromatobia ovivora rufopectus (Cresson) (Ichneumonidae) at two depths in the cocoon: 1) just under
the cover, and 2) on the host egg mass. Successful parasitism was scored as the number of ichnewnonid larvae found in the host egg mass just prior to pupation.
The wasps were maintained in the laboratory at 23-25 0 C and 50-60% RH. They were provided with white refined sugar, a mixture of honey and yeast, and water. Colony size was maintained by the addition of new wasps collected from the field. Their eggs were obtained by allowing a number of wasps to attack a cocoon produced in the laboratory. The cocoon was then opened, and the white, elongate wasp eggs were removed using a fine brush and forceps. Known numbers of these eggs were then transferred to other laboratory produced cocoons 1-3 days old, and placed in either of the two test locations.
The cocoons were obtained from spiders held in the laboratory in 50 x 50 x 10 cm cages (Witt 1971) or in 500 ml jars with screen tops. Oviposition and cocoon construction in both of these enclosures was normal.
The cocoons of.M. lemniscata are attacked in the egg stage by two major predators, a Tetrastichus sp. wasp (Eulophidae) and the neuropteran Mantispa, viridis Walker (Mantispidae) (Hieber 1984) (Table 6-1). Both of these predators utilize all of the eggs in a cocoon during their attacks. However, they attack the cocoons in
Table 6-1. Numbers of Mecynogea lemniscata cocoons attacked by the eulophid wasp Tetrastichus sp., the mantispid Mantispa viridis, and ants/ unknown predators for the years 1981 to 1983. The cocoons were collected in late August in 1981 and 1982 at the end of the reproductive season, and in July, August, and December in 1983. The percentages of attacked cocoons are in parentheses.
Sample Total No. Eulophid Ants/
Year Size Attacked Wasp Mantispid Unknown
1981 290 26 21 (80.8%) 0 (0%) 5 (19.2%)
1982 252 29 17 (58.61) 5 (17.2%) 7 (24.1%)
July 148 13 10 (76.9%) 2 (15.4%) 1 ( 7.7%)
Aug. 97 14 12 (85.7%) 0 (0%) 2 (14.3%)
Dec. 308 64 32 (50.0%) 12 (18.8%) 20 (31.2%)
a Cocoons contain either larvae, prepupae, and/or shed exuviae
with emergence holes.
b Cocoons contain either a larva, a pupa, or a pupal case and
c Cocoons are characterized by a large chewed hole, and the
absence of the flocculent silk layer, pupal cases, pupae,
shed exuviae, or parasite.
different ways. Mantispa viridis deposits its eggs in the habitat, removed from the host, and the emerging triungulid larvae actively search out and burrow into previously constructed cocoons. These larvae are obligate cocoon attackers, as opposed to the mantispid larvae that ride on female spiders and attack the egg mass just prior to cocoon construction (see Redborg and McLeod, In press, for a review of Mantispid biology). The Tetrastichus wasp either deposits eggs into the cocoon, or into the upper layers of the cocoon cover where the emerging larvae burrow through the cocoon and attack the egg mass (Austin, In press). Both the wasp and the mantispid are never found together in the same cocoon. The cocoons of M. lemniscata are also occasionally attacked by ants, which also remove all of the eggs or spiderlings.
The single line suspension system of M. lemniscata cocoons
functions to prevent predation on the egg stage. Egg predation was significantly greater than control for both the cocoons placed on
the branches (X = 47.40, df = 1, p < 0.001) and those placed on
the ground (X = 23.49, df = 1, p < 0.001) (Table 6-2). In both cases, the principle predator appeared to be ants. Modification of the cocoon cover also had a significant effect on egg predation (X2 = 38.67, df = 1, p < 0.001). In this case, however, the principle predator was not ants, but the mantispid M. viridis. In no case did the wasp attack any manipulated cocoons containing eggs, suggesting that the position of the cocoon in the web or the integrity of the cocoon cover are important to the wasp.
Table 6-2. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the eggs of Mecynogea lemniscata. All cocoons were collected after 30 days.
Experimental Sample Successful No. Cocoons % Cocoons
Modification Size Survival Attacked Attacked
Control 175 153 22a 12.6%
Contact 24 6 18b 75.0%
Placement 26 12 14b 53.8%
Removal 32 12 20c 62.5%
a Of the 22 cocoons attacked, 2 were attacked by ants/ unknown
predators, 19 by the eulophid wasp Tetrastichus sp., and
1 by the mantispid M. viridis.
In both cases the cocoons were attacked by ants/ unknown predators.
c All 20 cocoons were attacked solely by the mantispid M. viridis.
The effect of the suspension system in protecting M. lemniscata spiderlings from predators was not as pronounced (Table 6-3). There were no differences in predation between the Control group and either the cocoons placed on the ground for 45 days or the cocoons placed in contact with the vegetation. The cocoons placed on the ground for 90 days, however, were preyed upon at significantly
higher levels (X = 70.40, df = 1, p < 0.001). These results probably reflect the generally lower densities of ants foraging on the vegetation and ground late in the year (pers. ob.). Cover removal also had no significant effect on spiderling predation when compared to control, again because the the cocoons are inaccesible on their support line.
The eggs of A. aurantia are attacked by four primary
predators; the wasp T. ovivora rufopectus (Ichneumonidae), the neuropteran M. viridis, and the flies Pseudogaurax signata (Loew) (Chloropidae) and Megaselia sp. (Phoridae) (Table 6-4). Of these, the two most common predators are the ichneumonid and the mantispid. These are obligate egg predators and probably account for most of the initial attacks in cocoons attacked by multiple predators. The attack behavior of the mantispid has been previously discussed. The ichneumonid T. ovivora rufopectus attacks A. aurantia cocoons by inserting its long ovipositor through the cover into the flocculent layer. Its eggs are deposited on or near the host egg mass, and the emerging larvae make their way to the host eggs and burrow into the mass to feed.
Table 6-3. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the spiderlings of Mecynogea lemniscata. All cocoons were collected after 90 days, except for those placed on the ground. They were collected after 45 and 90 days.
Experimental Sample Successful No. Cocoons Cocoons
Modification Size Survival Attacked Attacked
Control 259 239 20 a 7.7%
Contact 77 71 6a 78
Placement (45) 19 17 2 a10.5%
(90) 32 11 20 a62.5%
Removal 34 32 2 a 5.9%
aAll attacks were by ants/ unknown predators.
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The status of P. signata is less clear. This fly deposits eggs on cocoon surfaces. The eggs hatch and the emerging larvae push their way through the cover into the cocoon (Kaston and Jenks 1937, Kessel and Kessel 1937, Hickman 1970). The literature suggests that chloropid flies are obligate spider egg parasites (see Austin, In press), a fact supported by the large number of A. aurantia cocoons attacked by this fly in 1982. However, only 17 of 42 cocoons were singly attacked by this fly (Table 6-4). The remaining cocoons were attacked along with other predators (particularly mantispids), and in many cases the fly deposited its eggs in the emergence holes of these previous attackers. In addition, Hall (1937) lists this fly as attacking the egg masses of the praying mantis. This suggests that this fly is a facultative egg predator that attacks eggs in any container, as well as utilizing dead or damaged eggs from other attacks when located. The phorid is found alone in cocoons approximately 50% of the time and it is probably an obligate egg predator, attacking cocoons in the same manner as P. signata.
The observed levels of egg predation for T. ovivora rufopectus, P. signata, and M. viridis in this study are similar to the levels of attack reported by Enders (1974) and Tolbert (1976) for A. aurantia cocoons. The A. aurantia cocoons in this study were also attacked by birds, ants, and moth larvae, although it is unclear whether these attacks occured in the egg or spiderling stage. The level of bird attack was lower (9%) than that found by Tolbert (35-50%). Argiope aurantia cocoons containing spiderlings are attacked by ants and other spiders (particularly salticids which
use damaged cocoons as retreats), and by small rodents if cocoons fall to the ground.
The amount of damage done to the eggs in an A. aurantia cocoon by each of the four primary predators varies greatly (Table 6-5). The ichneumonid, on average, does the greatest amount of damage, destroying approximately 90% of the host eggs. By themselves, the chloropid fly and the mantispid attack roughly the same number of cocoons. However, the average mantispid attack damages approximately 20% more eggs than the average chlorpid attack. The phorid fly destroys approximately 97% of the eggs in a cocoon. Birds also damage the contents of the cocoon heavily (74%), but the damage is usually done in the spiderling stage.
Tromatobia ovivora rufopectus was successful in attacking 34 cocoons in 1982 (see Table 6-4). However, an examination of the covers of 167 cocoons collected that year (the other 18 covers were to damaged to inspect) for ichneumonid oviposition holes indicated that 92 cocoons were actually sampled by the wasps. Thus, 58 cocoons were sampled but had no eggs deposited in them, presumably because they were in the wrong stage for attack (Vinson 1976). The other 75 cocoons were either never found by the ichneumonids, or were found and rejected based on criteria which did not require insertion of the ovipositor to determine.
The number of cocoons containing pupal cases of P. signata was also not a valid measure of the total number of cocoons attacked by this fly (see Table 6-4). Of the 69 cocoons in 1982 with attached P. signata eggs, 27 (39.1%) were apparently unsuccessful in that
Table 6-5. The mean percentage of eggs which survive in an Argiop aurantia cocoon attacked solely by the ichneumonid Tromatobia ovivora rufopectus, the mantispid Mantispa viridis, the chloropid fly Pseudogaurax signata, the phorid fly Megaselia sp., or birds. SD is in parentheses.
No. Cocoons Percent Eggs
Parasite Sampled Surviving
Ichneumonid 35 10.2 (13.7)
Mantispid 15 39.1 (27.6)
Chloropid Fly 15 63.7 (35.6)
Phorid Fly 4 4.7 ( 9.5)
Birds 16 26.4 (28.3)
emerging larvae failed to gain entrance into the cocoon (no pupal cases inside the cocoon). The remaining 42 cocoons were successfully attacked and can be divided into two groups; 17 cocoons attacked by the fly alone, and 25 cocoons attacked by the fly and one or more other predators. Of the 17 singly attacked cocoons, 13 had eggs on the surface and a fewer number of pupae inside, 2 had more pupae than eggs present, and 2 had pupae and no eggs present. Of the remaining exhibiting multiple attacks, 11 had eggs on the outside and inside of the shell and contained less (8 cocoons) or more pupae (3 cocoons) than eggs. The other 14 cocoons had pupae, but no evidence of eggs could be found because the cocoon and its contents were heavily damaged.
The large number of cocoons attacked by t. signata in
conjunction with other predators suggests that shell damage may be advantageous to this fly (see Table 6-4). Indeed, the average rate of success (number of pupae/ number of eggs X 100) for the cocoons attacked by the fly alone was 29.8% (SD = 30.2%) (based on the 13 cocoons which had visible eggs). The average rate of success for the flies which attacked cocoons with other predators was 82-7% (SD = 19-74%) (based on 11 cocoons; the 3 with more pupae than eggs were counted as 100% successful). This level of success is significantly greater (t = 10-05, df = 22, p < 0.001) than that achieved by P. signata attacking cocoons alone.
It is difficult to determine whether the success rates for
mantispids and phorid fies attacking A. aurantia cocoons are due to an inability to locate the cocoons, or to the inability of the
respective larvae to penetrate the cocoon cover. The large increase in successful mantispid attacks on artificially damaged M. lemniscata cocoons suggests that mantispid larvae are generally abundant and probably have no trouble locating cocoons, but have difficulty penetrating the dense cocoon cover. This suggests that the cover of A. aurantia cocoons is also responsible for the relatively low rate of successful mantispid attack. Presumably, the larvae of the phorid fly are also hindered by the cover, although this is a rare parasite and the low numbers of successful attacks mnay be a reflection of low numbers of searching parasites. It seems likely that birds, once they find a cocoon, have little trouble in exploiting it.
In A. aurantia cocoons, the egg mass is closest to the cocoon cover at its top edge, farthest from the cocoon cover at its bottom, and approximately the same distance as the average ovipositor length from the cocoon cover in the middle of the cocoon (Table 6-6). The cocoons are suspended in the vegetation by a cloud of lines emanating from suspension line "deltas" on the surface of the cocoon (see Fig. 2-2a). There are significantly more of these deltas, and thus significantly more lines, in the middle third of the cocoon surface than in either the upper (t = 4.48, df = 140, p < 0.00) or lower third (t = 156.40, df = 140, p < 0.00) (Table 6-7). The distribution of ichneumonid drill holes matches the delta distribution; there are significantly more holes in the middle third than in either the upper (t = 4.69, df = 166, p < 0.001) or lower third (t = 3.92, df = 166, p < 0.001) of the cocoon (Table 6-7).
Table 6-6. Mean cocoon diameter, cocoon length, egg mass diameter, egg mass length, distance to the egg mass (X-Axis), and distance to egg mass (Y-Axis) for Argiope aurantia cocoons, and mean ovipositor length for the ichneumonid Tromatobia ovivora rufopectus. The two distance variables indicate the length of an ovipositor needed to deposit eggs directly on the surface of the host egg mass. SD is in parentheses.
Variable Size Mean (mm)
Cocoon Diameter 25 19.2 (2.2)
Cocoon Length 25 24.4 (2.4)
Egg Mass Diameter 25 10.2 (1.4)
Egg Mass Length 25 11.1 (I.4)
Distance to Egg Mass
from cover: X-axis 25 4.5 (0.7)
Distance to Egg Mass
from cover: Y-axis 25 6.7 (1.2)
Ovipositor Length 57 3.9 (0.5)
Table 6-7. The mean number of suspension line deltas and oviposition holes of the ichneumonid Tromatobia ovivora rufpectus in the upper, middle, and lower thirds of the cover of Argiope aurantia cocoons. SD is in parentheses.
Sample Upper Middle Lower
Size Third Third Third
Line Deltas 71 18.7 (6.2) 22.3 ( 9.2) 3.9 (3.7)
Oviposition Holes 84 2.4 (3.7) 9.5 (13.4) 3.4 (5.0)
The location of the egg mass within the cocoon and the distribution
of drill holes suggests that the ichneumonid prefers to attack the cocoon in this specific area.
For A. aurantia, egg predation due to the cocoon falling to the ground was not statistically different from the control in terms of the number of cocoons attacked (Table 6-8). However, the individual survival of eggs in each cocoon was reduced substantially. All of the cocoons on the ground discovered by predators suffered 100% egg mortality as compared to about 59.4% mortality for cocoons remaining in place in the vegetation. The cocoons on the ground that escaped attack were buried by falling leaves and were presumably hidden from predators. The level of predation on eggs in cocoons with modified covers was also not significantly different from the Control group. However, in all cases the cocoons were attacked by the chloropid fly P. signata, further suggesting that damaged cocoons appeal to this egg predator. The lack of mantispids in the modified cocoons is most likely related to the apparatus used to suspend the cocoons, which prevented the larvae from locating the cocoons, and not because of their inability to use them.
The effect of cover modification also had no effect on
predation in the spiderling stage when compared to the Control group (both controls were not significantly different from one another) (Table 6-9). Placement on the ground, mimicking suspension failure, resulted in significantly greater predation IX=24.37, df = 1,p on the ground (5.9%) was also much lower then for spiderlings in
Table 6-8. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the eggs of Argiope aurantia. All cocoons were collected after 30 days.
Experimental Sample Successful No. Cocoons. % Attacked Modification Size Survival Attacked Attacked
Control 21 10 11a 52.4%
Placement 10 2 8b 80.0%
Removal 17 10 7c 41.2%
a Of the 11 cocoons attacked, 8 were attacked by the chloropid
fly P. signata, 8 were attacked by the mantispid M. viridis,
1 by the phorid fly Megaselia sp., 1 by the ichneumonid T. ovivora rufopectus, and 5 by birds.
b Rodent (mice) damage. The cocoon covers were shredded and the contents destroyed.
All 7 cocoons were attacked solely by the chloropid fly P. signata.
Table 6-9. The effect of modification of the cocoon suspension system and cover on the incidence of predator attack on the spiderlings of Argiope aurantia. All cocoons were collected after 90 days, except for those in the bird damage group. They were collected after 150 days.
Experimental Sample Successful No. Cocoons 10 Cocoons
Modification Size Survival Attacked Attacked
Control 23 20 3 a 13.0%
Cover Removalb Control 15 14 1b 6.7%
Placement 19 2 17 c 89.5%
Removal 21 20 1d 4.8%
Damage 23 15 8e 34.8%
a Bird damage. Of the 3 cocoons, 2 had part of their cover
removed, and 1 had cover damage and the contents removed. b Bird damage.
cRodent (mice) or bird damage. Characterized by the cover being shredded and the contents removed. d Bird damage. Contents removed from the cocoon. e Bird damage. Of the 8 cocoons, 2 were removed completely,
5 had 25-50% of their covers removed, and 1 had the cover
damaged and the contents removed.