BULLETIN OF THE ALLYN MUSEUM
THE ALLYN MUSEUM OF ENTOMOLOGY
Number 40 10 November 1976
DIFFRACTION STRUCTURES IN THE WING
SCALES OF CALLOPHRYS (MITOURA) SIVA SIVA
Arthur C. Allyn
Director, Allyn Museum of Entomology
John C. Downey
Biology Department, University of Northern Iowa, Cedar Falls. Iowa,
Research Associate, Allyn Museum of Entomology
In preparing color photographs of several species of the genus Callophrys we
encountered difficulty in obtaining an acceptable reproduction of the green color
of ventral surfaces. In several cases, we could obtain little, if any, green color on
photographs even though it was visually quite prominent.
Previous scale studies (see Downey and Allyn, 1975), together with continued
research in Pieridae and Lycaenidae, indicate at least two types of scale structures
which produce discreet frequencies through interference or diffraction pheonomena.*
Only one of these is found in the Nearctic Callophrys siva. Morris (1975) reported
the green iridescence of the Palearctic Callophrys rubi was caused by a porous
element in the scale lumen, which has a mosaic of polygonal grains having an
ultrastructure of a cubic network. Our previous work using scan electron mi-
croscopy had revealed similar porous elements in other Lycaenidae.
Samples of the ventral surface of wings of Callophrys (Mitoura) siva siva (W. H.
Edwards) were examined with a Beckman DB-GT spectrophotometer and a JEOL-
SMU-3 scanning electron microscope. The spectrophotometer was equipped with
a reflectance attachment and reference beam attenuator. A small attachment was
also devised which permitted the sample to be placed on the attenuator at various
angles. Dried wings were coated for scanning in a Varian V-10 vacuum coater to
approximately 50 angstroms with 60/40 gold palladium.
*Numerous investigations have dealt with the general subject of iridescence which need not be reviewed here.
Such studies include: Anderson & Richards, 1942; Gentil, 1935 through 1964; Ghiradella, 1974; Ghiradella et
al, 1972; Lippert & Gentil, 1952, 1959; Mason, 1926, 1927; Onslow, 1921; Suffert, 1924.
Light microscopy indicated the green scales in siva produced also a light
brown color. This color was similar to the coloration of the ventral forewing scales,
and is assumed to be melanin deposited in the scale structure. Therefore it seemed
reasonable that, to more precisely determine the exact output of the green portion
of the scales, we could produce spectophotometric curves of both the green hindwing
scales and the brown forewing scales, and subtract the data of the brown from that
observed from the green scales. The two curves produced in this manner further
indicated the reasonableness of this approach. The resulting curve (Fig. 1)
indicates a very tight peak of response at 515 nm. The intensity varies slightly with
the viewing angle, but there is no wavelength shift. This was confirmed both
visually and spectrophotometrically. Essentially no reflection in the ultraviolet
Upon SEM examination we located a dislodged scale from which the ventral
(bottom or adwing surface) side had been peeled back, revealing for the first
time the mosaic of polyhedral grains in great detail (Fig 2 and 3). The dorsal
side of a typical green scale is shown in Figure 4.
Internal examination of these grains indicates that their structure is more
hexagonal in form than it is cubic and most closely resembles a honeycomb from
all planes. A honeycomb-like formation would permit a more uniform thickness
throughout the lattice than would a cubic form as postulated by Morris (1975).
Figure 1. Spectrophotometric curve derived
of Callophrys siva.
from ventral hindwing surface
In cross section the grains appear to be composed of perforate layers four to seven
deep, and connected by minute trabeculae (pillars). This could account for earlier
observations (W. Schmidt, 1942a: 199; Schmidt and Paulus, 1970: 235) on layering
of this structure. As indicated by both Figure 5 and the stereo pairs (Fig. 6),
the structure is a uniform lattice perforated in the vertical and both horizontal
planes, and having uniform lattice thickness in all directions. Such a formation
would permit the constant color seen from all viewing angles. This uniformity
contrasts sharply with the color variation with viewing angle as observed in the
ridge-line and ridge-shelf interference phenomena of the Morphidae scale structure.
The lattice parameters measured from the electron micrographs were 300 nm.
Figures 2 & 3. 2: Ventral hindwing scale of Callophrys siva with adwing
surface peeled back (1930x). 3: Porous elements in scale lumen (1930x). The differ-
ling horizontal orientation of the elements may be noted.
Figures 4 & 5. 4: Dorsal view of ventral surface green scale of Callophrys
si'a (6430x). The porous elements may be seen below the ridges and cross ribs which
partially obscure their true form and shape. 5: Detail of structure of a porous
element (19300x). The lighter knob-like protuberances are the pillars connecting
the element to the adwing membrane.
The density correction factor derived following Morris (1975) is 1.162. Using this
factor, the lattice parameter yields a theoretical frequency of 516 nm. The observed
peak output of 515 nm is within reasonable limits of the theoretical.
The grains were highly variable in size and orientation appeared to be random.
This latter fact would also permit a uniform output at approximately 515 nm when
viewed from any direction or by diffused light.
The general form of the scales throughout the expanse of the ventral surfaces
of C. siva was quite uniform. Other than the presence or absence of the diffraction
grains the only difference noted was a wider spacing of the ridges in the green
scales of the hindwing and the light brown scales of the forewing. (Figs. 7, 8, 9).
The hole sizes of the grains in M. siva is between those observed in certain
blue scales and copper scales of other species of Lycaenidae. This conforms with
the postulated volume diffraction method of color production.
Figure 6. Stereo-pair of porous elements (6000x). The four to five layered
depth of the elements can be seen.
i, ^Ji~lii .sit's1i yy^^Sa^i^^S^^^^^'^SK^^^ ^^R^^^^^^^
Figures 7, 8 & 9. Ventral surface scales of Callophrys siva showing general
similarity of form (185x). Porous elements occur only in the green scales. 7: Green
hindwing. 8: Brown hindwing. 9: Brown forewing.
1. The green color of the ventral side of the hindwings of Callophrys siva is
produced by a volume diffraction grating (porous elements) situated within the
2. The shape of the entire porous element is irregular and scattered in a
close order random manner in the scale lumen. The horizontal orientation of
each porous element is also random.
3. The extraordinarily narrow wavelength in the green (515 nm peak) from
the diffracting porous elements accounts readily for the difficulties encountered
in color reproduction since most color film has a relatively low sensitivity at this
4. The wavelength differences between siva and rubi (Morris, 1975) are
compatible with the observed differences in lattice parameters of the porous
Sincere appreciation is expressed to Dr. Tyler Estler of the Physics Department
of New College for his verification of the physics of the observed phenomena.
Special thanks to our associate, Dr. Lee D. Miller, for his many cogent suggestions
and proofing of the manuscript.
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