Photonic Crystal Structure of Wing Scales in <I>Sasakia Charonda</I> Butterflies

Abstract

The hindwings of the male Sasakia charonda charonda butterflies comprise iridescent purple-blue areas, iridescent white-pearl areas, yellow spots and red spots as well as brown background. We have examined the microstructure of their scales by scanning electron microscopy, for applying their photonic crystal structures to fine light manipulators such as reflection elements in laser diodes. The scales in the yellow spots, red spots and brown background have almost the same structure, which is an optical diffraction grating made of ridges with two cuticle layers. Their difference comes from the contained pigments. The scales in the iridescent purple-blue and white-pearl are also the same in structure. They have seven tilted cuticle layers lapped on the ridges, which also constitute a grating. The widths of the ridge and groove in the grating are different between scales of the two kinds. It is shown that the vivid iridescence is mainly attributed to multiple interferences caused between rays reflected from the seven cuticle layers with air gaps.

Photonic Crystal Structure and Coloration of Wing Scales of Butterflies Exhibiting Selective Wavelength Iridescence

“…Transmission image ( a ) and reflection image ( b ); ( c ) The hindwing taken at a low glancing angle under sunlight (adapted from [13]).…”

“…The difference between the B and W scales is only the content of melanin. The parameters of the diffraction grating of these scales are shown in Table 1 [13]. The B and W scales of the male S. charonda have seven cuticle layers piled on the ridges, that is n = 7.…”

“…The inset is a computer diffractogram from the area enclosed by the square; ( b ) The side view and top view of the cuticles piled on the ridges; ( c ) The cross-sectional view of cuticle layers. B scales in the purple-blue iridescent area have the same microstructure (adapted from [13]).…”

“…The females thereby can receive more heat energy, to be used for breeding, from sunshine than the male butterflies. Thus, the wing structure of the butterflies may give us a hint to design photochemical crystal devices for internal conversion or radiationless de-excitation where the UV radiation is transformed into heat [13]. …”

“…The microrib layers cannot cause the backscattering diffraction for any incident light, and consequently does not exhibit iridescence, which T. magellanus produces with tilted microrib layers. We have also found a highly tilted, multilayered arrangement in vivid iridescent scales in a male Sasakia charonda ( S. charonda ) butterfly [12,13]. The structure of its scales is similar to that in the A. meliboeus .…”