SummaryWe investigate the microstructure of scales in the wings of male Sasakia charonda charonda butterflies by scanning electron microscopy with the aid of optical microscopy. Six types of scales are identified: B1, W1 and R1 in brown background yellow spots and red spots, respectively; B2 in iridescent purpleblue and W2 in white pearl, both of which characterize the male and B3 in the wing edges. The B1, W1 and R1 scales are almost the same in structure and the B2 and W2 scales are almost the same. The difference among the B, W and R scales is in species and content of pigment. The B1, W1 and R1 scales have only two layers of cuticle lapped on the ridges. In contrast with them, the B2 and W2 scales have seven multilayers of cuticle piled on the ridge. The multiple interference of light that occurs among these cuticle layers, spaced with air layers, generates the significant iridescence of the B2 and W2 scales. Thus, the characteristic purple-blue of the male wings is ascribed to the combination of the structural and chemical colouration in the B2 scales with melanin. The photonic crystals of these scales may be applicable to fine light manipulators such as reflection elements in laser diodes. B3 has many holes between the ridges and no multilayers of cuticle on the ridges. These structures may play any role in aerodynamically easy flight and/or in drainage of wet wings.

We performed scanning electron microscopy observations of the scales on the dorsal surfaces of wings of male and female Chrysozephyrus ataxus butterflies. The male butterfly has curly scales on the blue or green wings. It was deduced that the interference of the selective incident rays with the wavelength in $560 nm ! ! $420 nm and $340 nm ! ! $250 nm occurs incoherently by layers $270 nm thick piled in the flat grooves which are enclosed by the ridges and ribs on the curled scale. The metallically glittering green-violet hues of the male wings is thereby attributed to the reflection of the human visible rays in $560 nm (green) ! ! $420 nm (violet). The vivid violet marks in the female's forewings were also explained as the result of reflection of the incident rays in $400 nm ! ! $300 nm from the layers $190 nm thick in the flat grooves on the dorsal scale. Although the monolayered cuticle structure was observed on the ridges of these scales, its contribution to the wing colouration must be less because of a small width of the ridges as compared with the flat grooves. The scales in the dark brown areas of the female wings are different in structure from these scales; they have not any layers but windows enclosed by the ridges and ribs. Most of the light through the windows is absorbed in the lower laminae containing the eumelanin. These results were deduced using data of a previous optical measurement by Imafuku et al. (Zool. Sci. 19 (2002) 175) and elucidated consistently their conclusion.

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.

Suppose that you look Arabic numerals on transparent glass windows or in the air. Which does it mean ‘85’ or ‘28’? You end up watching the mirrored images when you view from back side. So let us make the mirrored image invisible. To solve mirrored image problems of an aero signage which means images are floating in the air, our research group has utilized dye‐doped prisms and total internal reflection in optics. You know that water is generally colorless and transparent but the water in a bath tub is little blue. The color changes depending on the depth of water. The deeper, the darker water becomes blue. This is the reason why some color is generated in the dye‐doped material. In 2021, we presented a unidirectional observable display using letterings made by intaglio printings for arranging dye‐doped tiny prisms on a glass window. You can watch these colored letterforms from front and transparent pixels from back. But this 2021 model has a little defect that colors are less vivid because of color generating mechanism by twice reflection in conventional optical prisms using transparent materials for making transparent from back side. In this paper, the authors show an aero signage display using dye‐doped corner cube prisms which can lengthen light paths in the prism by multi‐times reflections to avoid the semi‐transparent problem which means colors are not vivid.