Fine structures of wing scales in Sasakia charonda butterflies as photonic crystals

Abstract

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.

Fine Structure of Wing Scales in <I>Chrysozephyrus Ataxus</I> Butterflies

“…The interference caused between cuticle-air layers on the narrow ridge is not so much effective for the colouration of the C. ataxus unlike those of the male Ancyluris meliboeus, 7) S. charonda, 17) and E. mulciber butterflies. 23) Furthermore, the curl of the scale, which is not observed in the scale of the other butterflies, limits the diffraction condition to a small area on the ridge, consequently, greatly reducing the reflected light.…”

“…The iridescent violet marks in the female's forewing in Fig. 1 Table 2, together with those of the iridescent blue scales of a male S. charonda 17) and a male E. mulciber.…”

“…SEM observations were performed in secondary electron detection mode in a range of 1$15 kV of the accelerating voltage, using a Hitachi SU6600 with a Schottky field-emission gun and a JEOL JSM-6700F equipped with a cold field-emission gun. The wings were coated with a sputtered gold layer about 10-20 nm thick to avoid charging effects, similar to our previous observations 16,17) or observed without coating at 1 keV of primary beam energy. The hues of the scale's colours were examined in an optical microscope (OM).…”

“…17) Figure 5 shows the scale at the edge of the female's wing. The edge scales have some common characteristics with those of S. charonda 16) and E. mulciber butterflies, 23) regarding split tails (Fig. 5(a)), narrow spacings of ridges (d 2 ) and ribs (D 0 ) and mono-or no cuticle layer on the ridges (Table 1 and Fig.…”

“…15) Recently, we have investigated the photonic crystal structure of wing scales in a male Sasakia charonda charonda (S. charonda, the great purple emperor) butterfly and elucidated the origin of vivid iridescence in its wing. 16,17) The iridescence is caused by the interference of incident light reflected with a kind of blazed diffraction grating of the scale that has high efficiency in a shorter wavelength range of 200 nm$450 nm.…”