Fine Structure of Wing Scales in Chrysozephyrus Ataxus Butterflies
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Materials Transactions, Vol. 52, No. 3 (2011) pp. 297 to 303 Special Issue on New Trends for Micro- and Nano Analyses by Transmission Electron Microscopy #2011 The Japan Institute of Metals Fine Structure of Wing Scales in Chrysozephyrus Ataxus Butterflies Jirˇina Mateˇjkova´-Plsˇkova´1;*1, Filip Mika2;*2, Satoshi Shiojiri3;*3 and Makoto Shiojiri4 1Centre for Nanomaterial Research, Faculty of Science, Palacky University in Olomouc, Slechtitelu 11, 783 71 Olomouc, Czech Republic 2Institute of Scientific Instruments of the ASCR, v.v.i., Kra´lovopolska´ 147, 612 64 Brno, Czech Republic 3Matsubara Junior High School, Kyoto 604-8812, Japan 4Professor Emeritus of Kyoto Institute of Technology, 1-297 Wakiyama, Kyoto 618-0091, Japan 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. [doi:10.2320/matertrans.MB201001] (Received August 4, 2010; Accepted November 24, 2010; Published February 25, 2011) Keywords: butterfly, wing scale, Chrysozephyrus ataxus, Thermozephyrus ataxus, photonic crystal, field-emission scanning electron microscopy 1. Introduction charonda (S. charonda, the great purple emperor) butterfly and elucidated the origin of vivid iridescence in its wing.16,17) The characteristic pattern and vivid colouration of wing The iridescence is caused by the interference of incident light scales of butterflies as photonic crystals have recently reflected with a kind of blazed diffraction grating of the scale become the subject of wide interest for excellent tool to that has high efficiency in a shorter wavelength range of manipulate light. Optical microscopy studies by Mason1–3) 200 nm450 nm. and a review of electron microscopy observations by The structures of the photonic crystals of the wing scales Ghiradella4) guide us to the earlier works on the structural would be applicable to fine light manipulators such as colours of butterfly wings. Vukusic et al. gave an explanation reflection elements in laser diodes, especially in blue, purple, of the origin of the bright green colouration of the scales of an and ultraviolet laser diodes. In fact, some investigators have Indonesian male Papilio palinurus butterfly5,6) and also the produced replicas of the photonic structures of butterfly colouration of a Costa Rica male Ancyluris meliboeus scales using atomic layer deposition technique18–20) and a butterfly, called a ‘living jewel.7) They observed patches of biotemplate method.21) Therefore, the structural investiga- different unusual multilayered microstructures on their wing tions of the butterfly scales are still required for achieving scales. Biro´ et al.8) and Ve´rtesy et al.9,10) found a sponge-like tuneable photonic properties in the artificial scales. Although structure named ‘pepper-pot structure’, which exhibits there are about 180 000 species of Lepidoptera (butterflies photonic crystal-like behaviour and significantly reduces and moths), relatively few have been subject to detailed the penetration of blue light into the body, in the scales of examination of wing scale structure. a blue coloured butterfly in Lycaenidae subfamilies. Prum Chrysozephyrus ataxus Westwood, 1851 (C. ataxus)isa et al.11) investigated the physical mechanisms of structural species of butterflies which belong to the subfamily of colour production in twelve Lepidoptera species from four Theclinae in the family of Lycaenidae (Fig. 1). C. ataxus has families and reported that all the species but for one are another name of Thermozephyrus ataxus because Chryso- appropriately nanostructured to produce visible colours by zephyrus is a synonym of Thermozephyrus. It inhabits Japan coherent scattering. Beside these butterflies, the structural (where it is called ‘kirishima-midori-shijimi’) as well as colours of the wing scales have been reported on butterflies the southwest of China and the northwest district of the such as Colias eurytheme L.,12) Morpho cypris,13) Pontia Himalayas. Its wingspan is around 3842 mm. The dorsal protodice, Colias eurytheme,14) and Pieris rapae (small surfaces of the male wings are metallically glittering green- white).15) Recently, we have investigated the photonic crystal violet with very narrow sharp black borders, and their ventral structure of wing scales in a male Sasakia charonda surfaces are silver white with several pale brown specks and a few orange-ringed spots in the hindwing ends. The dorsal *1Present address: Sadovske´ho14, 61200 Brno, Czech Republic surfaces of the female’s forewings are dark brown with violet *2Corresponding author, E-mail: [email protected] marks, and their ventral surface are brown with white bands. *3Present address: Shishigatani-Goshonodan 17, Kyoto 606-8422, Japan In this paper, we observe wing’s scales in C. ataxus 298 J. Mateˇjkova´-Plsˇkova´, F. Mika, S. Shiojiri and M. Shiojiri Table 1 The wavelength P (nm), the full width at half-maximum wavelength FWHM (nm) and the reflectance rP (%) at the peaks on the reflectance/wavelength curves for the male and female C. ataxus butterflies, which curves were measured by Imafuku et al.22Þ UV and blue region Green region Sex Wing P; ðrPÞ FWHM P; ðrPÞ FWHM P; ðrPÞ FWHM Dorsal Green-blue 257Ã 35Ã 341 Æ 589Ã 547 Æ 4 125Ã M. (25Ã)(39Ã)(36Ã) Ventral White 254Ã 25Ã A higher reflectance which increases with (26Ã) wavelength from 27%Ã at 300 nm to 42%Ã at 700 nm. Violet marks 252Ã 30Ã 395 134Ã Dorsal in forewings (13Ã)(43Ã) Dark brown 245Ã 68Ã No peak in 300700 nm, and low (8Ã) reflectance which decreases from 12%Ã at F 700 nm to 4%Ã at 400 nm. White part Similar to the ventral wing surface of the male. Ventral Brown part 248Ã 68Ã No peak in 300700 nm, and low (10Ã) reflectance which decreases from 20%Ã at 700 nm to 6%Ã at 360 nm. The values with asterisk were estimated from the curves in Fig. 7 of Ref. 22). butterflies by high-resolution scanning electron microscopy broad UVA peak at 395 nm with an FWHM of 134 nm. (SEM) with the aid of optical microscopy, and discuss their A small reflectance of 412% over a range of the visible microstructures related to wing colours, comparing with rays accounts for the dark brown background in the female those of other butterflies, especially of the S. charonda. wings. Figures 2(a), (b) and (c) reproduce optical micrographs of 2. Samples and Experimental Procedure the dorsal scales of the male butterfly taken by reflected light, reflected and transmitted light, and transmitted light, respec- In the present experiment we used a male and a female tively. A part of the scale reflects strongly the metallically C. ataxus butterflies belonging to a subspecies of kirishi- glittering light, and the rest is dark brown as a result of maensis which had been reared from eggs sampled at absorbing almost the whole light, as clearly seen in Fig. 2(d). kaminyu, Shiga, Japan. SEM observations were performed in Figure 2(c) indicates the scales to be brown intrinsically with secondary electron detection mode in a range of 115 kV of a pigment, maybe brown-black eumelanin. the accelerating voltage, using a Hitachi SU6600 with a Figure 3(a) shows a low magnified SEM image of the Schottky field-emission gun and a JEOL JSM-6700F equip- dorsal scales on the male wing surface. A rise of near the ped with a cold field-emission gun. The wings were coated mark (a) is a vein, where the scales are accumulated but they with a sputtered gold layer about 10–20 nm thick to avoid seems almost the same in shape as the scales in the cell. The charging effects, similar to our previous observations16,17) or cell means ‘an elongated area extending from the base of the observed without coating at 1 keV of primary beam energy. wing, enclosed by veins’. The scales look curled, which are The hues of the scale’s colours were examined in an optical clearly seen in an enlarged image in Fig. 3(b). Figure 3(c) microscope (OM). shows ridges and cross ribs in a scale, and Fig. 3(d) shows an enlarged image of the ridges and ribs. From observed SEM 3. Results and Discussion images we estimated the widths of ridges (d1) and grooves (d2), the spacings between the ridges (d) and between cuticles The wings of C. ataxus butterflies observed in this (D), the number of the cuticles piled on the ridge (n), and experiment are shown in Fig.