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 nm 450 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 38 42 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 257 35 341 589 547 4 125 M. (25 )(39 )(36 ) Ventral White 254 25 A higher reflectance which increases with (26 ) wavelength from 27% at 300 nm to 42% at 700 nm. Violet marks 252 30 395 134 Dorsal in forewings (13 )(43 ) Dark brown 245 68 No peak in 300 700 nm, and low (8 ) reflectance which decreases from 12% at F 700 nm to 4% at 400 nm. White part Similar to the ventral wing surface of the male. Ventral Brown part 248 68 No peak in 300 700 nm, and low (10 ) reflectance which decreases from 20% at 700 nm to 6% 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 4 12% 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 1 15 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. 1, where (a) and (b) are the the spacing between the cross ribs (D0) (see Fig. 3(f)). The dorsal surfaces of a male and female wings, respectively. results are shown in Table 2, together with those of the Imafuku et al.22) measured wing colours of Chrysozephyrus iridescent blue scales of a male S. charonda17) and a male butterflies with a spectrophotometer, and reported that the E. mulciber.23) The big difference between the present dorsal wing surface of a male C. ataxus shows a strong C. ataxus and the other butterflies is the relative width of reflectance when the specimen is tilted and that it appears the ridge (d1=ðd1 þ d2Þ¼d1=d) and the number of cuticles green to the human eye, reflecting ultra violet (UV) as well as multiply piled on the ridges (n). The interference caused green light. Table 1 was made using their results, especially between cuticle-air layers on the narrow ridge is not so much the reflectance/wavelength curves displayed in Fig. 7 in effective for the colouration of the C. ataxus unlike those their paper. The green hues in Fig. 1(a) correspond to the of the male Ancyluris meliboeus,7) S. charonda,17) and reflection of the very broad peak at 547 nm with a full-width E. mulciber butterflies.23) Furthermore, the curl of the scale, at half-maximum (FWHM) of 125 nm, accordingly. The which is not observed in the scale of the other butterflies, iridescent violet marks in the female’s forewing in Fig. 1(b) limits the diffraction condition to a small area on the ridge, are a result of the reflection light belonging to the very consequently, greatly reducing the reflected light. Fine Structure of Wing Scales in Chrysozephyrus Ataxus Butterflies 299
Fig. 1 Optical microscope photographs of C. ataxus butterflies. (a) The dorsal surfaces of wings of a male butterfly. (b) The dorsal surfaces of wings of a female butterfly.
Fig. 2 Optical microscope images of the dorsal wing scales of male C. ataxus. (a) Image of the reflected natural light. (b) Image of the reflected and transmitted light. (c) Image of the transmitted light. (d) Enlarged image of a part in (a). (e) Enlarged image of P in (d). (f) Enlarged image of Q in (d).
The images in Figs. 2(d)–2(f) reveal that the vivid colour- the surfaces whose normal is parallel to the incident light. If ation of the male C. ataxus mainly comes from the flat areas the groove areas cause multi-reflection between the layers 0 in grooves enclosed by the ridges and cross ribs (d2 D ) with air gaps, the reflection is observed when the following (see Figs. 3(c) and (d)). Figure 3(e) shows an image of the interference condition is satisfied, scale cut with a razor at room temperature. The image allows us to observe a broken cross-sectional view as well as the top 2ðnctc þ nataÞ¼m p ð1Þ view of the scales, revealing that the flat groove area is where nc, and na, are the relative refractive index of the scales composed of multiply piled seven layers. This multilayered and air, and tc and ta are the thickness of the cuticle layer and arrangement is responsible for the metallically glittering the air gap, respectively. An integer of m is the order of green-violet of the male wings. In the measurement shown in interference and P is the wavelength of the reflected light. Table 1, the incident light was applied to a wing piece and We can take nc ¼ 1:55 as an appropriate value for the layer 13) the reflected light that returned in the same course was substance in the scales and na ¼ 1. Assuming m ¼ 1 for the 22) measured. It means that the reflectance measured was from observed reflection peak of P ¼ 547 nm, ð1:55tc þ taÞ¼ 300 J. Mateˇjkova´-Plsˇkova´, F. Mika, S. Shiojiri and M. Shiojiri
Fig. 3 SEM images of the dorsal wing scales of male C. ataxus. (a) law-magnified image. (b) curled scales. (c) Ridges and cross ribs in a scale. (d) Enlarged image of the ridges and ribs. (e) Cross-section of a groove area enclosed by the ridges and ribs, revealing seven piled layers (marked by number). (d) Schematic of the fine structure of the scale.
274 nm and the corresponding wavelengths of the reflection 2nctc ¼ðm þ 1=2Þ P ð2Þ for m ¼ 2 and m ¼ 3 must be 274 nm and 182 nm, respec- tively. However, the observed peaks did not appear at 274 nm Taking m ¼ 1 for P ¼ 547 nm and m ¼ 2 for P ¼ 341 nm, 22) and 182 nm in the spectrum measured by Imafuku. This we obtain thicknesses of tc ¼ 265 and 275 nm, respectively. suggests that the multiple layers in the grooves cannot be We thereby acquire a reasonable thickness of the flat groove considered as the structure for the multiple interference but layers to be t ¼ 270 nm. Although we could not estimate they must be incoherent with each other. This is natural the thickness from Fig. 3(e) because the layer surfaces were because their surfaces are not completely parallel to each not perpendicular to the imaging plane of SEM, it is not other. Then, we consider the interference for one layer, where completely inconsistent with the image. The observed UVC the reflection takes place by following condition: of P ¼ 257 nm is then regarded as the reflection of m ¼ 3. Fine Structure of Wing Scales in Chrysozephyrus Ataxus Butterflies 301
Table 2 The roughly estimated widths of ridges d1(mm) and grooves d2(mm), the spacings d(mm) between the ridges and D(mm) between cuticles, the number of the cuticles piled on the ridge. D0(mm) is the spacing between the cross ribs.
0 Scale d1 d1=d (%) d2 dDDn C. ataxus M. Green-blue 0:4 0:5 15 2.5 3 0:8 1:50:8 1:2 1
F. Violet 0:25 11 2:1 2:3 1:4 0:9 1 Dark brown 0:3 20 1:2 1:5 2:2 0:6 1 S. charonda 0:6 67 0:3 0:9 1:0 7 E. mulciber 0:3 0:4 35 0:6 0:70:9 1:1 0.5 3 Data cited from the paper by Mateˇjkova´-Plskova et al.17Þ Unpublished data obtained by Dechkrong et al.23Þ
Thus, each layer reflects incoherently or kinematically with colouration is not ascribed to the multilayer optical interfer- others, like a particle of the mosaic structure in X-ray ence but to the interference between the top and bottom diffraction, and the reflectance intensity is a simple sum of surfaces of each layer. The rays reflected from different the reflection from each layer. Imafuku et al.22) mentioned layers is incoherent with each other, accordingly. that the wing surface shows a strong reflectance when the As mentioned above, we ascribes the observed strong specimen is tilted. This is simply explained as a geometric metallic glitter on the wing to the 1st order reflection of the result of the increase of the area on curl scales which is green-violet light. According to Ou-Yang et al.25) the typical normal to the incident light and of the number of the scales absorbance spectrum of soluble eumelanin includes a linear irradiated by the incident light as the specimen is tilted. When increase of absorbance from 800 to 600 nm and an exponen- we observe the wing obliquely at a glancing angle of , the tial increase of absorbance from 600 to 300 nm. The strong strong reflection is obtained if the diffraction condition reflection comes from areas satisfying the interference condition in the curl scales. The parts being out of the 2nctc cos c ¼ðm þ 1=2Þ ð3Þ interference condition look dark due to the absorption of the is satisfied where a ¼ 90 and sin a= sin c ¼ nc. Since incident rays (see Fig. 2(a)). The transmitted light through 1 sin a ¼ nc sin c 1 and 0 cos c 1, taking these layers (and also through the ventral scales) exhibits the t ¼ 270 nm the satisfied wavelength is calculated as brown hues as a result of the absorption of shorter wave- 558 nm 426 nm for m ¼ 1 and 335 nm length rays (see Fig. 2(c)). 256 nm for m ¼ 2. The human eye perceives only the Figure 4(a) shows scales in a violet mark in the female’s reflected light in a range of 558 nm (green) > > dorsal forewing. A vein is observed near the upper right 426 nm (violet) nm in the glancing angle 180 0. corner. Some scales in the cell are curled and the others are Each scale is highly curved so that some part of the scale almost flat exhibiting the slit top tails. The image in Fig. 4(b) satisfies the interference condition and gives strong reflec- (and also Fig. 4(d)) comprises two scales; one has flat layers tion, as seen in Fig. 2(a). That is the colour of the male in the areas enclosed by the ridges and the ribs and the other C. ataxus butterfly in Fig. 1(a). has no layers. The former is a scale exhibiting violet hues, There is a report on the curl scales in the wing of a and the latter is a scale on the vein and similar to the scales Chrysiridia rhipheus (the Madagascan sunset moth) by exhibiting dark brown. Figure 4(c) shows an enlarged image Yoshioka and Kinoshita.24) From SEM observation of a thin of the scale exhibiting violet, whose structure is similar to width of d1 ¼ 0:4 mm and a large separation of d2 ¼ 3:5 mm that of the male’s scales in Fig. 3 although it has many holes. of the ridge they assumed that the reflection of the scale is As shown in Table 2, the width of the ridges d1 and the ratio mostly due to the basal layer of the scale, and from cross d1=d are small and n ¼ 1 so that the contribution of the sectional transmission electron microscopic images of a scale cuticles on the ridges to the colour hues would be small. As that the basal layer consists of air-cuticle alternate layers all seen in Fig. 4(e), the plate grooves also take multilayered over the scale. These scales have the air-cuticle multilayered structure of triple layers. We then estimated the thickness structure and also have a deep groove structure which is of the layers, similar to the calculation for the male wing, formed between adjacent two rows of the regularly arranged using the data shown in Table 1 and the eq. (2). The observed scales. They concluded that this groove structure together strong intensity of P ¼ 395 nm with rP ¼ 43% can be with multilayer optical interference produces an unusual regarded as the first order reflection (m ¼ 1) from the layer optical effect through an inter-scale reflection mechanism; with a thickness of tc ¼ 191 nm. We do not take tc ¼ thereby the wing color changes depending on light polar- 319 nm for m ¼ 2 because no reflection corresponding to ization. The colouration of the male C. ataxus dorsal wing is m ¼ 1 appeared at 659 nm in the experiment. The thin different from that of the Madagascan sunset moth although thickness of the layers is confirmed by 15 kV SEM image in they have similar basal flat areas between the ridges. Any Fig. 4(a), where the underlying scales are recorded through iridescence does not occur between adjacent two rows of the their upper scales as indicated by arrowheads. An FWHM as arranged scales in C. ataxus wing as seen in Fig. 2(a). large as 134 nm is attributed to a large variation of the Although the SEM image in Fig. 3(e) indicates multiple thickness among layers and the curl of the scales. In any case layers of cuticle on the basal flat, we concluded that the vivid the possible reflection in the overall angle 180 0 302 J. Mateˇjkova´-Plsˇkova´, F. Mika, S. Shiojiri and M. Shiojiri
Fig. 4 SEM images of the dorsal wing scales of female C. ataxus. (a) law-magnified image. (b) Ridges and cross ribs in a violet scale (left) and a dark brown scale. (c) Enlarged image of the ridges and ribs in the violet scale. (d) Enlarged image of a part of (b). (e) Cross-section of a groove area, revealing triple piled layers (marked by number). (f) Cross-section disclosing the structure beneath. occurs only for the rays in wavelength ranges of 395 nm is 12% at 700 nm and 4% at 300 nm shown in Table 1. The 301 nm (m ¼ 1) and 237 nm 181 nm absorbed incident light, particularly UV radiation, may be (m ¼ 2). Hence, the female wing’s marks look violet in the converted to heat energy by melanin. The female can hence range of human visibility. A cross-sectional image shown in get more heat energy, to be used for breeding, from sunshine Fig. 4(f) reveals how these groove layers are firmly support- than the male butterflies, as we previously assumed for the ed by props on the back skin in the framework of the scale. big brown wings of the S. charonda females.17) The dark brown scales on the background and on the veins Figure 5 shows the scale at the edge of the female’s wing. 0 have not any layers but windows of d2 D ¼ 1:5 mm The edge scales have some common characteristics with 0:6 mm between the ridges and between ribs (see Figs. 4(b) those of S. charonda16) and E. mulciber butterflies,23) re- and (d)). Although some of the incident light may be garding split tails (Fig. 5(a)), narrow spacings of ridges (d2) diffracted by the two-dimensional grid constructed from the and ribs (D0) and mono- or no cuticle layer on the ridges ridges and ribs, most of the light is absorbed in the lower (Table 1 and Fig. 5(b)). They make the scales strong and laminar containing the melanin. This is the reason of no peak cause a function of drains which leave out rainwater from in a range of 300 nm 700 nm and the low reflectance which wing surfaces quickly when the butterfly gets wet. The colour Fine Structure of Wing Scales in Chrysozephyrus Ataxus Butterflies 303
Fig. 5 SEM images of the scales at the edge of the female’s wing. (a) law-magnified image. (b) Ridges and cross ribs in a scale.
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