Coloration using higher order optical interference in the wing pattern of the Madagascan sunset moth

Abstract

Colour patterns of animals' bodies are usually produced by the spatial distribution of pigments with different colours. However, some animals use the spatial variation of colour-producing microstructures. We have studied one distinctive example of such structurally produced colour patterns, the wing of the Madagascan sunset moth, to clarify the physical rules that underlie the colour variation. It is known that the iridescent wing scale of the sunset moth has the alternate air-cuticle multilayer structure that causes optical interference. The microscopic and optical investigations of various parts of the wing have confirmed that the thickness of the cuticle layers within the scale largely varies to produce the colour pattern. However, it varies in very different ways between the dorsal and ventral sides of the hind wing; the thickness gradually varies on the dorsal side from scale to scale, while the abrupt changes are found on the ventral side to form distinctive borders between differently coloured areas. It is also revealed that an unusual coloration mechanism is involved in the green part of the ventral hind wing: the colour is caused by higher order optical interference of the highly non-ideal multilayer structure. The physical mechanism of the colour pattern formation is briefly discussed with the several mathematical models proposed so far.

Figure 1

The wings of the Madagascan sunset moth from (a) dorsal and (b) ventral view. The eight areas with different colours indicated by red arrows are named from C1 to C8. (c) The close-up view of the area between green and purplish red areas that is indicated by the red square in (b). The arrows indicate the position of the scales of which reflectance is shown in figure 6. Scale bar, 2 mm.

Figure 2

TEM images of the longitudinal cross section of the iridescent cover scales of the eight areas named from C1 to C8 as shown in figure 1. Scale bar, 1 μm.

Figure 3

Reflectance of the green area named C5 in the ventral hind wing. A UV–Vis–IR spectrometer was employed for the measurement in this wide wavelength range.

Figure 4

Reflectance of six different single scales each for the eight areas of the wings shown in figure 1. The spectrum was measured by the combination of an optical microscope and fibre-optic spectrometer. The six scales examined are located next to each other in most cases. Since both the intensity of the light source and the detection efficiency become lower, the spectra look noisy in the longer wavelength range.

Figure 5

Spatial variation of the wavelength λ_p due to the first-order constructive interference depicted as the height of the red bars at various positions of (a) the dorsal hind, (b) the ventral hind, (c) the dorsal front and (d) the ventral front wings. These bars are drawn such that their heights are proportional to (λ_p−500) nm at their positions. The subtraction of 500 nm is simply for the emphasis of the difference. The wavelengths λ_p are determined as the average of the peak position in the reflectance spectra shown in figure 4 and from other spectroscopic measurements at various parts of the wings. When the first-order reflectance peak is out of the range of the fibre-optic spectrometer, it is estimated as double the wavelength of the second-order peak. The red bar on the r.h.s. corresponds to 500 nm.

Figure 6

Reflectance of seven single scales that are located in the region between the green and purplish red areas of the ventral hind wing. The spectrum was measured by the combination of an optical microscope and fibre-optic spectrometer. The numbers show the positions of the scales examined, as shown in figure 1c.

Figure 7

Chromatic diagram for the reflected light from the multilayer structure as a function of thickness of the cuticle layer. Reflectance is theoretically calculated by the recursive method. The locus at (x, y)=(0.24, 0.50) in the green region is for the multilayer structure observed in the scale in area C5, which consists of four cuticle and three air layers with the thicknesses d_c=270 nm and d_a=130 nm with refractive indexes of 1.55 and 1, respectively. The other loci are calculated for the multilayer structures of which cuticle layer thickness d_c is gradually decreased from 270 to 121.5 nm with intervals of 13.5 nm, keeping other parameters constant.

Figure 8

Theoretically calculated reflectance of the multilayer structure with the different degrees of ‘non-ideal’ character, keeping the wavelength of the first-order peak constant at 1110 nm. The figures on the r.h.s. indicate the ratio of the optical thicknesses of the cuticle and air layers inside the multilayer structures. The ratio 1.5 : 0.5 is the case in area C5 in the ventral hind wing.