Colouration principles of nymphaline butterflies - thin films, melanin, ommochromes and wing scale stacking

Abstract

The coloration of the common butterflies Aglais urticae (small tortoiseshell), Aglais io (peacock) and Vanessa atalanta (red admiral), belonging to the butterfly subfamily Nymphalinae, is due to the species-specific patterning of differently coloured scales on their wings. We investigated the scales' structural and pigmentary properties by applying scanning electron microscopy, (micro)spectrophotometry and imaging scatterometry. The anatomy of the wing scales appears to be basically identical, with an approximately flat lower lamina connected by trabeculae to a highly structured upper lamina, which consists of an array of longitudinal, parallel ridges and transversal crossribs. Isolated scales observed at the abwing (upper) side are blue, yellow, orange, red, brown or black, depending on their pigmentation. The yellow, orange and red scales contain various amounts of 3-OH-kynurenine and ommochrome pigment, black scales contain a high density of melanin, and blue scales have a minor amount of melanin pigment. Observing the scales from their adwing (lower) side always revealed a structural colour, which is blue in the case of blue, red and black scales, but orange for orange scales. The structural colours are created by the lower lamina, which acts as an optical thin film. Its reflectance spectrum, crucially determined by the lamina thickness, appears to be well tuned to the scales' pigmentary spectrum. The colours observed locally on the wing are also due to the degree of scale stacking. Thin films, tuned pigments and combinations of stacked scales together determine the wing coloration of nymphaline butterflies.

Artificial selection for structural color on butterfly wings and comparison with natural evolution

“…The measured absorbance spectra of our various scale types (Fig. 3D) do not match the melanin absorbance spectrum exactly (30,32,33), indicating that brown scales in B. anynana either do not contain melanin or, more likely, contain additional pigments besides melanin. Because the type and amount of pigments that exist in the lower lamina of scales are not known, we cannot accurately estimate the change of refractive index value caused by pigmentation.…”

“…Previous studies on butterflies showed that structural color can be produced by interference with light reflected from the overlapping lamella that build the longitudinal ridges, from microribs protruding from the sides of the longitudinal ridges, or from the lower lamina, which can vary in thickness and patterning (Fig. 1J) (29,30). However, it is not clear how the violet/blue color is produced in members of the two Bicyclus clades that separately evolved this color, whether B. anynana can be made to evolve the same violet/blue color via artificial selection, and whether it will generate the color in the same way as the other species.…”

Spiders have rich pigmentary and structural colour palettes

“…In reflected light, both pigmentary and structural colours are visible, but only pigmentary colours are visible under transmission illumination (Land et al, 2007;Stavenga et al, 2014;Vukusic et al, 1999).…”

It's Not a Bug, It's a Feature: Functional Materials in Insects

“…Each different morphology changes the way light interacts with the structure, as do local defects and disorder. Insect nanomorphologies range from ordered structures starting from thin films [309][310][311] and multilayer structures [277,282,304,312,313] to 3D photonic crystals to quasi-ordered and fully disordered structures, [314][315][316][317][318][319][320] each with different optical properties.…”