Species‐dependent microarchitectural traits of iridescent scales in the triad taxa of Ornithoptera birdwing butterflies

Abstract

Ornithoptera birdwing butterflies have blue, green, or orange iridescent scales in different species or subspecies. To understand the species-or subspecies-dependent scale color differences, we performed comparative morphometric analyses of iridescent scales from three closely related taxa: O. priamus priamus (green), O. priamus urvillianus (blue), and O. croesus (orange). The three types of Ornithoptera wings exhibited reversible color changes to longer wavelengths with different kinetics upon immersion in methanol, suggesting that their color differences are at least partly based on differences in the size of air cavities made by nanostructures. Cover scales of all three color types were visually semi-transparent glass scales that exhibited color when placed on a dark background. The dorsoventral differences in coloration were observed in single scales, suggesting the optical importance of scale surfaces. Scanning electron microscopy of cover scales in cross section revealed that all color types exhibited finely sculpted tapered ridges and thick, irregular basal multilayers containing tandemly clustered granular objects and air cavities. Scale thickness, ridge height, and multilayer thickness were significantly different among the three color types, and granular object size was significantly different between orange scales and blue and green scales. We conclude that each of the three taxa of Ornithoptera butterflies possesses unique quantitative size values on tapered ridges and irregular multilayers with granular objects and air cavities to express unique structural color. These species-or subspecies-dependent structural colors might have evolved via quantitative shifts in these microarchitectural traits rather than via changes in the basic developmental or architectural plan for color expression.

Self-Similarity, Distortion Waves, and the Essence of Morphogenesis: A Generalized View of Color Pattern Formation in Butterfly Wings

“…Our laboratory also obtained similar results using Junonia and other butterflies (Kusaba and Otaki 2009;Iwata and Otaki unpublished data;Kazama et al 2017). Fig.…”

The Height of Chitinous Ridges Alone Produces the Entire Structural Color Palette

“…[10] We focused on the variation of the ridge height [11] inspired by recent evidence supporting the idea that the alteration of the vertical dimension (i.e., the thickness of the lower lamina) is a previously unconsidered evolutionary strategy to explore different colorations in butterflies. [12][13][14][15] The structures produced in the synthetic photocurable resin were transferred to a silicone elastomer through soft lithography and transformed into chitinous structures through a…”

“…[ 10 ] We focused on the variation of the ridge height [ 11 ] inspired by recent evidence supporting the idea that the alteration of the vertical dimension (i.e., the thickness of the lower lamina) is a previously unconsidered evolutionary strategy to explore different colorations in butterflies. [ 12–15 ]…”

Feather morphological predictors of angle-dependent color changes in parrot plumage

“…60,61 We urge more researchers with interests in feather color mechanisms to carefully investigate surface variations (i.e. using scanning electron microscopy), as has been done well in the literature on color mechanisms in butterfly wing 62,63 and spider facial 64 scales.…”