Multi-scale dissection of wing transparency in the clearwing butterfly Phanus vitreus

Abstract

Optical transparency is rare in terrestrial organisms, and often originates through loss of pigmentation and reduction in scattering. The coloured wings of some butterflies and moths have repeatedly evolved transparency, offering examples of how they function optically and biologically. Because pigments are primarily localized in the scales that cover a colourless wing membrane, transparency has often evolved through the complete loss of scales or radical modification of their shape. Whereas bristle-like scales have been well documented in glasswing butterflies, other scale modifications resulting in transparency remain understudied. The butterfly Phanus vitreus achieves transparency while retaining its scales and exhibiting blue/cyan transparent zones. Here, we investigate the mechanism of wing transparency in P. vitreus by light microscopy, focused ion beam milling, microspectrophotometry and optical modelling. We show that transparency is achieved via loss of pigments and vertical orientation in normal paddle-like scales. These alterations are combined with an anti-reflective nipple array on portions of the wing membrane being more exposed to light. The blueish coloration of the P. vitreus transparent regions is due to the properties of the wing membrane, and local scale nanostructures. We show that scale retention in the transparent patches might be explained by these perpendicular scales having hydrophobic properties.

Ridge and crossrib height of butterfly wing scales is a toolbox for structural color diversity

“…Variation in the thickness of this lower lamina across nymphalid butterflies changes the color hue produced by thin-film interference 13,14,15,16 . Similar thin-film structural coloration has been described in scales formed of a fused upper and lower lamina in extant butterflies 17,18 and primitive Lepidoptera 19,20 . This current simple model, however, is incomplete as it does not consider the contribution of the upper lamina (in non-fused scales) for color generation.…”

The BTB-ZF gene Bm-mamo regulates pigmentation in silkworm caterpillars

“…Pigments are synthesized by insects themselves and form solid particles that are deposited within the cuticle of the body surface and the scales of the wings ( Futahashi et al, 2010 ; Matsuoka and Monteiro, 2018 ). Interestingly, recent studies have shown that bile pigments and carotenoid pigments synthesized through biological synthesis are incorporated into the body fluids and fill in the wing membranes of two butterflies ( Siproeta stelenes and Philaethria diatonica ) via hemolymph circulation, providing color in the form of liquid pigments ( Finet et al, 2023 ). These pigments form colors by selective absorption and/or scattering of light depending on their physical properties ( Gürses et al, 2016 ).…”

Recent progress on structural coloration

“…Typically, the colors produced by humans, such as those seen in pigments or dyes in daily life, fall into this category [3,4] . Structural colors including butterfly wings [5–7] , bird feathers [8] , and beetle shells [9] provide an extraordinary alternative that nature has evolved. As depicted in Fig.…”