Hydrophobicity in clearwing Lepidoptera: impact of scale micro and nanostructure, and trade-off with optical transparency

Abstract

In opaque butterflies and moths, scales ensure vital functions like camouflage, thermoregulation, and hydrophobicity. Wing transparency in some species - achieved via modified or absent scales - raises the question of whether hydrophobicity can be maintained and of it dependence on scale microstructural (scale presence, morphology, insertion angle, and coloration) and nanostructural (ridge spacing and width) features. To address these questions, we assessed hydrophobicity in 23 clearwing species differing in scale micro and nanofeatures by measuring static contact angle (CA) of water droplets in the opaque and transparent patches of the same individuals at different stages of evaporation. We related these measures to wing structures (macro, micro, and nano) and compared them to predictions from Cassie-Baxter and Wenzel models. We found that overall, transparency is costly for hydrophobicity and this cost depends on scale microstructural features: transparent patches are less hydrophobic and lose more hydrophobicity with water evaporation than opaque patches. This loss is attenuated for higher scale densities, coloured scales (for erect scales), and when combining two types of scales (piliform and lamellar). Nude membranes show lowest hydrophobicity. Best models are Cassie-Baxter models that include scale microstructures for erect scales, and scale micro and nanostructures for flat scales. All findings are consistent with the physics of hydrophobicity, especially on multiscale roughness. Finally, wing hydrophobicity negatively relates to optical transparency. Moreover, tropical species have more hydrophobic transparent patches but similarly hydrophobic opaque patches compared to temperate species. Overall, diverse microstructures are likely functional compromises between multiple requirements.

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

“…An ultimate, adaptive hypothesis would be that retaining scales in transparent wing regions aids in generating structural colours [ 12 ], self-cleaning [ 65 ], aerodynamics [ 66 ] and thermoregulation [ 67 ]. Here, we demonstrate that the upright scales of the transparent regions in P. vitreus confer increased hydrophobicity to the wing, a function that might be generalized to most clearwing butterflies and moths [ 68 ].…”