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

Abstract While scales ensure hydrophobicity in opaque Lepidoptera, their reduction in clearwing species question hydrophobicity and its dependency on wing microstructures (scale presence, morphology, insertion, coloration) and nanostructures (scale ridge features). By measuring static contact angle (CA) of water droplets at different stages of evaporation in opaque and transparent patches of 23 clearwing species with varied microstructure, we establish for the first time that transparency is costly for hydrophobicity, a cost depending on microstructure: transparent patches lose more hydrophobicity with water evaporation than opaque patches, a loss 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. Models best predicting CA include microstructures for all scales, and nanostructures only for flat scales. We reveal for the first time that wing hydrophobicity negatively relates to optical transparency. Finally, tropical species have more hydrophobic transparent patches but similarly hydrophobic opaque patches compared to temperate species. All findings are consistent with the physics of hydrophobicity, especially on multiscale roughness, and underline that wings are evolutionary compromises.