Abstract
Transparent animals are often thin, which raises the question of their fragility. In Ithomiine butterflies, a tribe with closely-related opaque and transparent toxic species, we explore the variations of wing membrane thickness and interpret these variations as potentially selected by optical and mechanical constraints. We find that species with partially transparent wings have a thinner membrane in the transparent zone than in the opaque one, which likely helps light getting through. Yet, we find that more transparent species have a thicker membrane in their transparent zone, and that species with a higher wing proportion occupied by transparency have a thicker membrane in their opaque zone, and a greater difference in thickness between opaque and transparent zones. These results agree with predominant mechanical constraints selecting for the maintenance of overall wing resistance, especially in species with a large wing proportion occupied by transparency. Although optical constraints are likely at play, the influence of membrane thickness on light transmission is likely limited, and the thicker membrane in more transparent species is likely offset by the antireflective effect of nanostructures which are present in all of these species. Despite their transparency, the fragility of the wing membrane in these species is likely limited.