Convergence in light transmission properties of transparent wing areas in clearwing mimetic butterflies

Müllerian mimicry is a positive interspecific interaction, whereby co-occurring defended prey species share a common aposematic signal that advertises their defences to predators. In Lepidoptera, aposematic species typically harbour conspicuous opaque wing colour pattern, which have convergent optical properties, as perceived by predators. Surprisingly, some aposematic mimetic species have partially or totally transparent wings, which raises the question of whether optical properties of such transparent areas are also under selection for convergence. To answer this question and to investigate how transparency is achieved in the first place, we conducted a comparative study of optics and structures of transparent wings in neotropical mimetic clearwing Lepidoptera. We quantified transparency by spectrophotometry and characterised clearwing microstructures and nanostructures by microscopy imaging. We show that transparency is convergent among co-mimics in the eyes of predators, despite a large diversity of underlying micro- and nanostructures. Notably, we reveal that nanostructure density largely influences light transmission. While transparency is primarily produced by modification of microstructure features, nanostructures may provide a way to fine-tune the degree of transparency. This study calls for a change of paradigm in transparent mimetic lepidoptera: transparency not only enables camouflage but can also be part of aposematic signals. Significance Transparency in animals has long been associated to camouflage, but the existence of aposematic mimetic Lepidoptera with partly transparent wings raises the question of the role of transparency in aposematism. Here, we undertake the first comparative analysis of transparency features in mimetic Lepidoptera. We show that transparency is likely part of the aposematic signal, as light transmission properties are convergent among co-mimics. We also reveal a high diversity of wing structures (scales and wing membrane nanostructures) underlying transparency, which enables fine-tuning the degree of transparency. This study, at the interface between physical optics and evolutionary biology, sheds light on the evolution of transparency in aposematic mimetic lineages and may promote bioinspired applications for transparent materials such as antireflective devices.