Visible beyond Violet: How Butterflies Manage Ultraviolet

SIMPLE SUMMARY: Many animals, including insects, evolved sensitivity to ultraviolet light (UV). The presence of UV photoreceptors in the visual systems of many animals shows that UV-reflective traits are as important as other visible cues. Ultraviolet patterns on the surfaces of butterflies are among the most intricate UV-reflecting structures in the animal kingdom and they have been recognised and studied for many years. These patterns are often involved in intraspecific and interspecific interactions as signals of species identity or a cue reflecting the individual’s biological quality. In this review, we summarise the current knowledge about the significance of UV patterns in lepidopteran reproduction, including their role in mate choice and intrasexual competition. We focus on the mechanisms that produce UV colouration, discuss UV pattern variation in response to changing ecological factors, their adaptive function, and generally evaluate the evolutionary significance of communication in the ultraviolet spectrum. ABSTRACT: Ultraviolet (UV) means ‘beyond violet’ (from Latin ‘ultra’, meaning ‘beyond’), whereby violet is the colour with the highest frequencies in the ‘visible’ light spectrum. By ‘visible’ we mean human vision, but, in comparison to many other organisms, human visual perception is rather limited in terms of the wavelengths it can perceive. Still, this is why communication in the UV spectrum is often called hidden, although it most likely plays an important role in communicating various kinds of information among a wide variety of organisms. Since Silberglied’s revolutionary Communication in the Ultraviolet, comprehensive studies on UV signals in a wide list of genera are lacking. This review investigates the significance of UV reflectance (and UV absorption)—a feature often neglected in intra- and interspecific communication studies—mainly in Lepidoptera. Although the text focuses on various butterfly families, links and connections to other animal groups, such as birds, are also discussed in the context of ecology and the evolution of species. The basic mechanisms of UV colouration and factors shaping the characteristics of UV patterns are also discussed in a broad context of lepidopteran communication.