Jewel beetles, Buprestidae, modulate bright structural colours by multileveled optical engineering and surface sculpting

The elytra and pronotum of numerous Buprestidae, or jewel beetles, are brightly coloured due to stacked chitin and melanin plates that together act as optical multilayer reflectors. The difference in refractive index of chitin and melanin results in refractive index profiles, the dimensions of which determine the local reflectance spectra and thus the beetles' colour pattern. Building on the exemplary case of the jewel beetle Chrysochroa fulgidissima, the refractive index profiles of several buprestid species were derived from transmission electron micrographs. The profiles were implemented in a matrix transfer model that allowed calculation of the multilayers' reflectance spectra, which were compared with measured spectra. The elytral surface of jewel beetles varies from smooth to highly sculpted. With a smooth surface, the elytra act as specular reflectors, but undulating surfaces cause spatially broadened reflections, resulting in a matte appearance, as is particularly the case in Chysochroa gratiosa, an extreme example of elytra with strongly sculpted, multilayered elytra.