Diffusive structural colour in Hoplia argentea

Abstract

Nature's nanostructures can bring about vivid and iridescent colours seen in many insects, notably in beetles and butterflies. While the intense structural colours can be advantageous for display purposes, they may be also appealing to predators and therefore constitute an evolutionary disadvantage. Animals often employ absorption and scattering in order to reduce the directionality of the reflected light and thereby enhance their camouflage. Here, we investigate the monkey beetle Hoplia argentea using microspectrophotometry, electron microscopy, fluorimetry and optical modelling. We show that the dull green dorsal colour comes from the nanostructured scales on the elytra. The nanostructure consists of a multi-layered photonic structure covered by a filamentous layer. The filamentous layer acts as a spatial diffuser of the specular reflection from the multilayer and suppresses the iridescence. This combination leads to a colour-stable and angle-independent green reflection that likely enhances the camouflage of the beetles in their natural habitat.