A Literal Elytral Rainbow: Tunable Structural Colors Using Single Diamond Biophotonic Crystals in Pachyrrhynchus congestus Weevils.

The brilliant colors of many insects arise from the interference of incident light with complex nanostructured biomaterials that are present in their cuticle. Here, the rainbow-colored spots on the elytra of a snout weevil, Pachyrrhynchus congestus pavonius (Coleoptera: Curculionidae), are investigated using synchrotron small-angle X-ray scattering, scanning electron microscopy, microspectrophotometry, and photonic bandgap modeling. It is shown that the iridescent scales present in the rainbow-hued spots are due to a 3D photonic crystal network of chitin in air with a single diamond (Fd-3m) symmetry. In many insects, different orientations of photonic crystal domains are used to create various hues. In this weevil, however, both the chitin volume fractions as well as the lattice parameters of the biologically self-assembled single diamond nanostructure are varied to achieve the remarkable tuning of the structural colors across the visible light spectrum on a scale-by-scale basis. Uncovering the precise mechanism of color tuning employed by this weevil has important implications for further structural and developmental research on biophotonic nanostructures and may provide fresh impetus for bioinspired and biomimetic multifunctional applications, as synthesis of photonic crystals at visible length scales is currently challenging.