Photonic Amorphous I-WP-Like Networks Create Angle-Independent Colors in Sternotomis virescens Longhorn Beetles

Structural color originates from the interference of light with periodic structures that feature characteristic length scales on the order of the wavelength of visible light. Long-range order in photonic structures usually causes iridescence, and increasing disorder renders colors angle-independent. Random disorder distributes scattering intensity over all wavelengths, producing white in the absence of absorption. Various non-iridescent, vivid color patterns are found in Sternotomini longhorn beetles. Herein, Sternotomis virescens is investigated, where elytral scales produce a green-blue color pattern on otherwise black elytra. Combining focused ion beam scanning electron microscopy (FIB-SEM) tomography, ultra-small-angle X-ray scattering (USAXS), structural modeling, and full-wave optical simulations, it is found that the color originates from amorphous photonic networks based on sub-units resembling the I-WP unit cell, a triply-periodic minimal surface with body-centered-cubic symmetry. This work provides insights into how quasi-order produces stable colors in S. virescens longhorn beetles, highlights the advantages of volumetric imaging using FIB-SEM tomography of porous nanostructured materials, and raises interesting questions about the formation mechanisms of amorphous structures in vivo.