Not only a matter of disorder in I-WP minimal surface-based photonic networks: Diffusive structural color in Sternotomis amabilis longhorn beetles

Abstract

The diverse colors of longhorn beetles arise from either pigmentary absorption or interference of light with various photonic nanostructures. Variations in structure, order, and/or material composition give rise to diverse optical signals. Here, we investigated the colors of the longhorn beetle Sternotomis amabilis sylvia (Cerambycidae: Lamiinae). By combining optical microscopy and detailed bulk ultrastructural analysis of the colored scales that are the basis of the multicolored patterns of bluish-green and orange markings, we document polycrystalline networks based on the triply periodic minimal surface, Schoen’s I-WP, in the bluish-green scales. In contrast, amorphous quasi-ordered networks are found in the orange scales. The optical signal from the photonic networks is further altered by absorbing pigments. Ridged, micrometer-sized protrusions diffuse reflected light and suppress iridescence in all scale types. We discuss the pivotal role that order and disorder play in these photonic structures and support our understanding of the function of the scale geometry with full-wave optical simulations. Detailed knowledge about visible light interactions within intricate mediums, such as those observed in beetle scales, is highly relevant to current challenges in the design and synthesis of photonic nanostructures operating in the visible regime.