Bouligand Structures Underlie Circularly Polarized Iridescence of Scarab Beetles: A Closer View

Abstract

The iridescent metallic green beetle, Chrysina gloriosa, selectively reflects left circularly polarized light, and exhibits iridescence. The exoskeleton of C. gloriosa is decorated by a polygonal texture: hexagonal cells (∼10 micron) coexist with pentagons and heptagons. We find that the fraction of pentagons and heptagons computed using Voronoi analysis increase with an increase in curvature, implying that it is energetically favorable to create disordered patterns for higher curvature surfaces. In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the size of the core and spatial distribution of color changes when the angle of incoming incident light is varied. Using confocal microscopy, we observe that these cells consist of nearly concentric, nested arcs that lie on surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. In this paper, we describe in detail how the microstructure referred to as Bouligand structure provides the basis for the morphogenesis, and for generating the intricate optical response of the exoskeleton of the beetle.