Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)

Abstract

Members of the order Coleoptera are sometimes referred to as 'living jewels', in allusion to the strikingly diverse array of iridescence mechanisms and optical effects that have arisen in beetles. A number of novel and sophisticated reflectance mechanisms have been discovered in recent years, including three-dimensional photonic crystals and quasi-ordered coherent scattering arrays. However, the literature on beetle structural coloration is often redundant and lacks synthesis, with little interchange between the entomological and optical research communities. Here, an overview is provided for all iridescence mechanisms observed in Coleoptera. Types of iridescence are illustrated and classified into three mechanistic groups: multilayer reflectors, three-dimensional photonic crystals and diffraction gratings. Taxonomic and phylogenetic distributions are provided, along with discussion of the putative functions and evolutionary pathways by which iridescence has repeatedly arisen in beetles.

Brilliant camouflage : photonic crystals in the diamond weevil, Entimus imperialis

“…Photonic crystals with a diamond structure seem to be restricted to beetles and weevils (Coleoptera) [1], whereas photonic crystals with a gyroid structure have so far only been encountered in lycaenid and papilionid butterflies [6,25,27]. In nascent butterfly wing scales, specific interactions of the cell plasma membrane and the intracellular smooth endoplasmic reticulum initiate the development of single gyroid networks by cubic membrane folding via a double gyroid intermediate [27,37].…”

“…The colorations of the elytra of weevils and beetles are especially diverse [1,2]. Here, we investigate the diamond weevil, Entimus imperialis, which is endemic to the neotropical regions, especially southwest Brazil [31].…”

“…Structural (or physical) coloration is due to nanometre-sized structures with periodically changing refractive indices, causing coherent light scattering. Pigmentary coloration is by far the most common in the animal kingdom, but structural coloration is widely encountered as well, and not seldom structural colours are modified by spectrally filtering pigments [1,2,9].…”

“…Animal coloration is due to spectrally selective light reflections on the outer body parts [1,2]. The resulting coloration mostly serves a biological function in intraand interspecific signalling [3,4], thus improving mating chances [5], but it can also be used for camouflage in the animal's native habitat [6 -8].…”

An introduction to biomimetic photonic design

“…E. magnificus exhibits a chitin matrix within its scales which is periodically perforated with air holes (figure 2). This 3D structure acts in a similar manner to a 1D structure, a coloured reflection results from coherent scattering from the periodically-arranged scattering centres that make up the crystal [3]. Each scale comprises discrete domains; these are regions where the orientation of the same 3D crystalline structure is changed.…”