Structure and pigment make the eyed elater's eyespots black

PeerJ. 2020 Jan 13;8:e8161. doi: 10.7717/peerj.8161. eCollection 2020.

Abstract

Surface structures that trap light leading to near complete structural absorption creates an appearance of "super black." Well known in the natural world from bird feathers and butterfly scales, super black has evolved independently from various anatomical structures. Due to an exceptional ability to reduce specular reflection, these biological materials have garnered interest from optical industries. Here we describe the false eyes of the eyed elater click beetle, which, while not classified as super black, still attains near complete absorption of light partly due to an array of vertically-aligned microtubules. These cone-shaped microtubules are modified hairs (setae) that are localized to eyespots on the dorsum of the beetle, and absorb 96.1% of incident light (at a 24.8° collection angle) in the spectrum between 300-700 nm. Filled with melanin, the setae combine structure and pigment to generate multiple reflections and refractions causing light to travel a greater distance. This light-capturing architecture leaves little light available to receivers and the false eyes appear as deep black making them appear more conspicuous to predators.

Keywords: Aposematic; Beetle; Color; Deimatic; Eyespot; Melanin; Scattering; Spectrum; Startle; Super black.

Grant support

This work was supported by the National Science Foundation (DBI # 1458045), and by Virginia Tech, Department of Entomology, College of Life Sciences, and by a USDA NIFA Hatch Project (VA-160028). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.