Structure and pigment make the eyed elater’s eyespots black

Victoria L Wong,Paul E Marek

doi:10.7717/peerj.8161

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.

Highlights

Black in nature is often achieved by pigments that absorb most visible light (Zhang et al, 2017; Hsiung, Blackledge & Shawkey, 2015)
Based on visual examination with the light microscope, A. oculatus is generally black with white irregularly-shaped spots speckled across the body
The beetle is generally clothed with V-shaped seta of varying hue and texture, and the irregularly-shaped spots, white eyeliner, and eyespots are made up of this seta

Summary

Introduction

Black in nature is often achieved by pigments (e.g., melanin) that absorb most visible light (Zhang et al, 2017; Hsiung, Blackledge & Shawkey, 2015). Black pigment is overlaid by a glossy surface thereby imparting specular reflection increasing at angles normal to the illumination source, for example in many beetles (Seago et al, 2009). Super black in butterflies, birds, and snakes is usually achieved by structural absorption (Vukusic, 2009) of most (≥99%) light. (But, there are instances of ‘‘pseudo’’-black achieved through additive mixing of structural green and magenta iridescence Seago et al, 2009). In some of these instances, super black structures evolved

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