Switchable reflector in the Panamanian tortoise beetleCharidotella egregia(Chrysomelidae: Cassidinae)

Abstract

The Tortoise beetle Charidotella egregia is able to modify the structural colour of its cuticle reversibly, when disturbed by stressful external events. After field observations, measurements of the optical properties in the two main stable colour states and SEM and TEM investigations, a physical mechanism is proposed to explain the colour switching on this insect. It is shown that the gold colouration (rest state) arises from a chirped multilayer reflector maintained in a perfect coherent state by the presence of humidity in the porous patches within each layer, while the red colour (disturbed state) results from the destruction of this reflector by the expulsion of the liquid from the porous patches, turning the multilayer into a translucent slab that leaves a view on a deeper-lying pigmental red substrate. This mechanism not only explains the change of hue but also the change of scattering mode from specular to diffuse. Quantitative modelling is developed in support of this analysis.

Mechanism of variable structural colour in the neon tetra: quantitative evaluation of the Venetian blind model

“…the Hercules beetle (Dynastes hercules, [28,29]) and the Panamanian tortoise beetle (Charidotella eregia, [30]), physiologically control the amount of water in porous three-dimensional structures in their elytra. The infiltration or expulsion of water causes colour variation, since the refractive index of the pores is changed and, consequently, the amounts of light scattered, reflected and absorbed are altered.…”

Tunable structural color in organisms and photonic materials for design of bioinspired materials

“…The tortoise beetle can reversibly change its shell color from metallic yellow to a more diffusive red within 90 s when disturbed [33]. Its shell contains a double layer bearing the structural color in the outer part and red pigment in the inner part.…”

Natural photonics for industrial inspiration

“…Multilayers produce optical effects in living and fossil beetle cuticle from highly metallic colours ('ideal' system) to rather dull greens (non-ideal system in combination with scattering; Parker et al 1998a;Vigneron et al 2005Vigneron et al , 2007. They are also responsible for the colours reflected from the wings of many butterflies , where layers of chitin (nZapprox.…”

“…Parker 1999b; Vigneron et al 2007). However, this means of camouflage can only be achieved in an environment with diffuse light to prevent a strong, direct reflection from the Sun.…”