Switchable reflector in the Panamanian Tortoise Beetle Charidotella 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.