A study of the morphology and mechanism behind color change in Eurypepla calochroma (Coleoptera: Chrysomelidae)

The ability of changing color is a phenomenon documented across Coleoptera. However, it appears to be most dramatic in members of Cassidinae (Chrysomelidae). Thus far, a color changing mechanism has been studied in the cassidine species Charidoella egregia. This species has a metallic gold resting state while it changes to a dull red when porous patches in the cuticle dry, revealing the red-pigmented epidermis below. Another type of color change is observed in the cassidine species Eurypepla calochroma.

Populations of two morphotypes within this species were found, one consisting of a pale yellow cuticle with brown spots and another of individuals with a metallic green cuticle. The metallic green individuals have a metallic green resting state and change heterogeneously to a metallic blue cuticle; however, no color change was observed in the pale yellow individuals with brown spots. In order to compare the mechanism of this color change in Eurypepla calochroma, a field study was performed in South Florida for collecting and observation. Metallic green individuals changed their dorsal color distinctly to metallic blue within 2–20 minutes after physical disturbance. This study documents the pattern of color change and implements various microscopical techniques in comparative morphology. Based on histological sections of the pronotum and elytra, the metallic green color is derived from an endocuticular multilayer reflector in combination with a hydrated cuticle. A similar condition is found in Charidotella egregia, although the multilayer reflector is based in the exocuticle. In the pale yellow morphotype individuals with brown spots, this endocuticular multilayer reflector is absent. However, unlike Charidotella egregia, the color change of metallic green to blue in Eurypepla calochroma is hypothesized to be caused by changes in hemolymph pressure exerted in the cuticular layers.