Credit: A. R. PARKER ET AL.

Coloured beetles are of two basic types. Some have a layer of pigment in their exoskeleton (pigmentary colours), while others rely on the interaction of light with features in their integument (structural colours). These structurally reflecting colours can be very different — Calloodes grayanus (pictured right), for example, appears a weak green in colour, whereas Anoplognathus parvulus is a strong metallic gold. How are these different colours produced?

Writing in the Journal of Experimental Biology (201, 1307-1313; 1998), Andrew R. Parker and colleagues provide the answer. In both cases, the structural colour arises from so-called ‘multilayer reflectors’ — layers of a transparent material separated by layers with a different refractive index. Reflected beams from all of the layers interfere, and when this interference is constructive for a particular wavelength of light, that colour is observed.

But the similarity ends there. Parker et al. have found that the C. grayanus reflector is composed of regularly spaced layers of the same optical thickness, overlaid by an irregular transparent layer that scatters light. The resulting reflection is diffuse, appearing the same when viewed from any direction; this green colour seems to provide excellent camouflage against a leafy background.

The layers that make up the A. parvulus reflector, however, decrease in spacing (and consequently in optical thickness) with depth in the structure. The resulting metallic gold colour can be seen only from certain directions, but it makes A. parvulus very conspicuous nonetheless, perhaps allowing it to be recognized by members of the same species. Unfortunately, its striking vesture could also mark it as a tasty morsel for predators.