Bright, iridescent beetles are surprisingly well camouflaged

THE JEWEL beetle is so called for a reason. When illuminated, the insect’s carapace dazzles with a sparkling green prismatic spray. That this bright colouration sends some sort of signal to the world has long seemed obvious. Most researchers assume that the signal in question is either a warning to predators or a display of fitness to mates. Work published this week in by Karin Kjernsmo at the University of Bristol suggests, however, that the jewel beetle’s iridescence is neither of these. It is instead an “antisignal”, intended not to show the animal off, but to hide it.Much animal colouration is the product of pigments. A jewel beetle’s hue is different. It is generated by tiny structures that diffract light falling on them, creating an interference pattern from which colours emerge. Dr Kjernsmo questioned the idea that jewel beetles and species like them use their colours to attract mates, because such sexual selection usually results in only the male being brightly coloured, rather than both sexes, as with jewel beetles. She also questioned the idea that beetle colouration is a warning. For such a warning to work an organism must either be toxic to consume or dangerous to approach. Jewel beetles are neither. Nor do they mimic any known creature with these properties, which is the other way that something which looks like warning colouration can evolve (albeit as a bluff, rather than the real thing). There is, however, a third variety of defensive colouration. This is crypsis, in which colour acts as camouflage.