New research has found that the vivid and colourful iridescence of beetles actually works as a form of disruptive camouflage.

Experiments proved that, counterintuitively, iridescence does make beetles more difficult to see for predators and humans in a natural environment.

Iridescence is a form of "structural colouration." Rather than coming from pigments, it's produced by nanostructures on scales, feathers or an exoskeleton that refracts light at different wavelengths. This produces the familiar shimmery metallic sheen in colours like green, red, blue and purple. It's known in a range of animals, including birds like peacocks and pigeons, molluscs and many insects.

The many colours of the jewel beetle. (Karin Kjernsmo)

The colourful jewel beetle

In birds, where colourful iridescence is a feature of male plumage, it plays an important role in sexual display and mating. The function is less clear in insects such as the jewel beetle, where it is thought not to be a sexual characteristic because both sexes are iridescent.

This has presented a bit of a puzzle, as it was thought the bright, shimmering colours would seem to make insects more obvious to predators, and thus be maladaptive.

Karin Kjernsmo, an evolutionary and behavioural ecologist from the University of Bristol, in England, set out to test a theory proposed over 100 years ago, that suggested iridescence was in fact a form of camouflage.

Making sense of iridescence

Kjernsmo and her colleagues devised a field experiment in which they manufactured model beetles by attaching colourful beetle wing cases to a tasty mealworm, which they then placed on leaves and plants at their study site. They also put out a group of fake beetles with wing cases in different non-iridescent, solid colours.

Through an experiment, evolutionary and behavioural ecologist Karin Kjernsmo found that predators were less successful at finding iridescent beetles, and that they were attacked far less than solid but ordinary coloured beetles. (Amanda Buckiewicz)

By monitoring predation by birds on these false beetles, they found that the predators were indeed less successful in finding the iridescent beetles. They were attacked far less than all the other solid but ordinary coloured beetles in the experiment, even a leaf-matching green one.

Humans join the hunt

To eliminate the possibility that the birds were simply avoiding, rather than missing, the iridescent beetles, human volunteers were asked to go looking for their model beetles. Human eyes largely missed the iridescent beetles as well. Only 17 per cent of iridescent beetles were found compared to over 80 per cent of the solidly coloured beetles.

A jewel beetle pendant. (Amanda Buckiewicz)

This all supports the surprising conclusion that iridescence in these vividly coloured beetles is a kind of camouflage. Kjernsmo characterizes it as a "dynamically disruptive camouflage," which really means the camouflaged object can't be perceived clearly.

The iridescence distorts light in a way that shape and size of an object cannot be perceived. The predator is confused because it can't really tell what it is looking at, and tends to not recognize the beetle as food.

The iridescent jewel beetle (University of Bristol)

Written and produced by Mark Crawley.