How butterfly wings use living colour

The brilliant colours of a butterfly's wing are generated in the same way as the high-definition pictures of the trendiest plasma-screen TVs, scientists have found.

For the past 30 million years, African swallowtail butterflies have used the principles of light-emitting devices to generate the vivid colours of their wings, says Peter Vukusic, a physicist at Exeter University.

A new study shows butterflies' wings are coated with an ultra-thin layer of molecules that form microscopic air spaces where fluorescent pigments absorb ultraviolet light and re-emit it as vivid patches of blue and green.

The air spaces themselves have complex, multi-layered mirrors at the bottom to force light out through the top surface of the wings. Further analysis, published in the journal Science, has found that the airspaces are arranged in such a precisely uniform manner that the fluorescent light is prevented from leaking out sideways, to make sure the colours are even brighter and clearer.

Dr Vukusic said that the butterfly had essentially invented a biological version of the light-emitting diode (LED) millions of years before they were developed by electrical engineers. "It's amazing that butterflies have evolved such sophisticated design features which can so exquisitely manipulate light and colour. Nature's design and engineering is truly inspirational," Dr Vukusic said.

The wings of African swallowtails are coated in micro-scales - regular air-pockets within the stiff material that makes up the insect's external skeleton. "The function of the micro-scales is identical to those in the LED; they prevent the fluorescent colour from being trapped inside the structure and from being emitted sideways," Dr Vukusic said.

"The scales on the wing also have a specialised mirror underneath, again very similar in design to that in the LED. This mirror upwardly reflects all the fluorescent light that gets emitted down towards it."

The result is a very efficient system for the fluorescent emission of light that gives the butterfly significant control of the direction in which the light is emitted, Dr Vukusic said.

"The fact that nature and technology have converged on this pretty analogous device is amazing," he said.