A closer look at seemingly drab, transparent insect wings has revealed realms of previously unappreciated colour, visible to the naked eye yet overlooked for centuries.
Until now, the wing colours of many flies and wasps were dismissed as random iridescence. But they may be as distinctive and marvelous as the much-studied, much-celebrated wings of butterflies and beetles.
"Given favourable light conditions, they display a world of brightly patterned wings that are apparently unnoticed by contemporary biologists," wrote researchers led by University of Lund entomologists Ekaterina Shevtsova and Christer Hansson in a December 3 Proceedings of the National Academy of Sciences paper.
Wasp and fly wings are made from two compressed layers of transparent chitin, with light bouncing off both layers and mixing to produce colour. The same is true of oil slicks and soap bubbles, and scientists considered transparent wing coloration "a soap bubble iridescence effect, with randomly changing colours flashing over the wing surface," wrote the researchers.
Instead, the researchers found that surface variations in chitin filtered out the iridescence. Remaining colours proved to be stable, and were visible from almost any angle. They differed consistently between species and sex.
Generations of biologists seem to have missed this partly because they didn't look for it, and partly because the colours are most evident against a dark background. Against a white background, they're invisible - which is exactly how most entomologists study transparent wings.
"You hold the wing up against the light, so you can see the veins," said study co-author Daniel Janzen, an evolutionary ecologist at the University of Pennsylvania. "If you're looking through a microscope, you try to get a clear view behind the wing. It's the antithesis of getting wing colour."
The researchers studied wings under microscopes, against black backgrounds. But once Janzen, who breeds wasps for his research on caterpillar-parasite symbioses, started to look, colours could be seen by the naked eye as wings passed over insects' black bodies.
"They flash like little diamonds," he said.
The researchers think the coloration has specific functions, particularly for mating, just as it does in butterflies and beetles and other insects with better-appreciated markings.
The patterns will also help scientists distinguish between species difficult to differentiate in other ways. Already the researchers used transparent wing colours to identify three new species of wasp.
According to Janzen, at least a dozen other orders of insects, spanning dragonflies and cockroaches and grasshoppers, have transparent wings likely to be as colorful as those of wasps and flies.
"I envision taxonomists going back to their animals, and looking at them in a new light," he said. "It's like discovering a whole new piece of the animal."
Images: 1) Fruit fly against white and black backgrounds./PNAS. 2) Patterns in fly wings (top half) and wasp wings (bottom half)./PNAS. 3) Composite image of fly against white and black bacgrounds./PNAS. The images are all true-color, modified only by a 10 percent increase in colour saturation.
Citation: "Stable structural colour patterns displayed on transparent insect wings." By Ekaterina Shevtsova, Christer Hansson, Daniel H. Janzen, and Jostein Kjærandsen. Proceedings of the National Academy of Sciences, Vol. 108 No. 1, January 4, 2011.
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