Treehoppers are masters of mimicry. On their backs, these small, odd-shaped relatives of cicadas sport outgrowths called helmets, which resemble seeds, thorns, caterpillar poop, and even ants. Developmental biologists have now traced the origin and evolution of helmets, showing that treehoppers have achieved what no other insect has in more than 300 million years: a third set of wings, which are deeply modified to form the helmet.
"This is a beautiful example of how evolution works, creating novelty by modifying existing developmental programs," says Michalis Averof, an evolutionary developmental biologist at the Institute of Molecular Biology and Biotechnology in Heraklion, Greece, who wasn't involved in the research.
Helmets extend from just behind the head on the first segment of the thorax, the middle part of the insect body. Entomologists have long thought that the helmet is a folded extension of the thorax's exoskeleton, its hard outer skin. Like wings, helmets have veins, which prompted one researcher in the 1950s to suggest that the helmet was a modified wing. But entomologists dismissed his ideas as inconclusive. After an amateur entomologist friend complained that in trying to catch treehoppers he often wound up holding onto just the helmet, however, Nicolas Gompel and Benjamin Prud'homme began having second thoughts. "One wouldn't expect an insect could afford to lose part of its thorax, even to escape capture and stay alive," says Prud'homme.
The two developmental biologists from the Institute of Developmental Biology of Marseilles-Luminy in France and their colleagues took a close look at five treehopper species. To their surprise, they found that, as with wings and legs, hinges linked the helmet to the body, making it a flexible appendage, readily ripped off.
When they traced the development of a treehopper, Gompel and Prud'homme found that the helmet started off as two separate buds of tissue on the back of the nymph and later fused. The helmet's cellular arrangement resembles that of wings, and the two structures unfold in the same way during development, the researchers report online today in Nature. Three genes active in the developing wing were active in the developing helmet, suggesting that the same genetic program shapes both. "These similarities indicated the helmet and the wings share a common origin, both developmentally and evolutionarily, despite their anatomical and functional differences," says Prud'homme.
The earliest insects had finlike dorsal appendages on most if not all of their body segments, most likely for swimming. Over time, ever more sophisticated body plans evolved, so that for the past 300 million years, insects have just two sets of wings, on the second and third segments of the thorax. Genes called Hox genes are active in the first segment of the thorax to make sure that no wings form there. Treehoppers, which evolved 50 million years ago and very early on had helmets, have now proved to be the exception. The Hox genes are still active, but somehow the wings still form. And because those wings were extra and not needed for flying, they were free to diversify into a variety of disguises, Prud'homme and Gompel suggest.
"These bugs have been right under our noses for so long, and no one has noticed that their helmet is a highly modified pair of wings," says Jim Marden, an evolutionary biologist at Pennsylvania State University, University Park. "Amazing things are yet to be discovered all around us."