May 6, 2011

Treehopper Camouflage Evolved From Ancestral Wing

The origin and evolution of treehopper 'helmets' has been traced by developmental biologists to show that they have achieved what no other insects have done in more than 300 million years; they have developed a third set of wings, which have been modified to form the helmet.

Treehoppers are small, odd-shaped relatives of cicadas and are masters of disguise. They have outgrowths called helmets that could resemble anything from seeds to thorns to caterpillar poop and even ants.

The extravagant headgear of the treehoppers are actually wing-like appendages that grew back some 200 million years after evolution had supposedly cast them aside, according to the study published in Nature.

This reverse evolution challenges some very basic ideas of what makes an insect an insect, the researchers say.

Each insect's thorax is divided into three segments; each segment has a pair of legs. In most cases, insects also have two pairs of wings, one on the middle segment and another at the rear of the thorax.

To this day, no insects have been found to have functional wings in the first segment near the head, however, their ancestors did.

"Primitive insects 350 million years ago had wings on all of their body segments," says Benjamin Prud'homme, lead author of the study and researcher at the Development Biology Institute of Marseille-Luminy in France.

"We don't know if they were all for flight, but we do know -- from fossil records -- that these wing-like structures were present on each and every body segment," he says.

Prud'homme explains that for the next 100 million years, wings on the first segment of the thorax and the abdomen dropped away completely from the insects.

Suddenly, about 50 million years ago, the cicada-like treehoppers once again sprouted wing-like structures from the top of the first segment of the thorax, which have resembled thorns, antlers and aggressive ants and are referred to as helmets.

These helmets have long been assumed by experts to be armor-like expansions of the treehoppers' exoskeletons, reports AFP.

However, when Prud'homme and his colleagues carefully observed the treehopper's development into adulthood, they found that the helmet began as a pair of buds, which were attached to its sides, articulated like wings, and then fused together as they grew.

"This is the only known example of a modern insect that has grown a third pair of wings," Prud'homme says. "It is a modification of the basic body plan of insects."

Researchers can only speculate on how this has happened. They suggest that certain genes, for 200 million years, have prevented wing-like structures from emerging on the first segment of an insect's thorax, and that these genes have lost their inhibiting capacity. However, experiments on other insects have found that these genes still retain their repressive powers.

Regardless of how this happened, researchers say that the evolutionary process found a way to put the renewed appendages to use, speculate the researchers.

"This extra pair of wings was not needed for flight, but nor did it prevent it," Prud'homme says. "So it became raw material for evolution to play with."

Because these wings were extra wings that were not needed for flying, they developed into camouflage disguises for the treehoppers, suggest the researchers.

Armin Moczek, professor at Indiana University, writes in Nature that the study shows ""how development abilities can be lost or silenced over millions of years, only to be redeployed to contribute to the evolution of a complex and beautiful appendage."

"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."


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