Gears allow planthopper to super jump in the right direction, report says

New research provides more evidence that gears, once thought to be a man-made invention, actually evolved in nature. A study published in "Science" shows that the jumps of an insect are made possible by gears in its hind legs. (Images and video courtesy of Malcolm Burrows)
September 12, 2013

A jumping insect has gears, scientists discovered, a rare instance in which man and nature independently converged on the same idea.

It was not easy to verify. The planthopper (Issus coleoptratus) is tiny, just a bit larger than a flea. And it jumps extremely fast — with an acceleration of 200 Gs, a level close to the highest ever survived by a human.

But neurobiologist Malcolm Burrows and engineer Gregory P. Sutton, both of the University of Cambridge, used a high-speed camera attached to a microscope to capture the bugs in action. They put the tiny test subjects on their backs on sticky wax and gently rubbed their bellies to provoke them to move their hind legs as if jumping.

They found that the insects have toothed gears at the base of their hind legs that mesh and rotate to synchronize the timing of each limb’s release during a jump.

“It’s remarkable that these gears look so similar to the gears man has designed, even the individual teeth are so similar,” said Burrows, author of a study that was published online Thursday in the journal Science.

With this mechanism, the motion of the hind legs is locked and coupled together. The time difference in the movement of the legs is as short as 30 millionths of a second, allowing the insects to leap faster and farther.

Without synchronization, the body will rotate instead of going straight forward, Burrows said. Using the most extreme case, a one-legged planthopper will spin helplessly in the same spot without going anywhere.

A two-legged insect will be able to leap when its timing is off, but precious energy is wasted on rotational motion instead of a straight-ahead trajectory. When fleeing from a hungry predator, the lost distance could make the difference between reaching a safe branch and becoming dinner.

Using a catapult mechanism, the legs move very quickly. In preparation for a jump, the legs are cocked, similar to pulling back a bowstring in archery.

“You suddenly let go, and the arrow goes much faster than if you were to throw it directly,” he said.

The planthoppers have gear strips on the base of the hind legs, with about 10 teeth on each.

But the gears are somewhat like training wheels on a bike. They exist only in the nymphal stage before the insect becomes an adult. Eventually, they molt away, and adults use friction between two parts of their legs to synchronize jumps instead.

Gearlike structures seen previously in animals were merely ornamental. For instance, the spiny turtle has a spiky-edged shell that makes it look as if there is a gear wheel lying on the turtle’s back.

“It’s a wonderful example of the exquisite use of mechanisms in nature that solve problems in a very simple way,” said Robert Full, a biologist at the University of California at Berkeley, who was not involved in the study. Full’s research focuses on taking ideas from animals for engineering and design inspiration.

He discovered that geckos stick to walls using millions of tiny foot-hairs, and labs have been working on mimicking the lizard’s extraordinary characteristics to create an adhesion technology.

Biologist Anna N. Ahn of Harvey Mudd College, who works in a similar field and who also was not involved in the study, called the new finding “fantastic.”

“Nowadays, bio-inspiration is sort of a catchy term,” said Ahn, but gears are unusual in that man and nature independently came upon on the same idea.

How gears evolved in the planthoppers is unknown, but Burrows speculates that they could have begun as smaller bumps within the legs that grew larger until they became full-fledged teeth.

Kim is a freelance science writer based in Philadelphia.

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