Young insect legs have real meshing gears

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People’s proud invention of gears was preceded by mindless evolution: The tiny points on the legs of juvenile planthopper insects move like intermeshing cogs.

Those cogs in young Issus coleoptratus planthoppers touch at the upper parts of the legs, says neurobiologist Malcolm Burrows of the University of Cambridge in England. And when the planthopper leaps, gear teeth on one leg catch the teeth on the other in sequence. Meshing cogs get legs quickly moving in sync, enabling energy-efficient leaps, Burrows and Gregory Sutton at the University of Bristol in England say in the Sept. 13 Science.

“Imagine that we’ve got on our thighs — heaven forbid — projecting teeth from a gear wheel,” Burrows says.

Insect gear teeth are microscopic marvels, with up to a dozen along a curving strip not more than 400 micrometers long. Teeth in the insect’s hard outer covering stick out 30 micrometers at the most, each tapering to a point.

The teeth synchronize the legs, Burrows concludes. Planthopper legs don’t lift up and down on the sides of the body in vertical planes as a grasshoppers’ do. Instead the Issus legs, tucked under the body, angle in such a way that they need to move at the same time. Otherwise a powerful forward thrust would go wobbly and weak.

High-speed cameras revealed that a young planthopper’s two rear legs start moving within about 30 millionths of a second of each other, Burrows reports. That kind of near-synchrony “would be an incredibly difficult problem for the neurons to solve,” he says. Just one spike of activity for a nerve cell lasts some 300 times longer than the observed leg-to-leg delay. Then the nerve signal would have to travel and a muscle would have to react. Gears work much faster.

Before a jump, gears roll so that the top ends of their strip of teeth interlock as the legs cock, the cameras showed. A planthopper jumps with more power than direct muscle action supplies. The insects get a boost by squeezing muscles ahead of time to bend the outer skeleton, just as an archer bends a bow before loosing the arrow. When the insect finally jumps, the gear teeth roll together down the curve.

During the final molt to adulthood, the teeth disappear. Yet grownups shoot through foliage with ballistic jumps just as juveniles do. Adults are bigger and heavier, Burrows says, so perhaps leg-to-leg friction syncs motions without the need for gear teeth.

Gears aren’t the only mechanical devices that have evolved in living organisms, says Alexander Riedel of the Karlsruhe State Museum of Natural History in Germany.  Hinges abound, he says. He and his colleagues have described a beetle leg joint like a screw that turns in a nut.

A closeup of gears at the top of a young planthopper’s rear legs shows their intermeshing movements during a jump, and then slowly moving into position for a jump. (Images were captured at a rate of 5,000 frames per second and replayed at 30 frames per second.)
Credit: Courtesy of Malcolm Burrows