Photograph by Malcolm Burrows
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Gears on an insects leg.
Photograph by Malcolm Burrows

This Insect Has Gears in Its Legs

The image above is an extreme close-up of a common British insect called a planthopper. You’re looking at it from below, at the point where its two hind legs connect to its body. In the middle, you can clearly see that the top of each leg has a row of small teeth, which interlock together. As the planthopper jumps, the teeth ensure that its legs rotate together and extend at the same time.

This insect has gears.


It’s a steampunk bug!

The gears are found on many young planthoppers but Gregory Sutton from the University of Cambridge first discovered them on a common British species called Issus coeleoptratus. “We didn’t have to go to some obscure monastery in Outer Slaubvinia to find these things,” he says. “We had to go to a place called The Garden, in The Backyard. Either the most complicated gearing in nature happens to be in our backyard, or there is stuff that’s vastly more intricate and complicated that hasn’t been found yet.”

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A baby Issus coleoptratus.

Sutton has been working with Malcolm Burrows from the University of Cambridge for the last 10 years, to study the movements of jumping insects like fleas, locusts, leafhoppers and pygmy mole crickets. When they filmed young planthoppers taking off, they saw that the hind legs would always move within 30 microseconds (millionths of a second) of each other. Such extreme coordination makes sense—the slightest difference in timing would send the insects spinning off to the side. But how could they achieve such tightly synchronised movements?

The nervous system can’t be involved. In 30 microseconds, a neuron can barely begin to fire, much less trigger something that tweaks the insect’s movements.

The answer lies on the insect’s undersides. Back in the 1950s, other scientists noted that young planthoppers have small bumps on their trochanters—the first segment of the legs, which connect to the hip-like coxa. They were only found on the hind legs, and not the other pairs. No one knew what they were for. No one seemed to care. “It was one of those odd little footnotes in anatomical books,” says Sutton.

Burrows and Sutton discovered the function of the bumps by planthoppers that had been restrained on their backs. The insects would try to jump whenever the duo gently prodded their abdomens. Just before their legs shot out, their trochanters would squeeze together. The bumps engaged and rolled against each other, exactly like man-made gears. “I was gobsmacked,” says Sutton.

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A close-up of the planthopper’s gears.

Gears allow two machines to rotate together in opposite directions. That’s exactly what the planthopper’s trochanter bumps do. Sutton tested this by pulling on the tendons of its jumping muscles with some forceps (“It’s the Serious Edition of Operation”, he says.). Even if he only pulled one tendon, both legs would extend because the gears transmitted the motion of one trochanter into the other.

“Then, we got really lucky because we saw a few jumps where the gears wouldn’t engage perfectly,” says Sutton. When this happened, one leg was partially extended before the gears finally snagged and the planthopper’s nigh-perfect coordination was ruined.

Wait! It gets better. These gears are training wheels!

The planthopper nymphs lose them when they become adults. But the adults don’t shoot off in uncoordinated spins—if anything, they’re better jumpers than the youngsters. Their hind trochanters make much closer contact with each other, and Sutton thinks that the friction between them helps to keep them in time. “We’re kind of sure about that, but not entirely sure,” he says.

“This is to our knowledge the first time that proper, engaging, counter-rotating gears have been seen in the animal kingdom,” says Sutton. Crocodiles have cog-like teeth in their heart valves, and the wheel bug and cog-wheel turtle have teeth on their shells. But none of these structures actually act like gears. “You never see one cog-wheel turtle sidle up next to another, engage their shells, and spin in opposite directions,” says Sutton. “If you did, I want you to call me. If I see that on your website, and I haven’t been called, I will be an angry man.”

The discovery is astounding in itself, but Sutton—a mechanical engineer—thinks that they could help us to make more effective machines at incredibly small scales. The teeth of most modern gears harken back to the 18th century, when mathematician Leonhard Euler designed a shape that could be easily cut by a machine. It’s called an involute and it looks like a hill with a plateau at the top. It has been a standard part of gears ever since.

But the planthopper’s gear teeth look more like a shark fin. “What we have is a prototype for a tooth shape for a high-speed, one-directional gear that’s not constrained by the machining techniques of the 18th century,” says Sutton.

Modern machines, such as 3-D printers, could easily create gears with these shark-fin teeth. Sutton is really excited by the prospect, and suspects that they may perform better in very small machines. “Modern machinery often doesn’t work at very small scales,” he says. “Friction doesn’t matter so much when you have two big gears next to each other but when you get small, friction starts killing you.”

The planthoppers might help to solve that problem. “We’re still being impressed and shocked by what we find in the back garden,” says Sutton.

Reference: Burrows & Sutton. 2013. Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect. Science

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