Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Issus coleoptratus
Habitat: throughout western Europe, often hanging around on ivy plants
For a disconcerting experience, consider how mechanical you are. Humans may be conscious beings with higher feelings, but really we're just fancy machines with joints, motors, valves, and a whole lot of plumbing.
All animals are the same. Hundreds of gizmos have evolved in nature, many of which our engineers merely reinvented. Nature had rotating axles billions of years ago, in the shape of bacterial flagella. And weevil legs beat us to the screw-and-nut mechanism.
The insect Issus coleoptratus is another animal with an unexpected bit of machinery hidden in its body. Its larvae are the first animals known to have interlocking gears, just like in the gearbox of a car.
In high gear
I. coleoptratus is a type of planthoppers – a group of insects known for their prodigious jumping. It takes off in just 2 milliseconds, and moves at 3.9 metres per second. "This is a phenomenal performance," says Malcolm Burrows of the University of Cambridge. "How on earth do they do it?"
Burrows first ran into the larvae of I. coleoptratus in a colleague's garden. "We were poking around and there were these bugs, jumping around like crazy." He took a closer look, and noticed that each larva had meshing gears connecting its two hind legs. The gears had been seen before, by a German biologist called K. Sander, but his 1957 paper isn't even on the internet.
The bulb at the top of each hind leg has 10 to 12 teeth, each between 15 and 30 micrometres long. Effectively, each hind leg is topped by a biological cog, allowing the pair to interlock, and move in unison.
Working with Gregory Sutton of the University of Bristol, UK, Burrows filmed the gears at 5000 frames per second and confirmed that they mesh with each other (see video, top).
Great timing
The two hind legs moved within 30 microseconds of each other during a jump. Burrows and Sutton suspect that the gears evolved because they can synchronise the leg movements better and faster than neurons can.
Other animals have gears, but not gears that mesh, says Chris Lyal of the Natural History Museum in London. "When you look at [I. coleoptratus's gears], you wonder, why can't anything else do that?" he says.
The German study from 1957 claims that all 2000-odd planthoppers have gears. "I've looked at about half a dozen, and they all have them," says Burrows. "I'd be hesitant to say no other animal has them," says Burrows. "But they haven't been described."
Twitching legs
There's one other mystery. Only larval planthoppers have gears. When they become adults, they lose them. "Adults still jump very well," says Burrows, and their legs are synchronised, but instead of gears, they rely on friction between the trochanters. "Why do they throw away a neat mechanism?"
It might be that gears are easily broken, and as soon as one tooth is sheared off, the mechanism doesn't work as well. "For an adult that can't moult again, that's going to be a problem." Friction may be a crude way to synch your legs, but it might also be more robust than fragile gears.
Journal reference: Science, DOI: 10.1126/science.1240284
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