ENLARGE Image courtesy of Malcolm Burrows

Synchromesh, leafhopper style

Gears, one of the six simple machines, fasten hinges to the door, manufacture toasters and tablets, and drive your pet Ferrari to shocking speed. But in the realm of biology, we hear precious little about gears, or inclined planes, screws and pulleys.

Until now. Tomorrow, Science describes how insects called leafhoppers use gears to synchronize their rear legs so they can jump 100 times their length. “I’d been interested in how insects move, and then got interested in how they meet the demand of moving really fast, powerfully,” says Malcolm Burrows, emeritus professor of zoology at the University of Cambridge in the United Kingdom.

The quest to find Olympic-level jumpers lead Burrows to a large group of insects called the planthoppers, AKA leafhoppers.

When Burrows focused on a leafhopper called Issus coleoptratus, he found that nymphs (an early stage of development) can jump about 300 to 400 millimeters — more than 100 times their length.

To jump accurately, the nymphs fire the rear legs within 30 microseconds of each other. Nerve cells cannot deliver such split-second timing, Burrows says, yet without it, the jumps would be off target, and the hoppers could go hungry at best or become somebody’s lunch at worst.

Images courtesy Malcolm Burrows

Video courtesy Malcolm Burrows

All in the timing

Although it looks like the gears are delivering power, Burrows and Gregory Sutton of the University of Bristol (United Kingdom) found that they are actually synchronizing the leg muscles, which are “cocked” and then tripped to create the movement.

During their partial rotation, the gears move at a speed of more than 33,000 revolutions per minute! With an acceleration of about 700 gs, the ‘hopper quickly reaches a velocity of 3.9 meters per second.

Video courtesy Malcolm Burrows

For all the beauty of this evolved design, here’s an oddity: it’s absent in the adults, which can jump even further than the youngsters.

“It’s interesting, puzzling” why the gears don’t form in the adult stage, says Burrows, even though adults still need to synchronize their legs. (Insects, remember, remake their bodies several times during development.)

One reason why this “remarkable gear mechanism” would be absent in the adults is because gear teeth can break. “The only thing we can come up with that seems reasonably rational” is that gearboxes fail when their teeth are stripped teeth, Burrows says.

Just as a stripped transmission won’t help a Hudson-ful of Oklahoma bank robbers outrun the cops, a stripped gearbox would not help a leafhopper avoid getting eaten.

Survival recipe: Jump fast, jump far!

Developing leafhoppers get new gears every time they molt as they grow, and so might survive a short period with gear damage, Burrows says. “But if you are an adult and going to live for months or a year, the probability that you would be eaten by predator increases enormously.”

Nature — or more accurately evolution — is a pretty good designer, Burrows adds, and 20 families of leafhoppers have these gears. “Given the diversity, and widespread occurrence, this feature must have been around for a long, long time. Nature thought of this long before we existed.”

Although Burrows could not find a comparable example of natural gears, he found other natural counterparts for artificial mechanisms. Some insects, for example, use a Velcro-like structure or a snap to bind the wings during flight. “There might be some hidden literature or some other animal you don’t know about,” he says, “but so far, this is the first example I know about of an animal using interacting gears.”

Crickets, he notes, strike a series of bumps on the wing to make their call, “But it is not interacting gears doing something functional, like synchronizing the movement of two appendages.”

– David J. Tenenbaum









