Fleas leap from feet, not knees

An investigation of how the impressive jumpers take off finds an answer

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HOP TO IT Jumping fleas launch off from their equivalent of feet (the tibiae and tarsi), not their knees (the trochantera), new research shows. G. Sutton and M. Burrows/Journal of Experimental Biology 2011
A decades-old debate about how the animal kingdom’s most renowned jumper jumps appears to be settled.

No, not rabbits or frogs. Fleas. Using new tools like high-speed video, researchers with the University of Cambridge in England have shown that fleas take off from their tibiae and tarsi — the insect equivalent of feet — and not their trochantera, or knees. The researchers report their conclusion in the March 1 Journal of Experimental Biology.

Regardless of how fleas do it, the insects have always been famous jumpers, says study coauthor Gregory Sutton. “There are even fairy tales that talk about how magnificent fleas are at jumping,” he says. And it’s not surprising—fleas jump far. Some fleas—only a few millimeters long—can jump well over 10 centimeters, according to one study. Adult hedgehog fleas (Archaeopsyllus erinacei) go from resting to midair in about 1 millisecond, says Sutton, a mechanical engineer at Cambridge.

No known muscle can generate anywhere near the power needed to launch a flea so far, Sutton says. In the late 1960s, researchers discovered that the bugs aren’t jumping with just their muscles. Instead, they spring. Before fleas launch, they store energy in a naturally springy protein hidden away in their bodies called resilin, then release it in one big bound.

But where the spring power goes from there wasn’t clear. One camp said the force moves down to the knees, the other said the feet. “They argued about it,” Sutton says, and for years the technology didn’t exist to put the matter to rest. It did, however, for Sutton and biologist Malcolm Burrows. With a little “flea wrangling,” the researchers were able to collect 51 slo-mo clips of leaping hedgehog fleas. The St. Tiggywinkles Wildlife Hospital Trust in Buckinghamshire, England, donated the fleas right off the backs of hedgehogs, the researchers note. The team also drew up mathematical models to simulate bug leaps on paper and eyed flea anatomy up close using a scanning electron microscope.

Each avenue of exploration came up feet. For starters, flea knees never even touched the ground in about 10 percent of the jumps, Sutton says. With or without knee contact, the fleas still jumped with the same speed and acceleration. The team also found long spikes on the flea tibiae and tarsi — good for traction, perhaps — but only short hairs on the knees. The jumps Sutton and Burrows watched on film also matched the predictions in their feet-jumping but not knee-jumping mathematical models. 

“It seems like a pretty good nail in the coffin,” says Dan Dudek, who studies biomechanics at Virginia Tech in Blacksburg. But studies like these are more than just flea circuses, he says. Faced with the limitations of human manufacturing, many biologists and engineers have turned to living organisms for inspiration. Dudek studies the resilin protein, which he says is more resilient than any man-made spring. “Certain animals are interesting and worth studying in their own right,” he says. “But understanding that force generation, translation and control may make it easier to understand a jumping robot.”

Sheila Patek, a biologist at the University of Massachusetts Amherst, agrees. But, she says, many organisms — from spore-shooting fungi to stinger-shooting jellies — have independently evolved fast and furious traits. Engineers have the option of copying not just one animal but picking and choosing among the best of the entire evolutionary spectrum, she says.

And humans may have barely touched on what animals can do, Sutton says. The legendary abilities of jumping fleas, too, may be just that — a fairy tale. Both locusts and bugs called froghoppers jump faster and with wider ranges than the itchy flea, he says.  “Given how many insect species there are,” he says. “I find it hard to believe that the fastest insect lives in the front garden.”

It takes about one millisecond for a hedgehog flea to go from resting to jumping, but high-speed video technology lets researchers see it in super slo-mo. The technique helped settled a decades-old debate about how fleas jump. Credit: G. Sutton and M. Burrows/Journal of Experimental Biology 2011

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