A wee flea can propel its 1.8-millimeter-long body 1.9 meters per second in a single leap. How do these mighty mites do it?

The biomechanics of the flea leap have been something of a small controversy in the field of entomology. The core power for the jump comes from a coil in the animal's thorax, but two competing theories emerged over the decades as to how that stored energy hits the ground: Either the spring's recoil transfers energy to the fleas' knees, which push the animal directly off the ground, or the energy travels farther down in the legs through a system of lever-like leg segments to the shins and finally the toes, which provide the launch points.

"We were always very puzzled by this debate," Gregory Sutton, a post-doctoral researcher at the University of Cambridge's zoology department, said in a prepared statement. Data from each side "were consistent with each other, but we couldn't understand why the debate hadn't been settled."

Sutton and lab head Malcolm Burrows, who had previously spent more of their time working with locusts, turned their high-speed cameras on a small fleet of hedgehog fleas (Archaeopsyllus erinacei) to try to settle the debate once and for all.

The task, however, proved to be rather more challenging than simply slowing down film footage. For starters, the small subjects, weighing 0.7 milligrams, had to be persuaded to jump—either by "a light touch of a paintbrush" or when their environment transitioned from dark to light, Sutton and Burrows noted. Finally managing to focus the camera on these fidgety insects, the researchers captured 51 jumps from 10 different fleas.

But not all of the fleas pushed off from the ground in the same way. In most cases, the fleas' knees started on the ground but came off at about 0.6 millisecond before takeoff, before peak body acceleration was achieved. And in 12 percent of leaps, fleas' knees did not touch the ground at all, suggesting the toes as the main source of power.

A scanning electron microscope revealed that the knee surface was smooth—in contrast to the spine-covered, grip-ready toe and shin. These small points of friction provide for a much better launch.

Kinetic modeling, however, seems to have finally pushed the toe-thrust theory onto solid ground. These computer simulations show that both leap tactics result in the correct initial velocity of 1.35 meters per second. But only the toe-only design hit the top acceleration close to that of the 1,500 meters per second per second seen in the flea films. A similar jumping mechanism can be found in froghopper insects, Sutton and Burrows noted.

The new assessment also highlights the fleas' decidedly un-circus-worthy limited jump repertoire. They seem to only shift their leaping distance by altering their velocity at takeoff—rather than shifting takeoff elevation—making their trajectory relatively predictable. Such a constricted capability is "puzzling because it would seem to be in its best interest to jump in a wide range of unpredictable directions" as the fleas depend on their big bounds to both escape predators and hop onto their own prey, the researchers noted in the paper. "It is ironic that a famous jumper like a flea has such a restricted jump."

The Journal of Experimental Biology published the findings on February 10.


Image of dog flea courtesy of iStockphoto/olikim; video courtesy of Malcolm Burrows/Gregory Sutton