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Fleas’ ‘Feet’ Unleash That Spectacular Leap
The New York Times, February 10, 2011

One moment, the 10 fleas were happily sucking blood from hedgehogs being treated at Tiggywinkles Wildlife Hospital in Aylesbury, which, of course, is in England.

The next, they were being plucked from their comfortable home and transported to Cambridge, where they ended up in a glass box with a Styrofoam floor. From time to time, bright lights would flood the box, so that a high-speed camera could film them. And the fleas did what fleas do in times of crisis: they jumped.

When fleas jump, it is no ordinary leap. The insects can shoot as high as 38 times their body length, about three inches. And the acceleration is so intense that fleas have to withstand 100 Gs, or 100 times the force of gravity. “You and I pass out if we experience five Gs,” said Malcolm Burrows, an expert on insect jumping at the University of Cambridge.

Dr. Burrows and his Cambridge colleague Gregory Sutton obtained the fleas from Tiggywinkles to try to answer a question that had vexed naturalists for centuries: how fleas manage their spectacular jumps. In a paper published Thursday in The Journal of Experimental Biology, they report that the insects turn themselves into catapults, storing up energy that they release as they push off the ground with what passes, in fleas, for feet.

Dr. Burrows and Dr. Sutton are only the latest in a long line of naturalists who have pondered these insects. In 1664, the British physicist Robert Hooke put a flea under a microscope for the first time. Hooke thought the wingless fleas jumped with all six legs. “These six leggs he clitches up altogether, and when he leaps, springs them all out, and thereby exerts his whole strength at once,” Hooke wrote.

Since it takes about a thousandth of a second for a flea to hurl itself into the air, Hooke was obviously making a tremendous guess. It took three centuries for scientists to gather the first hard data about jumping fleas. In the 1960s, Eric Lucey, a biologist at the University of Edinburgh, filmed the insect with what was then the most advanced high-speed camera, at a thousand frames a second. Dr. Lucey showed the film to Henry Bennet-Clark, an expert on insects who was also at the University of Edinburgh at the time. Dr. Bennet-Clark realized that the fleas were generating a hundred times more power than their muscles could actually provide.

He noticed that just before leaping, the flea bends the closest segment of its hindmost legs toward the body for about a tenth of a second. When Dr. Bennet-Clark dissected fleas, he discovered that their leg muscles were connected to pads of stretchy protein called resilin. In 1967, Dr. Bennet-Clark and Dr. Lucey proposed that the fleas stored energy in springlike tissues, which they then released.

In 1972, another British naturalist, Miriam Rothschild, published a similar study and came to much the same conclusion. But Ms. Rothschild and Dr. Bennet-Clark disagreed about how the force from the spring actually lifted the flea off the ground. Ms. Rothschild argued that fleas planted a kneelike joint called the trochanter to jump. Dr. Bennet-Clark, on the other hand, thought the fleas pushed off from the footlike segments at the end of the legs, called the tarsi.

Unfortunately, their films were too blurry to determine who was right. In 2009, Dr. Burrows reread the original papers and decided it was time to attack the question anew, using 21st-century technology. Over the course of a week, Dr. Sutton filmed 51 jumps, which were then analyzed on a computer.

The scientists noticed that the fleas sometimes jumped with trochanters and tarsi both planted on the Styrofoam. But sometimes only the tarsi made contact. “These two jumps appear to be the same,” said Dr. Sutton.

It seemed that, as Dr. Bennet-Clark had argued, only the tarsi mattered. And Dr. Sutton and Dr. Burrows got the same result when they developed mathematical models of the forces produced in flea jumps. The actual jumps matched Dr. Bennet-Clark’s hypothesis. So it seems that fleas leap by channeling their stored energy down to the tips of their legs.

“Their work is pretty bulletproof,” said Steven Vogel, an expert in biomechanics at Duke University.

Dr. Sutton and Dr. Burrows suspect that flea springs are more complex than originally thought. They’ve studied another jumping insect, the froghopper, and found that its resilin pads are layered over hard cuticle. This sandwich structure stores lots of energy, while the stretchy resilin keeps the hard cuticle from cracking. The scientists suspect fleas use the same biomechanical trick.

Dr. Sutton thinks that superior springs are just one of several important lessons fleas can teach engineers. They might also learn how to build robots that can leap over rough terrain. “Insect jumping is incredibly precise and incredibly fast,” said Dr. Sutton. “If you could build a robot that could do that, it would be fantastic.”

But Dr. Sutton acknowledged that some of the most important secrets of fleas remain to be worked out. No one knows how fleas lock their springs in place and then release them, for instance. And no one knows how fleas snap their two rear hindmost legs at the same time. If they weren’t so precise, the insects would spin wildly off course.

“If you’re half a millisecond off, you’re done, and we have no idea how they do it,” Dr. Sutton said. “It’s one step at a time -- we’re just going have to take on the next problem and solve that.”

Copyright 2011 The New York Times Company. Reprinted with permission
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