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April 23, 2024

One small leap for a flea is one giant biomechanical feat

Below, excerpts from Carl Zimmer's New York Times Science section story about what's behind the flea's amazing jumping ability.


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... to try to answer a question that had vexed naturalists for centuries: how fleas manage their spectacular jumps. In a paper published in The Journal of Experimental Biology, they reported 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.

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 spring-like 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.

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."

Below, the published summary of the paper cited above.

Biomechanics of jumping in the flea

It has long been established that fleas jump by storing and releasing energy in a cuticular spring, but it is not known how forces from that spring are transmitted to the ground. One hypothesis is that the recoil of the spring pushes the trochanter onto the ground, thereby generating the jump. A second hypothesis is that the recoil of the spring acts through a lever system to push the tibia and tarsus onto the ground. To decide which of these two hypotheses is correct, we built a kinetic model to simulate the different possible velocities and accelerations produced by each proposed process and compared those simulations with the kinematics measured from high-speed images of natural jumping. The in vivo velocity and acceleration kinematics are consistent with the model that directs ground forces through the tibia and tarsus. Moreover, in some natural jumps there was no contact between the trochanter and the ground. There were also no observable differences between the kinematics of jumps that began with the trochanter on the ground and jumps that did not. Scanning electron microscopy showed that the tibia and tarsus have spines appropriate for applying forces to the ground, whereas no such structures were seen on the trochanter. Based on these observations, we discount the hypothesis that fleas use their trochantera to apply forces to the ground and conclude that fleas jump by applying forces to the ground through the end of the tibiae.

April 23, 2024 at 12:01 PM | Permalink


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