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The Velvet worm (Peripatus sp.) is believed to be an evolutionary "link" between annelids and arthropods. It is a predatory creature that spins a web to capture it's food. This one was photographed in the Peruvian Amazon. Dante Fenolio / Science Source
The Velvet worm (Peripatus sp.) is believed to be an evolutionary "link" between annelids and arthropods. It is a predatory creature that spins a web to capture it's food. This one was photographed in the Peruvian Amazon.
Dante Fenolio / Science Source
The velvet worm hunts by night. The sun would dry it up and kill it, is the problem. But the worm doesn’t see very well. Or hear very well. So to maximize its chances of catching dinner, it sprays streams of quick-hardening slime in a wide arc, hoping to catch crickets or spiders who happen to be in the firing solution. It’s…strange. “It’s like casting a very broad net in front of your head,” says Gonzalo Giribet, a biologist at Harvard University. “It’s a very clever mechanism for something that doesn’t see very well in the dark.” Now it turns out that the clever mechanism relies on some pretty sophisticated physics.
Biologists used to think the goo-spray was straightforward physiology, a wiggle of muscles. But no: To send a spray that wiggles back and forth as fast as the one in this video, the worm would need muscles that could contract every 20 milliseconds. And they don’t. Even the worm’s quickest-moving muscles, located in the jaw, can only twitch in half a second. So how does all that spray spray?
The trick, it turns out, is fluid dynamics. A liquid flowing fast through a flexible tube (like the worm’s slime cannons) won’t always be a perfectly straight stream. Small, random deviations in the flow nudge the tube slightly to one side or the other. When the tube rebounds and tries to straighten out, that reorients the stream in the other direction. These opposing forces push each other harder and harder, and soon, the tube is flexing back and forth—like an untended garden hose writhing when the faucet is wide open. Physicists call this behavior an elastic instability, and it causes the velvet worm’s slime tubes to oscillate up to 60 times per second. “Somehow, evolution led to an animal that uses this elastic instability to capture prey,” says Andrés Concha, a physicist at Adolfo Ibañez University in Chile, who led the new analysis.
No one had ever looked at the physics of how velvet worms shoot before, so to show that their idea was right, Concha’s team took high-speed video of the velvet worms in action (to get them to shoot, they tickled the worms using a fine brush). Then they built a model slime-shooter. When they streamed water through the mock-up tubes, they triggered the same back-and-forth fluctuations seen in the velvet worms. Considering that the velvet worm has survived for some 500 million years, it must be a pretty good hunting strategy.
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