The tropical velvet worm, a slow-moving nocturnal hunter with poor vision, has a secret weapon -- if a potential meal such as a termite or cricket gets anywhere near, the worm unleashes an instantaneous torrent of sticky slime.
The quick-hardening slime can entrap the hapless victim, and the worm increases its chances of success by firing the slime in a wide arc from openings in its head, with the spray rapidly oscillating in all directions, researchers have found.
"It's like casting a very broad net in front of your head," says Harvard University biologist Gonzalo Giribet. "It's a very clever mechanism for something that doesn't see very well in the dark."
Scientists have been puzzled by how the worm, a slow moving creature, could cause the spray to oscillate so rapidly from side to side, much faster then the worm could possibly move its head.
The trick, the researchers found, is sophisticated physics and fluid dynamics.
Small, random fluctuations in the worm's slime "cannons" cause them to move slightly one way or the other. They then rebound, sending the slime in a different direction, and the effect is then repeated, faster and faster, until the slime tube is rapidly oscillating, like a untended garden hose flailing around when water pressure is high.
The phenomenon, knows by physicists as elastic instability, can have the worm's slime tubes moving side to side as fast as 60 times per second, casting their hunting "net" over a wide area.
"Somehow, evolution led to an animal that uses this elastic instability to capture prey," says study leader Andrés Concha, a physicist at Adolfo Ibañez University in Chile.
While mechanisms for squirting venom or some other liquid are common among animals, creating anything other than a straightforward arc of liquid usually requires an active movement and some degree of control, the researchers say.
The velvet worm, however, seems happy to let a basic fact of physics do the work for it.
"Our work shows that this is indeed the case, and chalks up one more example of how evolution has co-opted a simple physical principle for a behavioral response," says study co-author and Harvard researcher L. Mahadevan.
The researchers have published their findings in the journal Nature Communications, and suggest their study could inspire scientists to create new microfluidic devices with applications in medicine and industry, where they could create either fine droplets or fibrous nets, or be used to mix substances together in an industrial or laboratory setting.