Weird worm uses elastic body to sling slime

By exploiting laws of fluid dynamics and elasticity, velvet worms fire a predatory jet of slime that oscillates faster than muscular action allows.

One worm’s slimy technique is as ingenious as it is disgusting, scientists have discovered.

Discharge is a well-utilized mechanic in the biological world – vertebrates urinate, squid use jet propulsion to swim, and archer fish can shoot insects out of the sky with a well-aimed stream of water. But biological jet streams are usually straight; very few animals can produce streams that fly every which way.

One such group of animals, known as velvet worms, are the subject of new research by Andres Concha, of Adolfo Ibáñez University in Chile. His study, published today in Nature Communications, shows how these slime-mongers exploit natural laws to do the dirty work for them.

Velvet worms comprise an obscure phylum called Onychophora. These soft-bodied animals boast several rows of legs, and some even bear live young. But most notably, the velvet worms hunt and defend themselves with a curious tactic – jets of sticky mucous-like goo shot from an opening on the tops of their heads to immobilize their enemies.

The bodies of velvet worms are equipped with branched slime glands. Upon sensing prey or danger, an individual can expel a rapid jet of the sticky fluid from openings called oral papillae, thus immobilizing the target. While projecting slime, the oral papilla can oscillate three or more times in just a fifteenth of a second – this way, the slime covers a greater area and is more likely to ensnare prey.

It was long-assumed that complex neuromuscular structures must be behind such rapid movement. But Dr. Concha and his team were skeptical. Upon examination, it appeared that the worm’s muscular system would be incapable of producing such an action.

And as it happens, they were right. By utilizing anatomical imaging, theoretical analysis, and high-speed videography, they determined that a velvet worm doesn’t need a muscular system to oscillate its oral papilla at all. Instead, it takes advantage of the elastic instability of its flimsy body.

“This papilla has an accordion-like structure that makes it a bendable object,” Concha said. “As soon as the flow inside the system reaches a critical speed, the papilla begins to oscillate without the need of muscular contractions. In short, the velvet worm squirt system is a ‘syringe’ (the reservoir) connected to a flexible "needle" (the oral papilla).”

Like a garden hose left unattended, rapid fluid motion causes the papilla to thrash and undulate as it spews slime. This may seem unreliable as a hunting tactic, but it’s actually quite efficient, according to Concha.

“Why use a complex neuromuscular system to move the oral papilla when it will oscillate by itself as long as the papilla is flexible enough?,” Concha said. “To cite Julian Monge [co-author]: ‘It is not so different from a man trying to catch a fish with a net, so that he overcomes his limitations as a fisherman. When you are soft-bodied, very slow, and can only come out at night when visibility is poor, having a ‘net’ that does the work for you is great.’”

“In particular, the production of glue is very costly for this worm,” Concha added. “In an attack, the worm could spend as much as 10% of its body weight. That is a considerable amount of energy. Thus, to be effective is extremely important.”

Nature is no stranger to passive systems like these. The Venus flytrap, for example, triggers a similar elastic instability to rapidly close its mouth on unsuspecting flies. But passivity has technological applications, too. Researchers are now investigating how nanofibers and microfluidic devices can utilize oscillation, elasticity, and passive systems. Scientists in many fields are finding out what engineers have been saying all along – that the most common solution isn’t always the simplest one.

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