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The Velvet Worm Squirts Slime at Crazy Angles and Amazing Speeds

Velvet worms apparently moonlight as fluid physicists.

​Velvet worms are delightfully idiosyncratic animals. They are so​cially complex, anatomically unique, and incredibly ancient, with evolutionary roots dating back well over 500 million years.

But as if that isn't enough zoological cred, these worms are also capable of squirting jets of slime at speeds and angles that seem to defy physics, an adaptation that has baffled biologists for decades. Now, some of the most bizarre properties of these slime jets are finally explained in a ​new study, which was led by physicist Andres Concha and appears today in Nature Communications.

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"The rapid squirt of a proteinaceous slime jet endows velvet worms with a unique mechanism for defence from predators and for capturing prey," wrote the authors in the study's introduction.

"However, to date, neither qualitative nor quantitative descriptions have been provided for this unique adaptation," the team added.

Attack by a Peripatus Solozanoi lasting 80 miliseconds. Credit: Credit: Cristiano Sampaio-Costa, Bernal Morera-Brenes, Julian Monge-Najera, Andres Concha

As you can see in the above video, the velvet worm's slime jet doesn't just blast straight forward, as the vast majority of biological squirting mechanisms do. Instead, the jet has a weblike arc, suggesting that the worm uses some kind of oscillation process to sculpt its trajectory.

It is one of only three animals known to eject these oscillating jets, and the other two— spitting cobras and spitting spiders—clearly move their fangs and heads to produce the effect. Velvet worms, in contrast, hold their heads still while shooting slime, making the mechanism behind the jet unclear.

Concha and his colleagues studied high-speed videos of the velvet worm's slime squirt, and realized that the animal has evolved some interesting mechanical tricks. The team discovered that the worms use a syringe-like anatomical system to amplify the oscillating effect of the slime as it exits the animal.

The synthetic simulacrum. Panels are (from left to right) actual experiment, the oscillation amplitude, and the speed at which liquid is flowing. Credit: Cristiano Sampaio-Costa, Bernal Morera-Brenes, Julian Monge-Najera, Andres Concha

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The effect is similar to the pattern of water ejected from an unattended garden hose when water suddenly passes through the worm's slime reservoir. The results suggest that velvet worms innately know how to balance fluid inertia against elastic resistance to produce the most effective squirts.

"If [the slime papilla] were too short they would not oscillate [because they are] way too rigid," Concha told me over email. "If they were to be extremely long, then [their] own weight will bend them."

"That takes you to the natural conclusion that via natural selection there should be an optimal length that allow them to capture prey in an efficient way," he added.

To confirm their observations, the team built a synthetic version of the velvet worm's papilla out of soft elastomer. Not only did they recreate the same oscillating effect with the simulacrum, they realized that it has potential as a technological technique in producing micro-drops for pharmaceuticals, filters, or tissue regeneration, among other applications.

Slime papilla opening obtained by scanning electron microscopy. The bar is 100 microns. Image: Cristaino Sampaio-Costa

"[U]sing flexible micro pipes, you could have spontaneous motion of the system," Concha said. "[T]his would allow to better mix products or drugs at the micro scale."

In the meantime, there is still plenty to learn about the velvet worm's unique capabilities. For example, Concha and his colleagues would like to study the molecular composition of the slime, and whether it can be artificially recreated. They are also interested in the genes behind the adaptation, and the morphogenesis of velvet worms.

"There is a lot of room for future research," Concha told me.

Given how unconventional these velvet worms have proven to be so far, that does not come as a surprise. After all, any animal that casually learns how to manipulate fluid dynamics for sliming purposes likely has other fantastic tricks up its sleeve.