The velvet worm is an incredible evolutionary feat in a small tubular body. These worms have between 13 and 43 pairs of stubby, hollow feet that are fluid-filled, lack joints, and have hard, retractable claws made of chitin to grip uneven terrain. Oh, and they spray slime out of two openings called oral papillae.
The milky-white goo is squeezed through long slime glands as its muscles contract, shooting the substance into the air and ensnaring prey. The glands themselves tunnel deep inside the velvet worm's body and comprise over 10 percent of the its body mass. The creature can propel this slime up to a foot away, although they usually squirt it only a couple of centimeters to increase accuracy.
Video Credit: NatGeoWild
Scientists used to think that this spraying of adhesive mucus was due to muscles in the papillae wiggling about. But as it turns out, it’s a lot less directed than that. While the slime tubes look like they are spraying with incredible precision, they’re actually not.
In a new paper published in Nature, researchers found that the movement is in fact the result of fluid dynamics, not muscles: The papillae oscillate like an unattended garden hose that flails about when water surges through it. When the substance is propelled through its glands, the papillae shake and send the slime squirting in a sprinkler-like fashion.
As the authors note in their paper, “The interplay between fluid forces and the papilla elasticity produce the characteristic oscillatory waving motion used to capture prey, and obviate the need for any fast-moving controlled muscles.”
To strengthen their point, the researchers made a mimic of the worm’s papillae using elastic tubes and sent water rushing through them. To get the tubes to wave back and forth in a similar manner, the water needed to reach a speed of about 8.6 meters per second (28 feet per second). Check out the video below.
Once the slime has left the worm's body, it quickly hardens over any prey unfortunate enough to get sprayed. It’s a quirky evolutionary mechanism, but it seems 500 million years can do that to you.
Understanding the velvet worm’s unique abilities may influence “future oscillating microfluidic devices,” noted the researchers, and even inspire new “micro and nanofibre production."
Credit: Andrés Concha, et al. Image: (d) Velvet worm’s oral papilla accordion structure - scale bar: 200 μm, (e) Opening of the oral papilla - scale bar, 100 μm.