Scorpions have strange joints that can simultaneously bend and twist

Scorpions’ tail joints both bend and twist at once

Nature Picture Library / Alamy

Scorpion tails can simultaneously twist and bend thanks to unusual joints, which could inspire new kinds of robots.

A detailed analysis of the scorpion tail reveals that its joints move simultaneously in ways similar to both a door hinge and a rotating wheel, providing for highly precise sting strikes, all the while allowing body tissues to run through its hollow structure.

“Nobody has ever seen a joint like this before, so it’s really fascinating,” says Alice Günther at the University of Rostock in Germany.

After investigating dozens of scorpions representing 16 species, Günther and her colleagues ran microscopic computed tomography (CT) scans of the five tail segments of a laboratory-bred adult female Mesobuthus gibbosus scorpion, a species that has tails typical of the vast majority of scorpions. They used the images to create 3D digital and print models providing more practical views of the arachnid’s tail joint, which evolved into its current form 400 million years ago.

They found that the first four segments have an unusual design. One end of each segment has two small knob-like elevations that look like they could insert into a complementary socket to make a stable hinge joint. Instead, however, the knobs latch into a circular rim in the connecting segment. Because the system is not fixed, the segments can slide and twist along that rim as if they were on a rail.

Only the fifth segment – the one with the stinger – has sockets for the two knobby joints. The stinger’s joint bends but does not twist, Günther says.

Because scorpions have an exoskeleton, their tail segments create a long, jointed tube through which run muscles, nerves and even the intestines all the way up to the anus at the fourth segment.

This makes the tail unique because it does what limbs do for other animals – like hunting and digging – while encasing major life systems, says Günther. As such, the tail’s design could be particularly interesting for industries that require transporting fluids through articulated arms, she says.

Doctors and surgeons could deliver targeted treatments into specific body parts through such devices, for example, or cables and other sensitive equipment could be protected inside jointed robots, says Fabrizio Scarpa at the University of Bristol, UK, who was not involved in the work.

Even so, the tube aspect is not nearly as exciting as the joint mechanics themselves, Scarpa says. “From my perspective, what’s really interesting is that unique bending and twisting junction mechanism, because it’s just so robust,” he says.

It could find use in soft robotics, in which robots are built with flexible materials, and in aeronautics, like for morphing airplane wings that respond to air forces by changing shape to keep the aircraft afloat, he says.

Journal reference: Journal of the Royal Society Interface, DOI: 10.1098/rsif.2021.0388

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