World

The Curious Case of the Spring-Loaded Web

Almost 150 years ago, on an October afternoon, Burt Green Wilder was strolling through the woods outside Ithaca, when he stumbled across a strange spider web attached to a hemlock branch. It was triangular, as if a wedge had been cut from a full web. And “instead of hanging loosely from the twigs, it was upon the stretch, as if constantly drawn by a power at one or the other end,” Wilder later wrote.

Wilder had many talents: He was a Civil War surgeon, a pioneering neuroscientist famous for his vast collection of preserved brains that included, eventually, his own, and a zoologist whose fondness for spiders led him to create a device that milked them for silk. Upon encountering a bizarre triangular web, such a person was almost guaranteed to prod it.

When he did, he was amazed that the structure “loosened with a snap,” and the triangle shot forward as if whatever was holding it taut had let go. A moment later, as Wilder watched, it started slowly stretching back to its original state. It was being pulled by a single strand extending from its tip, and by following that line, Wilder found the web’s creator—a tiny spider, no bigger than a grape seed, and camouflaged to resemble a tree bud. Hyptiotes cavatus. The triangle weaver.

Hyptiotes, the triangle weaver spider.
Hyptiotes holds on to a piece of its web. Credit: Sarah Han

Hyptiotes (pronounced HIP-tee-oh-tees) had been discovered a few decades earlier. But Wilder was the one who worked out how it builds and uses its web—an extraordinary, spring-loaded trap that allows it to rapidly ensnare large prey.

Spinning at night, it makes four radial spokes that converge at a point, an apex line that connects that point to a nearby branch, and sticky capture threads that run across the spokes. Once finished, it sits on the apex line, cuts it, and uses its own body to bridge the two separate threads. Its front legs grab the end leading to the web. The silk-making spinnerets on its backside grab the end anchored to the branch. Then, by walking its back legs along that anchor line, as a person hauling along a rope, it slowly winches the web taut. If an insect hits the web, Hyptiotes lets go with its back legs, allowing the web (and its body) to spring forward.

Thanks to this springing action, several of the sticky capture threads slam into the insect. And since Hyptiotes is still holding on to the anchor line with its spinnerets, it can repeatedly recock and relaunch the web. Again and again, the triangle pulls back and springs forward, each time entangling the insect even more. Finally, when the victim is well and truly trapped, the spider scampers over and starts to feed.

Read: [Spiders can fly hundreds of miles using electricity]

Wilder described all of this in 1875, and while others have studied Hyptiotes since, no one had studied the physics of its attack. Sarah Han from the University of Akron did so recently using high-speed video cameras, and the numbers she got came as a shock.

When the spider releases the anchor line, its body accelerates at 770 meters per second squared (more than 2,500 feet per second squared)—almost 80 times greater than a free-falling object, and 60 times greater than a sprinting cheetah. That’s only possible because the spider stores energy in its stretched web. If it tried to accelerate that hard on its own power, it would need 20 times its body weight in muscle.

Many animals use special energy-storing structures in their body to amplify the power of their muscles. Fleas compress a springy pad when they cock their legs, and the energy released when that pad expands powers the insects’ incredible jumps. Mantis shrimps—a group of aggressive crustaceans—use a similar structure in their arms to deliver the world’s fastest punches. Chameleons launch their super-fast tongue strikes by pulling the tongue back as if drawing an arrow on a bow, and then releasing it.

In all these cases, energy is stored within the animal’s body. By contrast, very few animals store energy in an external tool. Humans, with our bows, slingshots, catapults, and ballistae, are one. Hyptiotes, with its spring-loaded web, is another. The ray spiders (aka slingshot spiders) can also join the club: They spin traditional circular webs and then stretch the center back to create a spring-loaded cone. Symone Alexander from Georgia Tech, who studied one of these spiders, showed that they can achieve even greater accelerations than Hyptiotes. And “though Hyptiotes face their web, slingshots face away, indicating that there are perhaps some differences in prey-sensing and web-release strategies,” Alexander says.

“Spider webs have to intercept, stop and retain prey and each of these functions might be served by slightly different mechanical properties,” says Natasha Mhatre from Western University. Hyptiotes, however, has evolved a web that can “very rapidly transition between two very different states that each serve their functions really well—a tense web to stop prey and a looser web to entangle it.”

These traps only work if the spider can hold its web taut for hours—maybe days. Wilder noted as much, writing that “her powers and her endurance are, in proportion to her size, quite beyond what we are familiar with.” But even though Hyptiotes does have buff legs, which almost certainly help it to pull the web into place, it can’t possibly just be tensing its muscles for hours on end. It probably has a catch system to hold its limbs in place without effort, much as fleas and mantis shrimps do. But “we haven’t seen that anywhere,” says Han. “There must be something going on internally, and we plan on investigating it further.”

Read: [There’s a spider that makes milk]

Another mystery: Why have these spiders evolved a triangular web in the first place? How does that improve over the basic circular models? Han suspects that speed is a factor: Most web-building spiders have to run over and attack their prey when they feel the right vibrations, but Hyptiotes can instantly ensnare its victims just by letting go. And most spiders subdue their prey with venom, but Hyptiotes belongs to a group called the uloborids that have lost their venom glands. By using its web to thoroughly entangle a snared victim from afar, “it doesn’t have to encounter prey that might hurt it in a struggle,” says Han.

But then, without venom, how does it actually finish off the insect? Other uloborids have resorted to extreme measures: One species crushes its prey to death by wrapping it in up to 140 meters of silk. (That’s almost 460 feet, and not a typo.) Does a Hyptiotes victim also die from constriction?

“I’m not exactly sure what is killing it,” Han says. “It might just die slowly as they start to feed on it.”

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