Small Spiders With Big Appetites Use a Pulley System to Catch Large Prey
New research and videos show how spiders in the Theridiidae family hoist up prey 50 times their size
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It sounds like a scene out of a horror movie. A hapless victim stumbles into a snare that yanks them into the air, where they dangle helplessly for hours on end. But this isn't your average cinematic crime: the victim is a lizard and the predator is a tangle-web spider.
A new study on the tangle-web spider's snare-hunting style, published on February 3 in the Journal of the Royal Society Interface, presents video and analysis of how the spiders catch massive prey. The study was conducted in a black box that could highlight five spiders’ silk handiwork. Though the spiders could catch anything 50 times larger than the spider itself, up to the size of a mouse or small lizard, the researchers used orange-spotted roaches, which are a bit more than 1.5 inches long, as bait. Then the researchers watched how the spiders used their stretchy silk to slowly but steadily hoist captured roaches up, paralyze them with venom and start to feast.
The paper is "another example of how spiders use their silks as external tools to overcome muscle limits," says co-author Gabriele Greco, an engineer studying biomaterials at the University of Trento, to Inverse’s Tara Yarlagadda.
Tangle-web spiders, part of the Theridiidae family, get their name from sprawling bundles of silk they call home. They’re a far cry from the symmetrical, well-centered Halloween depictions of spiderwebs. But while net-like webs wait for insects to fly into them, tangle-webs have long, stretched-out tendrils of silk glued to the ground. When something small, like an ant, bumps into the sticky end, the silk disconnects from the ground, grabs the bug and springs it up into the air, Susan Milius writes for Science News.
The thing is, a lot of larger animals like mice and lizards also walk under tangle-webs and end up bumping into the trap wires. A tangle-web spider won’t let the trespasser just walk away. Instead, the first wire acts like an alarm system, alerting the spider that it’s time to take action.
"This is when the trap changes," Greco to Live Science’s Mindy Weisberger. "The spider is getting involved, too."
The researchers watched the spiders trap roaches. When one trips the wire, the spider starts making more silk, which it stretches out into hypertension thread and connects it from the web to the offending roach. Like an elastic band, the hyper-tense silk contracts, pulling upward on the roach.
The spider repeats this process over and over, attaching more threads to the roach until it lifts off of the ground. The strategy works a bit like a pulley system because the spiders take advantage of the energy in the stretched silk to do some of the lifting for them. The roaches struggled against the suspensions, but because the threads were no longer fully taut, they didn’t snap.
“In the end, all these threads create enough tension to lift the prey, and that is when the spider wins,” says Greco to New Scientist’s Leah Crane. “Then the prey cannot escape because it cannot grab the surface below.”
Once a roach loses contact with the ground, a spider can inject it with venom to paralyze it. Then it can lift it all the way up to the central den of the web and begin to eat.
The whole process requires five different kinds of spider silk—four to make the web and snares, and one to bind the paralyzed roach, per Live Science. The strategy is one way that small spiders overcome the limits of their tiny muscles and tackle big challenges. Other spiders have an impressive repertoire of skills at their disposal, too: some make lassos, trap doors, and slingshots, Science News reports.
But Greco tells Inverse that research on the exact mechanics of spiders’ different abilities is limited. Future research could help scientists better understand how the strategies evolved and how that affects spiders’ role in their ecosystems.