Some spiders use their silk to hoist helpless prey so it cannot escape

By Leah Crane

Emanuele Olivetti

Some spiders take on prey that’s far larger than they are, including  lizards. To keep their prey from running away, they use their webs as pulleys to lift the doomed animals off the ground.

Gabriele Greco and Nicola Pugno at the University of Trento in Italy used high-speed video to watch five captive spiders from the Theridiidae family – the most common type of spider found in human homes – catch cockroaches up to 50 times more massive than themselves. They found that the spiders seemed to be using their body weight to put tension on the silk threads to keep them taut before attaching them to the cockroaches. The spiders then continued to attach more and more threads to their prey until it was lifted into the air.

“In the end, all these threads create enough tension to lift the prey, and that is when the spider wins,” says Greco. “Then the prey cannot escape because it cannot grab the surface below.” Once the prey is off the ground and unable to run away, the spider can take its time to kill and devour it.

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The researchers found that the silk did not stretch much during lifting, possibly because the spiders had already stretched it out before attaching it to their prey. This allowed the threads to recover when the cockroaches struggled instead of permanently sagging. “This silk used to lift the prey, it’s very strong, comparable to steel, but it is as elastic as the normal silk you would use to make clothes” says Greco.

This is interesting because one might not expect such a relatively simple animal to know how to use tools to catch its prey in such a sophisticated way, he says. It may allow spiders to have an outsized impact on their ecosystems by eating all sorts of small animals instead of just bugs.

Journal reference: Journal of the Royal Society Interface, DOI: 10.1098/rsif.2020.0907

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