A family of spiders can often prey on their own weight by hooking silk lines to their quarry and raising the fleshy premium to the air.
Tangled spiders, of the family Theridiidae, are masters of the use of silk to amplify muscle power. Their networks are “a messy tangle,” says Gabriele Greco, who studies biological materials at the University of Trento in Italy. Inclined and intertwined silk threads in a cobweb scribble.
Filming how spiders hunt from these sips, Greco and Trento’s companion Nicola Pugno focused on the most spectacular scenarios: insect attacks that weighed up to 50 times more than the spiders themselves. However, website makers could win their battles thanks to skillful fighting, poison, and a lot of silk surrounding the prey. The winning spiders also attached several silk threads to their prey pack to bring the party to the net, Greco and Pugno reported on Feb. 3 in the Journal of the Royal Society Interface. However, curiously these threads are never completely pulled.
To analyze the weight-dragging movements of the spiders, the researchers installed laboratory boxes with black walls to facilitate the observation of white silk. Inside each box was a species of Theridiidae, either a triangular cobweb spider (Steatoda triangulosa) or a false black widow (S. paykulliana).
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In the wild, both species extend some threads from the tangle to the ground, anchoring the sticky end of the thread. When some small, edible creature, like an ant, hits the wire, it comes off the ground and snatches the small bite up to pounce helplessly into the air. Lunch!
However, what really interested Greco and Pugno was the news that these threads occasionally caught "giant" prey, including a snake and a mouse. To give food-trapping silk an extreme workout, the researchers used large cockroaches.
A single sticky-tipped chain can’t throw a heavy cockroach into the air, so when the prey hits the silk, the resident spider runs to add extra threads to its large catch. In the lab box tests, the spiders had to add thread after thread before the researchers saw the first upward jolt. Throughout the lifting process, the drag wires held some dice instead of stretching very tightly. That makes sense, Greco concluded, because these threads are pulled and criticized a lot by the fighting prey. Super tight wires can break.
Surrounded in a lab box, a small Steatoda spider tries to catch a much larger cockroach to feast on. Spiders have not evolved wheeled pulleys to lift massive loads, but tangled spiders like these have a silky alternative. The spider attaches one silk chain after another to the fighting cockroach until the silk bears the full weight of the cockroach. Then the spider continues to add ever shorter wires that pull the monster weight from the prey to the main part of the net.
What these spiders have evolved from is not a load pulley with a wheel, but joining a string of silk threads gives small animals their own way of lifting heavy loads in small increments. After about 400 million years of evolution, the shapes and uses of the spider fascinate researchers like Greco, who are looking for new materials.
The new study on silk-based weight lifting seems “very cool” to Symone Alexander, a chemical engineer with broad interests at the University of Auburn in Alabama. Rounded and striped nets of Halloween decoration may be the popular idea for spider silk, but animals have expanded places and ways of living with a wide range of silky innovations. Spiders evolved with silky trap doors, nets, lassos and – one close to Alexander’s heart – ultra-fast slingshots (SN: 3/6/19).
“Even in a single net there are different types of silk (and glue) that are used to make picture lines, capture lines and anchors,” he says. "Spiders are witty."