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Every year, Halloween enthusiasts adorn their homes with synthetic cobwebs. But humans aren’t the only creatures who decorate their abodes.
Spiders bedeck their webs with “stabilimenta”—various woven patterns that are typically made of a different kind of silk than the rest of the web. In a new study published Wednesday, researchers reported that they found that these “web decorations” may help the spiders detect certain vibrations that can help them find their prey.
Arachnologists have debated the function of these ornaments for decades. Early researchers in the field called such a decoration a “stabilimentum” (Latin for a “support”) because they believed the structures helped stabilize webs. But this hypothesis has been proven wrong, says Gabriele Greco, a bioengineer at the University of Pavia in Italy and co-author of the recent study, which was published in PLOS One.
Scientists have also proposed that stabilimenta shield spiders from harsh ultraviolet rays, convey water for the arachnids to drink, or can either visually attract or repel prey. There is just one function that researchers widely agree on: stabilimenta help spiders of some species hide from predators. But “there are many different types of [web] geometries,” Greco says, “and this makes the possibilities of new functions available to spiders.”
A creature landing or moving on a spider web generates force. This force can become a vibration that travels through the silk that the spider can perceive. While reading old papers, Greco was unable to find any research into how stabilimenta shape those vibrations.
He and his colleagues chose to study Argiope bruennichi—a large spider that has yellow, black, and white stripes and spins classic webs with spiral patterns—because the species is easy to find and one of the only spiders that produces stabilimenta in Italy, said Greco. He and his colleagues took images of six different types of stabilimenta in the forests of Sardinia over the course of two years.
There was the classic, or “normal,” stabilimentum—a dense, thick, zigzagging thread. Similarly, the “juvenile” stabilimentum that was produced by juvenile spiders zigzagged, but it was not as thick. The researchers called web decor that was woven on only one side from the center of the web “reduced.” “Drafted” stabilimenta appeared as incomplete or thin zigzags. And some webs had no stabilimenta at all. Lastly, the team categorized a “platform” stabilimentum: a thick and dense network of silk, woven in a symmetric pattern in the middle of the web.
Once these structures were photographed, the researchers created computer simulations to model how vibrations spread through the various webs. They tested the effect that different directions of impact had on each web.
In the simulations, the team first found that stabilimenta had no effect on waves from the force of an object landing perpendicular to the web or hitting it from the side toward the center. “Until here, I was happy because this is exactly what I expected,” Greco says.
But sometimes prey gets stuck in a web and thrashes from side to side, emitting vibrations parallel to the spiral. Greco was surprised to find that stabilimenta in the platform shape can play a huge role in transmitting that vibration. In the simulations, a platform stabilimentum made it possible for some vibrations to reach the web’s far side by improving connectivity among the threads—a process that, in nature, would help the waiting arachnid detect prey. (A similar but milder effect came through with other stabilimentum shapes.)
“This adds one more piece to the puzzle,” says Todd Blackledge, a biologist at the University of Akron, who was not involved in the study, “I think the really important take-home message of this paper is that they did not find a big, dramatic effect” of stabilimenta overall. He emphasizes that because the study was based on modeling, we still need to ask, “Does this really apply to real spiders in the real world?”
Greco sees this study as a starting point to categorizing stabilimenta’s effects on vibration, and next, he hopes to study such spooky effects in the wild.
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Spiders such as this Argiope bruennichi sometimes adorn webs with zigzagging stabilimenta. Pierluigi Rizzo (member of Aracnofilia – Italian Society of Arachnology) (CC-BY 4.0)
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