Some spider species that live in groups of thousands on enormous webs synchronise their movements to catch insects up to 700 times heavier than an individual spider
Spiders that hunt in packs use web vibrations to coordinate their attacks, allowing them to kill prey hundreds of times larger than they could on their own.
Of the 50,000 known spider species, just one or two hunt as a group, with thousands of individuals spread across webs that can span several cubic metres. When prey insects land on their web, the spiders synchronise their attack, moving as one to catch animals up to 700 times heavier than an individual arachnid.
To better understand how this works, researchers disturbed the webs of two colonies of Anelosimus eximius, a social spider species. They mimicked the movement of prey by creating vibrations in different parts of the webs, while filming the spiders’ movements.
The team then analysed the movements frame by frame, finding that the spiders pause their motion towards prey, and restart it, at the same time.
The stopping time corresponded with the amount of “noise” in the web, according to computer models of the spiders’ motion. The arachnids only stayed still for as long as they had to in order to distinguish the vibrations caused by their fellow spiders from those of their prey.
“It’s like when there are lots of people talking in a crowded room and then there’s this other noise, like a telephone that rings, and everyone has to hush to find the source of the noise,” says Raphaël Jeanson at the University of Toulouse, France. “Of course, the louder the telephone’s ring, the less people have to be quiet to find the phone and it’s the same thing with these social spiders.”
“Depending on the size of the prey – and the vibrations that the prey creates on the web – the spiders have to be more or less quiet and still in order to localise the prey without getting disturbed by the vibrations of other spiders that are moving around,” says Jeanson.
After the spiders come to a collective halt, the group starts moving when one or two individuals become mobile, with there being no sign of a leader among the pack. “We don’t know how it works exactly, but when one of them moves, it sets them all moving,” he says. “It’s really a snowball effect.”
Synchronised hunting means the spiders can catch butterflies, grasshoppers and other flying insects, which struggle to free themselves from the web.
The webs of these social spiders aren’t sticky, so the spiders have to act quickly to avoid their prey escaping. The stop-start approach does extend the hunt, but it may be time well spent. “If they all arrive at the same time, there’s a strength in numbers that’s really beneficial compared to a disordered arrival of individual spiders that get lost in the web along the way. There’s a clear advantage of synchronisation despite the costs of the waiting time,” says Jeanson.
The spiders also time when they eject an immobilising glue from their hind legs and bite their prey, injecting a venom.
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