Spiders known for elaborate circular webs have altered their spinning style in dark spaces to create apparent tripwires for walking prey.
Those that make circular webs are known as orb-weavers, and most of them trap mosquitoes, beetles and other flying insects in sticky spiral frame webs sparsely attached to outdoor structures, like tree branches. But European cave orb spiders (Meta menardi) anchor their webs to cave walls using twice as many silk strands, which appear to vibrate when tripped by unsuspecting crawlers, says at the University of Oxford.
鈥淭he idea is that, because it has that many more attachments to the wall, somebody crawling would be more likely to hit one of those attachments by mistake,鈥� he says.
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like centipedes, slugs and other spiders, which would be unlikely to get entangled in typical orb webs.
In 2021, Hesselberg鈥檚 鈥� those circling the web centre 鈥� and had more radial threads extending out as anchors. It made Hesselberg wonder if the spiders were adapting their web design for a different capture technique.
To find out, Hesselberg and his colleague Emily Brannigan observed the feeding behaviour of female orb spiders in the Creswell Crags Archaeological Park in Derbyshire, UK, for a total of six days. They released various prey 鈥� a mealworm, a large centipede and another cave spider 鈥� along the radial strands of the webs or near to them.
When these animals triggered one of the anchor strands, the cave spiders ran along an adjacent anchor strand towards the intruders. Vibrations from these 鈥渢ripwires鈥� were probably strong enough for the spider to sense from the middle of the web because of the reduced number of frame threads 鈥� which would otherwise have dampened the signal, says Hesselberg.
In theory, this would allow the spider to assault the prey, overpower it and drag it back to the web centre for a meal. But the studied spiders didn鈥檛 end up attacking the animals that tripped the wire 鈥� probably because, in these experimental cases, the potential prey were too big or poisonous. Such attacks may have been too risky outside the safety of the sticky web frame, says Hesselberg.
The researchers suspect the tripwire method might be just one of several tactics that these spiders have evolved to adapt to feeding in caves.
鈥淭his indicates that they have a much wider flexibility in their foraging strategies than the orbs outside do,鈥� says Hesselberg.
Ethology
