Spider silk is a most amazing and versatile material, and spiders put it to all sorts of uses. It helps them to climb, to travel from place to place and most famously, to ensnare their prey. But one group of spiders, the uloborids, use their silk in a unique way – as a murderous garbage-compactor.
Most spiders kill with venom and even those that pose no threat to humans pack enough poison to deal with insect prey. Their famous webs are simply elegant traps, designed to immobilise prey so that the spider can deliver a fatal bite. But the uloborid spiders have uniquely lost their venom glands and have to subdue their prey through other means.
Two years ago, William Eberhard from the University of Costa Rica discovered their strategy. He noted that these spiders will wrap their trapped prey in what seems to be an excessive amount of silk. Earlier studies showed that two species of uloborids spent over an hour enveloping their prey, more than a hundred times what other orb-weavers will devote to the task. In doing so, they used up a spectacular 80 metres of silk.
Those figures are even more astonishing because the spiders discard the silk after they are finished feeding. What could possibly justify such an exorbitant expense of time and energy? By watching one species, Philoponella vicina, Eberhard found that silken shroud compresses an insect with such force that its legs break and its eyes buckle inward. The spider crushes its prey to death by cocooning it in silk.
Do you want flies with your wrap?
P.vicina is an unfussy eater and Eberhard watched it in the wild as it tackled flies, wasps and beetles alike. High-speed cameras revealed its technique. Once an insect blunders into the web, P.vicina rushes over to it and shuttles her abdomen from side to side and uses her back legs to pull about 10-20 separate lines of silk out of it. All in all, she makes about 28,000 wrapping movements and cocoons the insect in over 140 metres of silk. Once imprisoned, the spider regurgitates digestive juices all over the silk and sucks up the fluids that remain, leaving behind a dry, hollow shell.
Eberhard found that the wrapping process exerted a tremendous amount of pressure on the trapped insect, enough to make a squashed package of a long-legged tipulid fly, far larger than the spider herself. The spider applied silk to the far edges of the flies’ bodies and the tension exerted by it eventually pulled them into a more compact and easily wrapped shape. To give you a sense of scale, the image on the right shows a wrapped fly next to one of its legs, which Eberhard removed before the spider set to work.
By scaring the spider away, Eberhard managed to dissect some of the wrapped insects under the microscope. The images revealed signs of massive damage. The long legs of the tipulid flies were bent and broken. Three fruit-flies had at least one eye that was buckled inward by the pressure. And at least five flies were killed outright.
Eberhard believes that the cause of death is suffocation, either by physically collapsing the insects’ airways, or by restricting the pumping body movements that some species use to breathe. It’s not unlike the strategy used by large constricting snakes like pythons, except uloborids use a material of their making rather than their own bodies.
By using its silk in an ingenious way, P.vicina can deploy forces that are well beyond the scope of its own body. Eberhard found that it takes over 360 milligrams of weight to dent the leg of a tipulid fly, and the spider herself weighs 14mg at most. Her legs are not particularly built for strength, but they don’t have to be. Each wrapping motion applies a tiny additional bit of tension to the insect and it’s the accumulation of these tiny forces that eventually crushes it. That’s why the uloborids spend so much time and silk on wrapping.
But why crush prey at all? A less substantial covering would be enough to keep the prey from struggling while the spider injected its digestive juices. Nor does the crushing process create wounds that the digestive fluids can seep into, for the spiders successfully eat hard-bodies species like beetles and ants that don’t crush when wrapped.
The answer may be to related to the spider’s grotesque feeding habits. Other groups inject digestive fluids into the bite wound but since uloborids have no venom and don’t bite, they slobber all over their cocoons instead. Eberhard thinks that the function of the apparently excessive wrapping is to compress the insect into a small enough volume that it can be spat on effectively.
Reference:Eberhard, W.G., Barrantes, G., Weng, J. (2006). Tie them up tight: wrapping by Philoponella vicina spiders breaks, compresses and sometimes kills their prey. Naturwissenschaften, 93(5), 251-254. DOI: 10.1007/s00114-006-0094-1
Image: by Akio Tanikawa