Science Questions

Why don't spiders run out of silk?

Sun, 3rd Dec 2006

Part of the show Naked Science Q&A and Polonium Poisoning


Kathy in Australia asked:

Why don't spiders run out of silk?


Spiders are really actually quite clever. Ancestrally they go back a couple of hundred million years, we think. They have glands at the back end of the spider and now it turns out also on their feet that make silk. And what scientists think, is that the glands on the back of their abdomen that make silk are just adapted limbs, where they used to have some legs. Silk is the reaction of proteins. So you have a chemical reaction going on at the back end of the spider that literally spins silk on demand. The spider eats something that has got protein in it. So when it goes and catches something in its web, it injects a venom into that insect that kills it by paralysing it. The insect is paralysed and doesn't die instantly so it remains fresh, the spider injects digestive juices which liquify an insect. And because an insect is like a husk, with a hard skeleton on the outside with the soft bits in the middle, the spider can literally suck the good bits out of the inside leaving behind a dry, wizened up shrivelled skeleton. That's why you see these sort of husks of insects left under spider webs. All the protein and goodness from inside the insect ends up inside the spider, the spider digests that, absorbs it, and then the proteins go to the back end of the spider. And they get turned into new web, amongst other things a spider needs to make. And some spiders have taken this a step further. What they've done is to make the process even more efficient, by eating their own web. This doesn't do them any harm because web is just protein. By eating their own web they're getting the proteins back into their body and they can then reuse them. Spiders' web is incredible stuff and it can absorb immense amounts of energy. It's got the tensile strength of steel. Scientists are now looking at ways of using it for bullet-proof vests, for example. If you can make this artificially in enough quantities you've got something with the tensile strength of a piece of steel, and the ability to stop bullets much better than a bullet-proof vest. Which means rather than police having to go around in these very thick outfits which restrict movement, if you could make it out of spider silk it would be a) lighter, b) a breatheable fabric, so it wouldn't make you so hot and uncomfortable, and c) it wouldn't restrict your movement so much.


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