Why does a spider's legs curl up when it dies?
October 6, 2006 5:35 AM   Subscribe

Why do spider legs curl up after death?
posted by unmake to Science & Nature (7 answers total)
They're basically hydraulic. Their natural state is curled up, and if I recall correctly pressure is exerted on a fluid to make them extend. When they die, the pressure is no longer exerted and they curl up.
posted by edd at 5:44 AM on October 6, 2006 [2 favorites]

This by the way is the question that inspires the title of this book. It's where I read about this, and where someone might go to check my answer is correctly recalled.
posted by edd at 5:50 AM on October 6, 2006

i also seem to remember hearing that spiders dont have muscles in the traditional sense.

its all basically hydraulic so this seems to fit in with what edd is saying. but i dont have a link :(
posted by moochoo at 5:55 AM on October 6, 2006

I was coming it to say the very same thing these 2 posters have said. Think of it as a Hydrolic system that no long has any preasure on it - it returns to its natural state, and the natural state of the spiders legs is curled up.

Really interesting to think about though considering how fast some spiders can move - I can't imagine the levels of pressure they must generate to move that quickly.
posted by captainzero at 7:19 AM on October 6, 2006

Good point, captainzero, some wolf spiders have very thick legs for their size, and I wonder if this is true for them.

Come to think of it, wouldn't spinning silk seem to require lots of hydraulic pressure, with all that viscous liquid coming out of very tiny nozzles? Consider also the recent discovery that tarantulas spin silk with their feet to be able to crawl up walls.

As Halloween approaches, I was just thinking about the extra-creepiness factor of spiders' abdomens, with that horrifyingly soft, throbbing, vibrating, vitality. I suppose that must ultimately stem from its function as a bellows to generate the necessary pressure.

One last thing; spiders have to inject their venom, which is also their digestive fluid, into living insects which must have their own internal pressures that the spider has to overcome. A rigid exoskeleton would seem ideal to contain pressures much higher than atmospheric; I wonder if anyone has tried to measure the range of internal operating pressures of insects.
posted by jamjam at 9:26 AM on October 6, 2006

On further thought, the legs of wolf spiders are probably thick because the restoring stroke depends on hydraulic pressure. Contracting the leg means pumping fluid out of the leg, if you want that to be as fast as possible, you'd better make the pipe big, and the same argument applies to re-extending the leg.
posted by jamjam at 10:03 AM on October 6, 2006

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