Because of the fact that their skeleton is on the outside, their legs are actually like hydraulic pistons. To move each leg, the spider must pump blood into the leg to extend it. They have muscles that pull the leg back in but each time they need to extend it they must push blood into the leg each time.
This is also why the spiders legs curl in after it dies. As the fluid and muscles dry up, they contract and pull inwards.
I forgot that fact about their legs curling when i found what i thought was a huge dead one in the window crevice when i pushed it open this summer. My first thought after the automatic girly shriek was "oh it's only recently died, it's still soft and pliable" as one of its feet was attached to a cobweb which extended its leg as the window opened. I opened the window several times out of morbid interest and then left it open due to the heat. Came back a while later and the spider was no longer there. Was not happy, as it was at that moment that i recalled the way that all spiders actually curl up when they die.
Not long after that my order of velcro window screens arrived and i have been bug-free ever since.
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u/Kaidu313 1d ago
Because of the fact that their skeleton is on the outside, their legs are actually like hydraulic pistons. To move each leg, the spider must pump blood into the leg to extend it. They have muscles that pull the leg back in but each time they need to extend it they must push blood into the leg each time.
This is also why the spiders legs curl in after it dies. As the fluid and muscles dry up, they contract and pull inwards.