Muscles attached on the inside of the exoskeleton contract to move the legs inward, but spiders don't have any muscles to extend the legs back out again. Instead, they have to force bodily fluids (mainly blood) into the legs to push them outward. If a spider loses too much body water, it can't generate the necessary hydraulic pressure to push its legs out. This is why you sometimes see spiders on their backs with their legs curled up.
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