Many creatures with exoskeletons basically have legs that are spring loaded to contract. And when they want to extend the legs, they use a hydraulic system to push fluid into the leg which straitens it out. These two things replace the “extend” and “contract” muscles of an endoskeleton and are the reasons given for why bugs will curl up when they die and dry out: without the hydraulics being controlled, there’s just the springy part curling the legs up.
This is something like blowing air into one of those blowout noisemakers making it unroll, and once the you stop blowing it rolls back up.
There’s also other invertebrates (slugs) that I don’t know, but they aren’t the insects/arachnids you mentioned.
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