Some invertebrates have a hydrostatic skeleton. This is a type of exoskeleton whose structure is maintained by the pressure of a fluid within the skeleton. The fluid pressure is maintained by muscular contractions.
Hydrostatic skeletons are found in many insect larvae and also in spiders. When a spider dies the fluid pressure is no longer maintained and this means the pressure cannot keep the legs extended. This is why, when you find a dead spider, the legs have contracted and are folded underneath the body.
A photograph of the cellar spider (Pholcus phalangoides). Spiders have a hydrostatic skeleton.
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