A dead spider with its legs pulled tightly toward its body appears smaller and shriveled. However, the rigid parts of its body do not reduce in dimension. The appearance of “shrinking” is instead a mechanical illusion caused by the loss of an internal support structure. This physical change upon death is a direct consequence of the unique way a spider moves and maintains its posture while alive.
Understanding Spider Movement Through Hydraulics
Spiders move their eight legs using a sophisticated internal system that operates much like a hydraulic machine. Unlike mammals and insects, spiders are missing extensor muscles in the two main joints of their walking legs (the femur-patella and tibia-metatarsus joints). To extend its leg, a spider must actively increase the internal pressure of a fluid called hemolymph, the arachnid equivalent of blood. This internal pressure is generated by muscles in the prosoma, the front section of the body to which all the legs are attached.
The prosoma acts as a pressurized chamber, pumping hemolymph into the limbs through specialized channels. This influx of fluid forces the joints to straighten out against the resistance of the flexor muscles. The system allows spiders to generate rapid, powerful movements by suddenly boosting this internal fluid pressure. The only muscles present in those primary leg joints are the flexor muscles, which pull the legs inward when the hydraulic pressure is reduced.
The Mechanism of Post-Mortem Collapse
The spider’s unique reliance on a pressurized fluid system is the direct reason for its post-mortem appearance. When a spider dies, the muscles responsible for generating and maintaining the high hemolymph pressure in the prosoma relax and fail. Since the heart also stops pumping, the entire hydraulic system goes offline, causing the internal fluid pressure to drop precipitously.
With the necessary fluid pressure gone, there is no force left to counteract the natural tension of the flexor muscles. The legs are immediately pulled inward by these unopposed flexor muscles, resulting in the characteristic curled-up posture. This is a purely mechanical collapse rather than an active process of shrinking.
This loss of hydraulic stability can also be observed in living spiders that are severely dehydrated or injured, causing them to exhibit the same curled-up pose. The flexor muscles may also contract further as the body enters rigor mortis, locking the legs into this folded position. This final, compact posture is a passive state, showing the default, non-pressurized configuration of the spider’s limbs.
Distinguishing True Shrinkage from Dehydration Effects
The appearance of a dead spider being smaller is largely due to the mechanical collapse of its limbs, but actual tissue loss does contribute to the illusion. A spider’s body is covered by a rigid, non-flexible exoskeleton, or cuticle, which defines its overall size and cannot shrink. The two main sections of the spider’s body are the hard prosoma and the softer, more flexible abdomen (opisthosoma). After death, the internal moisture, primarily water contained within the hemolymph and soft tissues, begins to rapidly evaporate. Since the abdomen is not protected by the same rigid plates as the prosoma, it is particularly susceptible to desiccation. As the internal volume of the abdomen decreases from water loss, the pliable abdominal wall shrivels and flattens. This deflation of the abdomen, combined with the extreme inward folding of the legs, causes the spider to look significantly smaller and more withered.