When a spider dies, it often assumes a distinctive curled-up posture, with its legs drawn tightly beneath its body. This common observation, frequently encountered in homes and outdoor spaces, is not random but a direct consequence of the spider’s unique biological mechanisms. Understanding the physiology of spider locomotion provides insight into why this particular post-mortem state occurs.
The Spider’s Hydraulic System
Spiders employ a specialized hydraulic system for movement, particularly for extending their legs. Unlike many other animals that rely on antagonistic pairs of muscles—one to flex and one to extend—spiders primarily use internal fluid pressure to straighten their limbs. Their circulatory fluid, called hemolymph, functions similarly to blood in vertebrates but also acts as a hydraulic fluid. The spider’s body can rapidly increase the pressure of this hemolymph within its cephalothorax, the fused head and thorax region. This pressurized fluid is then pumped into the legs, causing them to extend.
While spiders lack extensor muscles in most of their leg joints, they do possess flexor muscles. These flexor muscles are responsible for pulling the legs inward, allowing the spider to bend its limbs. The interplay between the outward push of hydraulic pressure and the inward pull of the flexor muscles enables spiders to move with precision, walk, and even perform powerful jumps. This hydraulic mechanism is a defining characteristic of spider locomotion, distinguishing it from that of many other arthropods.
What Happens When a Spider Dies
The curled-up position of a deceased spider is directly linked to the failure of its hydraulic system. Upon death, the spider’s heart ceases to pump, leading to a rapid loss of hemolymph pressure throughout its body. Without the continuous internal pressure to push the legs outward, the flexor muscles, which are still active or become rigid, pull the legs inward. This results in the characteristic contracting of the legs towards the body.
The flexor muscles of a spider are always in a state of slight contraction. When the opposing hydraulic pressure is removed, these muscles are unopposed, causing the legs to naturally curl. Additionally, the process of rigor mortis, the stiffening of muscles after death, can contribute to this inward curling, further locking the legs into their flexed position. This combination of lost hydraulic pressure and the action of flexor muscles explains the default curled posture observed in dead spiders.
The Appearance of the Deceased Spider
The typical appearance of a deceased spider is its legs drawn in tightly, often resembling a crumpled ball. This posture is a natural consequence of the biological mechanisms governing their movement. The legs are pulled close to the body due to the absence of the hemolymph pressure that once held them extended. This curled state is a common sight and serves as a visual indicator of the spider’s unique physiology.
In some instances, a dead spider may also be found flipped onto its back. This occurs because the legs curling inwards shift the spider’s center of gravity, making it unstable and causing it to topple over. The final curled appearance is a testament to how the spider’s internal systems, specifically its hydraulic leg extension, dictate its form both in life and in death.