A dead spider is often found with its legs curled tightly inward, lying on its back. This common observation has a clear scientific explanation rooted in the unique biological systems spiders possess. Understanding how spiders move provides insight into their posture after death.
Spider Movement Mechanics
Spiders employ a specialized method for leg movement, differing from many other animals. Instead of relying on opposing muscle pairs, spiders primarily use a hydraulic system to extend their legs. Their internal fluid, called hemolymph, acts as a hydraulic fluid, similar to blood in vertebrates. (Source 1, 6, 11) This hemolymph is pumped under pressure from the spider’s main body cavity, the prosoma, into their legs. (Source 1, 15, 19)
Many of a spider’s leg joints lack extensor muscles that would push the leg outward. (Source 1, 7, 9) Instead, these joints extend when hemolymph pressure increases, filling areas within the leg segments. (Source 1, 6, 7) While these joints primarily use hydraulics for extension, spiders do possess strong flexor muscles that pull their legs inward. (Source 1, 6, 9) This combination allows for powerful actions like gripping prey, climbing surfaces, and propelling themselves in leaps. (Source 9, 10, 20)
The Dying Process
When a spider dies, its hydraulic system ceases to function. The heart stops pumping, leading to a rapid drop in hemolymph pressure throughout the body. (Source 5, 11) Without this internal fluid pressure, the flexor muscles in the legs are no longer opposed. (Source 2, 3, 11)
These flexor muscles naturally contract, pulling the legs inward. (Source 2, 4, 10) This results in the curled-up posture seen in deceased spiders. Rigor mortis, the stiffening of muscles after death, also contributes. (Source 2, 3) As the spider’s body stops producing adenosine triphosphate (ATP), the energy for muscle relaxation, the flexor muscles remain in a contracted state, locking the legs into their curled position. (Source 9)
Why They End Up Upside Down
The inward curling of a spider’s legs influences its final resting position. As the legs retract tightly beneath the body, the spider’s weight distribution shifts. (Source 3, 11, 16) This change in the center of gravity makes the spider unstable when attempting to remain upright. (Source 3, 5)
The curled legs raise the spider’s body, creating an unstable base. This often causes the spider to topple over onto its back, where it rests in a more stable, inverted position. (Source 3, 5, 11) The combination of lost hydraulic pressure, natural flexor muscle contraction, and the resulting shift in balance explains why most dead spiders are found upside down with their legs drawn in. (Source 3, 11, 16)