Spiders do not possess knees in the same way mammals do. As arthropods, they utilize a unique and highly efficient method of locomotion fundamentally different from the muscle-and-bone lever system found in vertebrates. Their movement relies on a combination of internal fluid pressure and specialized muscles. Understanding how a spider moves requires a closer look at the structure of its legs and the innovative biological hydraulics that power them.
Anatomy of the Spider Leg
A spider’s leg is composed of seven distinct segments, moving outward from the cephalothorax (fused head and chest section). These segments are the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus, which is the final segment often tipped with claws. This segmented structure functions through hinge-like joints, not the complex ball-and-socket joints found in human knees.
The patella segment serves only as a joint connection between the femur and the tibia, not as a complex articulation like a mammalian knee. The entire limb system is encased in a rigid exoskeleton, which acts as the skeleton for muscle attachment and contains the hydraulic system. This arrangement allows for a propulsion method that bypasses the need for a full set of opposing muscles.
The Hydraulic System for Movement
The mechanism for a spider to straighten its legs is biological hydraulics. Spiders lack extensor muscles in the major joints of their legs, meaning they cannot actively push their legs outward using muscle contraction alone. Instead, they use pressurized fluid called hemolymph (the spider’s blood) to extend their limbs.
The spider generates pressure by contracting muscles located in its cephalothorax, where all eight legs attach. This muscular squeeze increases the internal fluid pressure, forcing the hemolymph into the legs. This rush of pressurized fluid acts like a hydraulic pump, mechanically straightening the joints and pushing the leg outward. This system allows for rapid and powerful extension, powering the swift motions required for walking, running, and jumping. This is particularly evident in species like jumping spiders, which can launch themselves over fifty times their own body length.
Muscle Function: The Flexion Mechanism
While leg extension is driven by hydraulic pressure, the opposing motion—flexion, or pulling the leg inward—is purely muscular. Spiders possess strong flexor muscles within the leg segments. These muscles contract to bend the joints, allowing the spider to pull its legs toward its body or grip surfaces tightly.
This separation of function conserves the spider’s muscular energy; the powerful push is outsourced to the hydraulic system, and muscles are only required for the controlled pull. The absence of extensor muscles in the main joints provides more space within the exoskeleton for the flexor muscles to develop. This makes them exceptionally strong for grasping prey or maintaining a grip on a vertical surface, serving as an energy-efficient adaptation for an animal with a rigid external skeleton.
Why Spiders Curl When They Die
The sight of a dead spider with its legs curled tightly underneath its body is a consequence of the hydraulic system’s failure. When a spider dies, its internal systems cease to function, including the muscular control in the cephalothorax that generates hemolymph pressure. The heart stops pumping, and the pressure within the body cavity rapidly drops.
Without the active hydraulic force to push the joints outward and hold the legs straight, the intact flexor muscles take over. The natural resting position of the joints, combined with residual tension or post-mortem contraction of the strong flexor muscles, causes the legs to snap inward. This curling is not a deliberate final act, but the physical mechanism of the spider’s locomotion system defaulting to its bent state once hydraulic support is gone.