Spiders possess muscles, but their method of locomotion is unique, relying on a hybrid system of muscle action and fluid dynamics. Unlike vertebrates, which use opposing muscle pairs to both extend and flex their limbs, spiders use muscles for only one half of the movement cycle. This specialized mechanism gives them the scuttling movement and the ability to perform high-speed maneuvers like jumping. Understanding how spiders move requires looking beyond simple muscle contraction to how they manipulate internal pressure.
Spider Muscle Anatomy and Function
Spiders have striated muscles, similar to human skeletal muscles, which attach to the inside of their exoskeleton. These muscles are responsible for all internal movements, including feeding, pumping hemolymph, and operating the legs. The primary difference from other animals is the distribution of muscles within the leg joints.
In the spider’s walking legs, the muscles primarily function as flexors, meaning they contract to pull the limb inward or bend the joint. The mechanical structure of major leg joints, such as the femur-patella and tibia-metatarsus joints, prevents muscle tissue from acting as an extensor. This anatomical constraint means muscular force can only be applied to bend the leg toward the body.
The flexor muscles are robust, providing the torque necessary to pull the legs in against the hydraulic pressure that extends them. Muscles controlling the most proximal joints, like the coxa-trochanter joint, are capable of both flexion and limited extension, providing the main power stroke for walking.
The Role of the Hydraulic System in Movement
The lack of extensor muscles is compensated by a hydraulic system that uses the spider’s internal fluid, hemolymph, to push the legs outward. Hemolymph is the arthropod equivalent of blood, circulating nutrients and oxygen, but it also serves as the hydraulic fluid for locomotion. This fluid is contained within the open circulatory system, flowing through the central body cavity, or prosoma, where the legs are attached.
To extend a leg, the spider rapidly increases the hemolymph pressure in the prosoma by contracting specific dorsoventral muscles. This muscular contraction compresses the fluid-filled body cavity, forcing the hemolymph into the legs through internal channels called lacunae. The influx of pressurized fluid forces the leg segments to straighten, acting as the primary mechanism for leg extension.
During intense activity, such as a quick escape or a jump, the pressure inside the prosoma increases dramatically. While normal walking requires hemolymph pressure in the range of 4–8 kilopascals (kPa), high-speed movements can generate pressures up to 60 kPa or even 130 kPa. This hydraulic mechanism allows the spider to achieve rapid, powerful extension without the weight penalty of large extensor muscles.
Coordinated Locomotion and Limitations
Spider locomotion is a precisely coordinated interplay between muscle-driven flexion and hydraulic-driven extension. When walking, the spider uses flexor muscles to pull a leg off the ground and swing it forward. It then relies on the surge of hemolymph pressure to rapidly push the leg out and down against the substrate for the power stroke. This synchronized action, often an alternating tetrapod gait, allows for stable and efficient movement.
The dependence on internal fluid pressure creates a specific vulnerability: if the hydraulic system is compromised, the spider loses the ability to push its legs outward. A puncture to the cephalothorax or severe dehydration can lead to a drop in hemolymph pressure. When pressure is lost, the flexor muscles pull the legs inward, causing the characteristic curled-up posture seen in a deceased or severely injured spider.
For explosive movements, such as a jump, the spider utilizes a sudden, maximal spike of hydraulic pressure to extend the rear legs simultaneously, launching its body forward. This rapid pressure burst illustrates the power and speed achievable by the hybrid system. This process maximizes force output and agility by substituting internal fluid pressure for a traditional muscular antagonist.