Spiders, eight-legged arthropods, navigate their environments with remarkable agility, whether scuttling, scaling, or leaping. Their unique locomotion involves specialized anatomy, an internal hydraulic system, and intricate coordination.
Leg Structure for Locomotion
A spider’s movement is rooted in the structure of its eight legs, attached to the cephalothorax. Each leg is comprised of seven segments: the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. These segments connect at various joints, allowing for articulation and a wide range of motion. The outermost segment, the tarsus, is often tipped with two or three claws for gripping different surfaces.
The Hydraulic Movement System
Spiders employ a unique hydraulic system to extend their legs, distinct from antagonistic muscle pairs in many other animals. Instead of relying on extensor muscles, spiders primarily use hydraulic pressure from hemolymph, their circulating fluid. This fluid is pumped from the cephalothorax into the legs, causing them to extend. While robust flexor muscles pull legs inward, extensor muscles are notably weak or absent in many major leg joints.
The cephalothorax acts like a finely tuned bellows, rapidly adjusting pressure to facilitate leg movement. This hydraulic reliance explains why a dead spider’s legs typically curl inward; without internal pressure, the flexor muscles contract unopposed. This system also enables spiders to perform powerful actions like jumping, by rapidly increasing hemolymph pressure in specific legs. In larger spiders, while hydraulics still play a role, muscular mechanisms, particularly flexors, become more dominant for rapid movements.
Coordinated Eight-Legged Gaits
Spiders coordinate their eight legs in specific patterns, known as gaits, to achieve efficient and stable movement. One of the most commonly observed gaits is the alternating tripod gait. In this pattern, the spider moves three legs from one side of its body (e.g., front-left, middle-right, back-left) while the other five legs remain on the ground, forming a stable support. This allows for continuous movement while maintaining balance, as a stable tripod is always in contact with the surface.
Different gaits are utilized depending on the spider’s speed and the terrain. For slower walking, spiders may use more varied patterns, but as speed increases, the alternating tripod gait becomes more pronounced. The spider’s nervous system plays a crucial role in orchestrating these complex movements, ensuring effective locomotion across various environments.
Adapting to Different Surfaces
Spiders possess specialized adaptations on their legs that allow them to adhere to and traverse a wide array of surfaces. At the ends of their tarsi, many spiders have dense tufts of microscopic hairs called scopulae. These scopulae consist of numerous fine hairs (setae), which are further covered in even smaller structures called setules or “end feet.”
This hierarchical structure creates an immense contact area, enabling strong adhesion primarily through van der Waals forces. This adhesive capability allows spiders to cling to smooth surfaces like glass, even supporting more than 170 times their own body weight. Additionally, the claws at the tip of each leg are used for gripping rough textures or their own silk, providing purchase where van der Waals forces might be less effective. Some species also secrete a fluid from their footpads, which can enhance traction and adhesion.