A spider does not possess paws, which are soft, fleshy structures typically associated with mammals. Instead, the spider’s leg ends in a highly specialized, exoskeletal apparatus. Spiders use a combination of claws and microscopic hairs at the end of their legs to navigate their environment, allowing them to achieve feats of climbing and adhesion.
The True Anatomy of a Spider’s Foot
A spider’s leg is composed of seven distinct segments, with the final one being the tarsus, which functions as the foot. Most spiders have either two or three hook-shaped claws at the tip of the tarsus.
The number of claws relates to the spider’s lifestyle. Web-building species typically have three claws, using the third, smaller central claw to maneuver and grip silk threads. Hunting spiders that roam often have only two claws. The tarsus is also covered in specialized, sensory hairs known as setae.
These setae are integral to the spider’s ability to walk on smooth surfaces. Many species that climb well have dense tufts of adhesive setae, called scopulae, located on the underside of their tarsus and the preceding segment, the metatarsus. These structures represent the true physical interface between the spider and the ground.
The Mechanism of Adhesion
The ability of spiders to walk on glass or ceilings relies on the scopulae. Each adhesive seta is tipped with thousands of tinier, hair-like projections called setules or “end feet.” This hierarchical branching structure dramatically increases the total surface area available for contact.
The collective action of these microscopic setules generates the powerful adhesive force through Van der Waals forces. These are weak, short-range intermolecular attractions that occur when molecules are brought into extremely close proximity. Because the setules are numerous and small, they can get close enough to a surface to engage these forces.
When the setules make contact, the cumulative Van der Waals forces produce an adhesive strength capable of supporting many times the spider’s body weight. For example, some jumping spiders can generate enough force to carry over 170 times their own weight while clinging to a surface. The spider controls this adhesion by peeling its legs away in a controlled manner, lifting the setules sequentially rather than all at once.
Hydraulic Movement and Leg Function
Unlike mammals, which use opposing muscle pairs to both flex and extend their joints, spiders lack the extensor muscles in their outer leg joints. They have muscles to pull their legs inward, but extending them requires a different system.
Spiders use an internal hydraulic system powered by their body fluid, or hemolymph. To straighten a leg, the spider rapidly increases the pressure of the hemolymph within its cephalothorax (the fused head and thorax region), forcing the fluid into the leg segments. This pressurized fluid acts like a hydraulic pump, pushing the leg joints outward to achieve extension.
The internal pressure required for movement varies, reaching 4 to 8 kilopascals (kPa) during normal walking, and surging higher during intense activities like running or jumping. This reliance on a hydraulic system explains why a dead spider’s legs curl inward; without active hydraulic pressure, the flexor muscles take over and the legs collapse toward the body.