Spiders, with their eight segmented legs, possess highly specialized structures at their tips, though not “feet” in the human sense. These adaptations allow them to navigate diverse environments, from scaling smooth surfaces to constructing intricate webs.
Anatomy of Spider Legs
Spider legs are complex appendages, each composed of seven segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The tarsus is the outermost segment, often considered the “foot” of the spider, though not in the human sense. Its tip typically bears two or three claws. Web-building spiders commonly have three claws, with a smaller middle claw, while hunting spiders often possess two. These claws are essential for gripping rough surfaces and manipulating silk.
Spider legs are covered in dense, microscopic hairs called setae. These setae can branch into finer structures known as setules. Spiders that climb smooth surfaces have specialized adhesive pads, called scopulae, covered in these fine hairs. These pads allow for strong adhesion through van der Waals forces. Some cribellate spiders, which produce woolly silk, also possess a calamistrum, a comb-like structure on the metatarsus of their fourth leg used to process silk.
How Spiders Move and Grip
Spiders utilize their leg anatomy to achieve diverse forms of locomotion and grip. When walking on rough surfaces, they use their claws to hook onto irregularities, providing stable traction.
For climbing smooth surfaces, such as walls and ceilings, spiders rely on the dense covering of setae and their finer setules. These microscopic hairs create numerous points of contact with the surface, enabling adhesion through van der Waals forces. This form of adhesion is temporary and reversible, allowing spiders to attach and detach their legs rapidly as they move. The collective force generated by these interactions is powerful enough for a spider to support its body weight, even upside down.
Spider leg movement is distinct from many other animals. While spiders have muscles to flex their limbs inward, the extension of their legs is primarily achieved through a hydraulic system. This system uses hemolymph, their bodily fluid, which is pumped under pressure from the cephalothorax into the legs. This fluid pressure causes the legs to extend, enabling rapid and efficient movement.
Specialized Leg Functions and Adaptations
Spider legs serve several specialized functions. Spiders use chemosensory hairs on their tarsi to “taste” and “smell” their environment, detecting prey, potential mates, and other chemical signals.
Legs are instrumental in silk manipulation. The claws on their tarsi, along with specialized bristles, are used to comb and process silk for constructing webs or wrapping captured prey.
For jumping spiders, their powerful leaps are facilitated by a rapid increase in hemolymph pressure in their legs. This allows them to propel themselves significant distances for hunting or escaping. Some spiders are also adapted for movement on water; they utilize surface tension and specialized hydrophobic hairs on their legs to walk across the water’s surface without sinking.
Web-building spiders avoid sticking to their own webs. They primarily walk on non-sticky silk threads, which form the structural framework of the web. Additionally, their legs possess a non-stick coating and are covered in setae that minimize contact with adhesive droplets, allowing them to navigate their sticky traps.