Spiders appear fuzzy or hairy, but the structures covering their bodies are not true hair in the biological sense. The bristle-like projections adorning spiders are known by the scientific term setae, which are extensions of the animal’s exoskeleton. These structures serve a multitude of complex biological purposes, acting as sophisticated interfaces between the spider and its environment. Understanding the nature of setae reveals how spiders sense, move, and defend themselves in the world.
Setae: The Spider’s Equivalent to Hair
Setae are fundamentally different from the hair found on mammals. A spider’s body is covered by an exoskeleton composed primarily of chitin, and the setae are direct outgrowths of this hard outer layer. The chitinous composition makes setae more like stiff bristles or spines rather than flexible mammalian fur.
Setae are formed from a single epidermal cell and are anchored in a flexible membrane that allows for movement. They are found across the spider’s entire body, including the legs, pedipalps, and abdomen. This structural distinction means that spider setae are not homologues of mammalian hair; they are an example of convergent evolution.
Sensory Roles of Spider Hairs
Many setae function as highly sensitive mechanoreceptors, allowing the spider to perceive its surroundings without necessarily seeing or touching them directly. These specialized hairs detect mechanical stimuli, translating minute movements into neural signals that the spider can process. This sophisticated sensory system is a primary way a spider navigates, hunts, and avoids predators.
Among the most remarkable sensory setae are the trichobothria, which are extremely fine and flexible hairs typically found on the legs. These structures are suspended in a socket and are exceptionally sensitive to minute air currents, acting as distance receptors. The trichobothria can detect air movement as slow as 0.15 millimeters per second, allowing the spider to sense the wake of an insect flying nearby.
The mechanism for detection is highly refined. When an air current deflects a trichobothrium, the mechanical energy is converted into an electrical nerve impulse at the base of the hair. This response is strictly phasic, meaning the sensors react to the speed and change of the air movement, making them ideal for detecting the turbulent signals generated by prey.
Physical Roles: Adhesion and Defense
Beyond their sensory functions, setae also perform important mechanical roles related to locomotion and protection. Adhesion is managed by dense patches of specialized setae called scopulae, typically located on the tips of the legs and feet of many hunting spiders, such as jumping spiders and tarantulas. These scopulae are covered with thousands of even smaller, microscopic branches known as setules.
The immense surface area created by these microscopic setules allows the spider to adhere to smooth surfaces, including glass, with remarkable strength. Adhesion is achieved through short-range intermolecular forces, specifically van der Waals forces, which arise from the close proximity between the setules and the climbing surface. This dry adhesive system enables the spider to climb vertically and even hang upside down.
Setae are also weaponized for defense, particularly in New World tarantulas, which possess urticating hairs. These hairs are barbed and highly irritating, covering the dorsal surface of the spider’s abdomen. When threatened, the tarantula can flick these hairs into the air by rubbing its hind legs against its abdomen, creating a cloud of microscopic irritants that causes discomfort to predators.