Spiders do not possess fur, a characteristic unique to mammals. While many spiders, particularly tarantulas, appear hairy, these coverings are structurally and compositionally distinct from mammalian fur. This “hairiness” stems from the dense array of specialized bristles covering their bodies and legs.
Beyond Fur: Understanding Spider Coverings
The body coverings of spiders are not fur, but rather structures called setae (singular: seta) or bristles. Unlike mammalian fur, which is composed of keratin proteins and grows from follicles within the skin, spider setae are outgrowths of their exoskeleton. This exoskeleton is primarily made of chitin, a tough, flexible polysaccharide. Therefore, spider setae are chitinous extensions, fundamentally different from the keratin-based hair of mammals.
These setae vary significantly in shape and size, ranging from fine, hair-like structures to more robust spines. Their appearance can lead to the mistaken belief that spiders have fur, especially in species like tarantulas where setae are numerous and densely packed. Under magnification, their distinct nature as rigid outgrowths of the external cuticle is revealed, rather than soft, flexible strands of fur. When a spider molts, these setae are shed along with the rest of the exoskeleton, as they are an integral part of this outer layer.
The Diverse Functions of Spider Hairs
Spider setae serve a wide array of functions for their survival and interaction with the environment. One primary function is sensory perception, compensating for limited eyesight. Many setae are mechanoreceptors, detecting vibrations, air currents, and touch. Specialized slender hairs called trichobothria are particularly sensitive to minute air movements, allowing spiders to detect approaching prey or predators even in darkness. Other chemically sensitive setae, often on their legs and mouthparts, function like taste and smell receptors, helping spiders identify mates and assess potential food sources.
Setae also play a significant role in adhesion and locomotion, enabling spiders to navigate various surfaces. Many spiders, particularly those that climb, possess dense mats of microscopic setae on their feet, known as scopulae. These structures are equipped with countless tiny, flattened tips called spatulae, which generate strong adhesive forces, allowing spiders to cling to smooth surfaces like glass or ceilings. Each scopulate hair can have hundreds of branches, further increasing the contact area. This intricate design enables them to walk vertically and even upside down.
Beyond sensory and locomotive roles, setae provide other benefits, including water repellency and defense. The fine structure and density of setae on a spider’s body can trap a layer of air, making their surface hydrophobic and helping them repel water. In some species, specialized barbed setae called urticating hairs are used as a defensive mechanism. When threatened, these spiders can flick these irritating hairs from their abdomen towards a perceived danger, causing irritation and discomfort to predators by embedding themselves in skin or mucous membranes.