Spiders often appear to have a fuzzy or bristly covering, leading many to wonder if these fascinating creatures possess hair similar to mammals. While outwardly resembling hair, these coverings are distinct biological features that serve various specialized purposes. Understanding these structures offers insight into the unique adaptations that allow spiders to thrive in diverse environments.
Understanding Spider Bristles
What appears as “hair” on a spider is fundamentally different from the hair found on mammals. These structures are technically called setae, which are specialized bristle-like extensions of the spider’s exoskeleton. Unlike mammalian hair, which is made of keratin and grows from follicles, spider setae are composed primarily of chitin, a polysaccharide, and proteins, forming a non-living part of their outer cuticle.
Setae are integral components of the spider’s external skeleton, meaning they do not grow from follicles but are rather outgrowths of the body wall. Their structure can vary significantly, ranging from simple, unbranched bristles to complex, branched forms. Some adhesive setae, for example, feature microscopic tips called spatulae or microtrichs, which are essential for their function. The chemical makeup of setae, rooted in chitin, distinguishes them from the keratin-based hair of mammals, highlighting a key biological difference despite superficial similarities. This structural distinction underscores how spiders, as arthropods, have evolved unique integumentary features suited to their lifestyle.
The Specialized Functions of Spider Bristles
Spider setae perform a wide array of functions, each tailored to specific aspects of their survival and behavior. Many setae are highly sensitive sensory organs, acting as mechanoreceptors. These include trichobothria, slender bristles that are extremely sensitive to air currents and vibrations, allowing spiders to detect approaching prey or predators from a distance.
Another important function of setae is adhesion, particularly through structures called scopulae. These are dense mats of setae found on the tips of a spider’s legs, especially in hunting spiders, enabling them to climb smooth surfaces like glass. The fine branches on these setae maximize contact with surfaces, generating adhesive forces through van der Waals interactions, which are weak intermolecular attractions.
Some tarantulas possess urticating hairs, a type of barbed setae on their abdomen, which they can kick off as a defense mechanism when threatened, causing irritation to predators. Setae on a spider’s spinnerets, known as spigots, are directly involved in the production and manipulation of silk for web-building. Specific setae on their legs also help spiders handle their delicate silk strands without getting ensnared in their own sticky webs.