What is the purpose of a spider?

External features

The bodies of spiders, like those of other arachnids, are divided into two parts, the cephalothorax (prosoma) and the abdomen (opisthosoma). The legs are attached to the cephalothorax, which contains the stomach and brain. The top of the cephalothorax is covered by a protective structure, the carapace, while the underside is covered by a structure called the sternum, which has an anterior projection, the labium. The abdomen contains the gut, heart, reproductive organs, and silk glands. Spiders (except the primitive suborder Mesothelae) differ from other arachnids in lacking external segmentation of the abdomen and in having the abdomen attached to the cephalothorax by a narrow stalk, the pedicel. The gut, nerve cord, blood vessels, and sometimes the respiratory tubules (tracheae) pass through the narrow pedicel, which allows the body movements necessary during web construction. Among arachnids other than spiders, the tailless whip scorpions (order Amblypygi) have a pedicel but lack spinnerets. Spiders, like other arthropods, have an outer skeleton (exoskeleton). Inside the cephalothorax is the endosternite, to which some jaw and leg muscles are attached.

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Spiders have six pairs of appendages. The first pair, called the chelicerae, constitute the jaws. Each chelicera ends in a fang containing the opening of a poison gland. The chelicerae move forward and down in the tarantula-like spiders but sideways and together in the rest. The venom ducts pass through the chelicerae, which sometimes also contain the venom glands. The second pair of appendages, the pedipalps, are modified in the males of all adult spiders to carry sperm (see below Reproduction and life cycle). In females and immature males, the leglike pedipalps are used to handle food and also function as sense organs. The pedipalpal segment (coxa) attached to the cephalothorax usually is modified to form a structure (endite) that is used in feeding.

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The pedipalps are followed by four pairs of walking legs. Each leg consists of eight segments: the coxa, attached to the cephalothorax; a small trochanter; a long, strong femur; a short patella; a long tibia; a metatarsus; a tarsus, which may be subdivided in some species; and a small pretarsus, which bears two claws in spiders that do not build webs and an additional claw between them in web-building ones. The young of two-clawed spiders often have three claws. The legs, covered by long hairlike bristles called setae, contain several types of sense organs and may have accessory claws. A few species use the first pair of legs as feelers. Spiders can amputate their own legs (autotomy); new but shorter legs may appear at the next molt.

Most spider species are generalists and have long generation times in comparison to prey, ergo there are no natural control species of spiders per se, according to the University of Michigan. This means introducing a given type of spider to an area -- or removing it -- can have little to no effect on most ecosystems. Indeed, studies synthesized and summarized by University of Maine researchers in 2003 show multiple spider species are more effective at reducing insect populations than single species. As such, it's hard to pinpoint a precise niche for a precise spider. They're no go, for instance, in stopping the singular, explosive outbreak of a single pest species. Still, spiders are far from superfluous to a given ecosystem when considered as a whole. If anything, new, different spider species are important for the simple fact that they're different from those already there.