Spiders and spiderlike animals belong to the class Arachnida. Arachnids differ from most other arthropods in having no antennae. All adult arachnids have four pairs of legs and have no wings. They usually lack mandibles, having instead fang-like mouthparts to pierce and break up prey.
Spiders are soft-bodied arachnids with two body parts: the fused head and thorax, called the cephalothorax, and the abdomen, also known as the opisthosoma. Spiders have four pairs of walking legs and a fifth pair of appendages, located just behind their mouthparts, known as pedipalps. Pedipalps are not used for locomotion but often assist in touching and maneuvering prey. Each male pedipalp is equipped with a special structure that is used to transfer sperm to the female. This apparatus makes the male palp enlarged; it is often described as resembling a boxing glove. The mouthparts, called chelicerae, each end with a fang. The fang is connected to the venom gland, which enables the spider to inject venom into its prey.
All but a few species of spiders are venomous. (A notable non-venomous example in the Southwest is the feather-legged spider.) While most spiders are venomous, few are dangerous to humans. In Arizona, only the widows (black widow) and the brown spiders (brown recluse types) have venom dangerous to people. Other species may bite and cause some local swelling, but unless one is allergic to the venom, no medical attention is necessary.
Venom in spiders has two functions: prey capture and defense. In its most common use, spiders bite their prey and inject venom, which immobilizes the prey and starts the process of digestion. Spiders have no teeth and rely on the venom to liquefy their prey in order that their stomachs, known as sucking stomachs, can draw in the meal. Some of the larger spiders, such as tarantulas and wolf spiders, have projections on the inside of their chelicerae that help break the prey into smaller pieces to aid digestion. These hunting spiders typically leave a small pellet of crushed exoskeleton after eating, whereas the smaller web-building spiders leave an empty shell of the former prey, neatly cocooned in silk.
The second function of venom is for defense. A spider that is threatened by restraint or by touching may bite. The spider controls the amount of venom injected and may inject none at all. A black widow could bite when its hiding place under a rock or a log is exposed. People are sometimes bitten by brown spiders that have taken up residence in clothes that have been left piled on the floor or in a tent overnight.
Spiders typically have eight eyes, although some species have a reduced number. The brown spiders have only six, while some cave-dwelling spiders have no eyes at all. Although most spiders have eyes, only one group, the jumping spiders, rely on their eyes to hunt. Other spiders use their eyes to orient when wandering or to detect motion in the initial stages of prey capture. Spiders are equipped with special hairs, mostly on their legs, through which they feel, taste and hear. Even at a distance, spiders can “feel” where their prey is by the displacement of air around these hairs.
The diverse use of silk is the most characteristic feature of spider, which have at least four, but more typically six, spinnerets (appendages that spin silk). A spider may have up to six types of silk gland, each producing a different kind of silk. It is actually a complex strand of proteins that is produced as a liquid, and solidifies under tension. Silk is used to build webs, catch food, line burrows, protect eggs, detect prey (as trip lines), and even to aid in dispersal. This dispersal phenomenon is known as ballooning. Spiderlings (baby spiders) climb to the top of an object or plant and let out silk. The spiderlings are so light that, as the air currents take up the silk, the spider is transported, sometimes miles away. Some of the larger spiders, such as tarantulas, are too heavy to balloon, but most smaller species have this ability.
Ecology and Life History
Spiders are predators. They survive by eating other animals such as insects, crustaceans, or even other spiders. Thus when spiders get together it can be potentially life-threatening. But spiders have developed special ways of communicating with each other to avoid cannibalism. Male spiders warn female spiders of their presence by plucking their webs, exchanging chemical signals, tapping the surfaces females are resting on, or producing audible drumming or scrapings. Each species has a specific code, and its use prevents most cannibalism.
Among insects, the primary predators on spiders are spider wasps in the family Pompilidae. Different species of wasps have different strategies, but the result is the same: the wasp lays an egg on the immobilized spider, and when the wasp larva hatches, it eats the spider. Most wasp species do not hunt particular spider species, though they may prefer one over another. Interestingly, the spiders rarely defend themselves when attacked by wasps.
Male spiders, which reach maturity at an earlier age than females, must search for a mate. With the exception of tarantulas, spiders typically mate one time during their lives. The average spider lives for approximately one year, but some of the larger wolf spiders live about two years; and tarantulas may live up to twenty years. The male web-building spider sometimes guards the web of an immature female from other males, mating with her immediately after the molt in which she becomes an adult. Males of other species must go through an elaborate courtship of song and dance before the females will accept them. Once the female has mated, she can store sperm for several months up to a year. She may produce 2 to 3 egg cases during that time. Depending on the species, a few to a few hundred eggs may be laid per egg case, but usually each subsequent egg case is smaller with fewer viable spiderlings.
Adaptation to Desert Life
Spiders are fairly tolerant of environmental extremes. They can adjust their body temperatures to be higher or lower than the ambient temperature by, for example, sunning to warm up, or by escaping into the shade or a burrow to cool down. High body temperatures threaten the loss of water as vapor through transpiration; a loss amounting to over 20 percent of the spider’s body weight is lethal. Spiders can drink, even from moist soil. Many spiders are nocturnal, which means that they are active only during the cooler, more humid parts of the day. Spiders can survive cold winter temperatures by moving to relatively warm micro-climates such as burrows or leaf litter; they curl up to reduce exposure, become rigid and reduce their meta-bolic rate.