The class Arachnida is a group of mostly terrestrial arthropods that includes spiders, scorpions, mites, ticks, harvestmen, pseudoscorpions, and sun spiders. Many arachnids have highly developed chelicerae (also found in pyc-nogonids and horseshoe crabs) in the head region, which usually take the form of a pair of pincerlike organs. It is possible that arachnids evolved from related pincer-bearing arthropods that migrated from the sea to the land. In doing so, arachnids have developed a cuticle, which reduces water loss, and their book gills have become book lungs.
Like all arthropods, arachnids have a hard exoskeleton made of chitin and jointed appendages, but unlike most of their relatives, true arachnids have no antennae. Their bodies are divided into a cephalothorax and an abdomen. The cephalothorax, called a prosoma, is usually unsegmented, and its upper surface is covered with a carapace. The lower region is usually protected by plates. The abdominal segments of most arachnids, apart from scorpions, are fused. In ticks and mites, both the cephalothorax and abdomen are fused to form a single body.
The first pair of appendages, called chelicerae, are used to grasp prey and to feed. The second pair, called pedipalpi, perform various functions, including grasping, killing, or mating, depending on the species. The remaining four sets of appendages are legs. Although most arachnids are carnivorous, they have no jaws and cannot chew. They feed by secreting or injecting digestive enzymes into the prey, which they then suck in.
The respiratory system of arachnids is made up of specialized breathing organs, called book lungs, and a network of tracheae, which are tubes that carry air from the exterior to the internal organs. Some arachnids have book lungs only, others have tracheae only, and still others have both. The circulatory system is usually open—that is, arteries carry blood from the heart into a series of blood spaces called the hemocoele. Blood is oxygenated as it flows past the book lungs on its way back to the heart. Some arachnids have a copper-based, oxygen-carrying compound called hemocyanin as the major respiratory pigment of the blood. The excretory system includes a pair of Malpighian tubules, which carry wastes from the blood to the gut, and coxal glands, which open to the exterior in the cephalothorax.
The brain consists of two bundles of nerve cells, or ganglia, above the esophagus, joined to more ganglia below it. Nerves from the ganglia run to various sensory organs, and there are sensory hairs over the body surface. Most arachnids have simple eyes—spiders usually have eight, scorpions have six to twelve. Even so, their eyesight is, in most cases, very poor.
Reproductive systems vary in arachnids, but the sexes are generally separate. Fertilization is by copulation, usually preceded by an elaborate courtship ritual. Sperm is often transmitted through a packet of sperm called a spermatophore, which is picked up by the female. When hatched, most arachnids are small versions of the adults; they do not metamorphose as do many insects. The young molt several times while growing.
There are more than 30,000 known species of spiders. The largest spiders are the tarantulas of South America (suborder Orthognatha), some species of which have a body 3 inches (8 centimeters) across with a leg spread of 7 inches (18 centimeters).
Spiders are all predators and feed mainly on insects. Larger spiders, however, sometimes feed on vertebrates such as fish. Most spiders have poison glands in the cephalothorax, and inject the poison by means of the chelicerae. The poison is used to paralyze or kill prey, or in self-defense. Only a few spiders have venom strong enough to endanger humans.
Many spiders produce silk threads, which they use to spin webs, catch prey, and make cocoons. Abdominal glands secrete the silk through organs called spinnerets. Not all spiders spin webs, however. Trap-door spiders (family Ctenizidae) dig a tunnel, which they line with silk. The tunnel is closed at ground level by a hinged door. When a small animal passes by, the spider jumps out and grabs it. Many wolf spiders (family Lycosidae) and jumping spiders (family Salticidae) do not trap their food but stalk their prey and then leap on it.
Spiders perform a courtship ritual before mating. Male wolf spiders, for example, wave their pedipalpi to attract a female. In many web-spinning spiders, the courting male plucks the threads of the web in a special way so that the female will not mistake him for prey. Before courtship begins, the male spins a “sperm web,” onto which he drops semen, and fills a reservoir at the tip of his pedipalp with sperm. During mating, he inserts the pedipalp into a special pouch in the female, in which the sperm are stored. The female lays the eggs later and sperm are released over them. She then wraps the fertilized eggs in layers of silk to form a protective cocoon, which she hides or carries around until they hatch.
Scorpions (order Scorpionida) have remained virtually unchanged for about 450 million years. They are the largest of the true arachnids, reaching 5 to 8 inches (12 to 20 centimeters) in length. Scorpions are nocturnal and live in tropical and subtropical regions. Their pedipalpi are powerful pincers used to grasp prey. The segmented tail has a sharp sting at the tip with a poison gland, which causes paralysis and death to insects and small mammals.
Mites and ticks (order Acarina) are small arachnids, usually less than one millimeter in length. Many are parasites, living on blood and tissue fluids, causing skin irritation and occasionally transmitting diseases to their hosts. Their chelicerae can pierce skin, making ticks, for example, difficult to dislodge.
Harvestmen (order Phalangida) are spiderlike animals with long legs and only two eyes. Pseudoscorpions (order Chelonethida), like true scorpions, have pincers, but no tail or sting. Sun spiders (order Solufugae), which can be up to 2 inches (5 centimeters) long, have simple elongated pedipalpi that make them look like ten-legged spiders.