Defining the spider Spiders belong to the phylum Arthropoda, along with insects and crustaceans. The order of spiders, Araneae—together with scorpions, harvestmen, and the large order of mites and ticks—make up the class of Arachnida. Spiders differ from other arachnids in having the body divided into cephalothorax and abdomen. About 43,000 spiders are known, but many have yet to be discovered and described. Habitat, physiological characteristics, and behavior Spiders are found in all terrestrial habitats on all continents. Some are even found on or in fresh water. They range from less than 1.0 mm long to tropical tarantula size with leg spread to 25 cm (10 inches). Spiders are more abundant in tropics than in cold countries. They feed on living prey, usually insects. Many spiders hunt their prey, some build burrows with trap doors, others sit camouflaged on plants, and many build webs to snare insects. Hunting spiders may have good eyesight, while web spiders have poor vision but an excellent sense of touch. Fine setae (or hairs) on their legs are sense organs for vibrations. Most spiders are solitary, but in warmer climates some spiders are social, many in a web. Enemies of spiders include some wasps, spiders and birds. The body of a spider consists of an anterior cephalothorax and a posterior abdomen joined by a pedicel. The cephalothorax contains the brain, stomach, and usually venom glands. The digestive tube, blood vessel, nerve cord, and some muscles pass through the pedicel. The abdomen contains the heart, digestive and reproductive systems, lungs or trachaea, tubes for air, and also silk glands. It is segmented in only one small group of spiders in southeast Asia. At the posterior end it bears usually six spinnerets with spigots, each with a different kind of silk. There are usually eight simple eyes on the cephalothorax, sometimes fewer. Attached to the cephalothorax anteriorly is pair of short jaws, each with a fang containing a venom duct; four pairs of legs; and between jaws and legs, a pair of short, leg-like palps. #spider

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