Pretty Dangerous Spiders
Spiders are air-breathing chelicerate arthropods that have two body segments, eight legs, and no chewing mouth parts. About 40,000 species have been identified. In spiders' bodies the usual arthropod segments are fused into two tagmata, the cephalothorax and abdomen, joined by a small, cylindrical pedicel. As in all arthropods the coelom is very small and the main body cavity is a hemocoel through which hemolymph delivers oxygen and nutrients to the tissues and removes waste products. The gut is so narrow that spiders cannot eat large lumps of solid matter, and spiders liquidize their food by flooding it with digestive enzymes and grinding it with the appendages around their mouths. In all except the most primitive group, the Mesothelae, spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax. Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure. Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of silk gland within their abdomens. Spider silk provides a combination of lightness, strength and elasticity that is superior to that of synthetic materials, and spider silk genes have been inserted into mammals and plants to see if these can be used as silk factories. As a result of their wide range of behaviors, spiders have become common symbols in art and mythology, symbolizing various combinations of patience, cruelty and creative powers.
jumping spider's main ocelli (center pair) are very acute. The outer pair are "secondary eyes" and there are other pairs of secondary eyes on the sides and top of its head.