; subphylum Chelicerata, class Arachnida)
Very large and diverse order of predatory arachnids most of which feed on arthropods, although the large tarantulas (Mygalomorphs) will catch nestling birds and other vertebrates, and some will catch tadpoles and small fish. The prosoma has a uniform dorsal carapace bearing two to eight eyes. The chelicerae are large but have a fang (rather than a pincer) which bears the opening of a poison gland lying entirely within the chelicera in primitive species, or extending into the prosoma. The pedipalps are leg-like and the complex structures of the male pedipalp are crucial in sperm transfer, the pedipalp introducing the sperm into the female epigyne with a pumping action. The abdomen (opisthosoma) is usually unsegmented and attached to the prosoma by a narrow, stalk-like pedicel. The abdomen is extremely manœuvrable, bears the spinnerets, and contains two pairs of respiratory organs (book lungs and posterior tracheae), the reproductive organs, and internal glands. Almost all species use their eight walking legs for running rapidly, and some jump by means of rapid rises in internal pressure. The legs have seven segments, the distal segment (pretarsus) having two comb-like claws and often a hook-like median claw for silk manipulation. Spider sensory apparatus varies from sensory setae, trichobothria, and slit sense organs to chemical and tactile receptors on the tarsi and pedipalpi. Vision is important and acute in the Salticidae (jumping spiders). In many species prey items are caught by special webs, sheets, or snares of silk which may be stationary or thrown, and may have drops of sticky adhesive; while other species hunt on the ground, pouncing on their prey. Poison and digestive juices are injected and the digested contents are sucked out. All spiders are dioecious, usually with smaller males, and exhibit elaborate courtship displays involving sexual signalling, nuptial gifts, and some risk to the males in some species. Most species enclose their eggs in a silken sac and guard the eggs and young. As well as using silk for prey capture and egg sacs, spiders may ‘balloon’, using long threads of silk to carry them as high as 1000 m or more, and may travel 100 km. Spiders are found in all terrestrial niches and there is one aquatic species, some shore dwellers, and others which hunt on the surface film of water, their tarsi being specially adapted with hydrophobic hairs. The more than 50 000 or so known species are found on all continents except Antarctica, and in all climates.
Subjects: Zoology and Animal Sciences.