Spiders are the dominant arachnids in most habitats. This group surpasses all others in number and variety of species, in complexity of behavior patterns, and in the range of habitats occupied. They are similar in morphology and behavior to some of the other arthropods, including the tailless whip-scorpions (Amblypygi) and pseudoscorpions (Pseudoscorpiones). The cephalothorax and abdomen are separated by a waist, which is formed by the constriction of the pregenital segment. The abdomen is composed of 11 segments. Two pairs ofbook-lungs open between the sternites of abdominal segments 1 and 2. The abdomen has specialized glands for producing and manipulating silk, which plays an important role in the foraging and dispersal ofthese arthropods. Spiders have poison glands in the chelicerae (mandibles); the second segment of the chelicerae forms a sharp, piercing fang with an opening at the tip to release the poison. Most spiders can give a poisonous bite, but few have mandibles strong enough to penetrate human skin, and when they do, the venom is usually harmless.

The food of spiders includes a variety of other arthropods, including sowbugs, millipedes, pseudoscorpions, harvestmen spiders, and other spiders. As predators their potential food is variable, but most species are adapted to a size and frequency of prey that occur in their habitat. Some attack large insects, while others capture small species. Some spiders capture diurnal insects, others hunt at night; some specialize in crawling insects, others in those that fly. The majority of spiders in the urban environment live around stones or leaf litter on the ground. Some excavate burrows in loose soil, while others find harborage under or around stones or logs, or in the moist layer of decaying leaves and plant litter. Some of the small-sized species are always wandering and may not construct a silken retreat. Large-sized species usually live permanently in a burrow and forage close to this site. Species that use large webs to trap their prey usually construct a silken retreat at the edge or close to the web. Vibration of the silk strands that connect the retreat with the web enables the spider to respond to a struggling prey. The daily life of web-building spiders centers on their ability to detect and respond quickly to vibrations in their web.

Digestion of food is initiated outside the body. After the prey has been immobilized by a poisonous bite or wrapped with silk, digestive fluid from the gut of the spider is applied to the victim. Within a short time, the spider sucks in the digestive fluid and the predigested body of the prey. This process is repeated until the prey has been consumed. Specific feeding behavior depends on the presence of cheliceral teeth. Theridiid and thomisid spiders lack cheliceral teeth and make only a small hole in their prey. Digestive fluid is passed in and out of this hole and the dissolved tissue is gradually consumed. After the meal the prey is an empty shell. Spiders with cheliceral teeth are capable of crushing their prey and consuming the predigested contents. After the meal the prey is an unrecognizable mass of small pieces ofcuticle.

Spider venom is a mixture of substances, mostly of neu-rotoxic polypeptides with molecular weights of 5-13 kDa. Biogenic amines and proteolytic enzymes are also present. Poison glands are a feature of nearly all spiders, and they are all potentially harmful, at least to their prey. Only 20-30 of the 30000 total species are dangerously poisonous to humans. The bite of the black widow spider is extremely painful at the site of the bite, but the physiological effects away from the site are potentially the most dangerous. These include accelerated heart beat, increased blood pressure, and paralysis of the diaphragm muscles, which often results in suffocation. The poison of the black widow spider consists of seven different proteins and is a neurotoxin that affects neuromuscular synapses. Bites of tarantulas (Theraphosidae) are relatively harmless, in spite of their reputation; generally, the bite is no worse than a wasp sting. The European tarantula (Lycosa tarentula) has a long history of causing severe and sometimes unusual reactions in people bitten, but the symptoms (including erratic dancing) of tarantism may be related to other causes or another spider. The bite of some tropical lycosid spiders, such as Lycosa erythrognatha of Brazil, can cause tissue lysis and necrosis at the site of the bite.

All spiders are capable of producing silk. They all possess spinning organs, which they use to form silk into egg-sacs, draglines when walking or hunting, and for building snares or webs to trap prey. Silk is a proteinaceous substance, fibroin; the amino acids alanine, glycine, and serine constitute 50-60% of the total fibroin. Silk of the orb weaver Nephia has a molecular weight of about 30 kDa as a liquid in the silk gland, and between 200 and 300 kDa as a solid silk thread. The tenacity of spider silk may be compared to that of nylon. The dragline of the garden spider, Araneusspp., has a tenacity of 7.8 g/denier; the corresponding value for nylon is 8.7. Spider silk can be stretched by 31%, compared with 16% for nylon; the length at which it breaks under its own weight can be as much as 80 km. The silk from different spider species is slightly different, and the silk from different glands of the same spider is different. Silk from pseudoscorpions (Neobisium maritimum) differs from that of spiders, primarily by the smaller amount of glycine and alanine compared to spider silk.

Mating procedure and the premating behavior of males and females are elaborate, and generally species-specific. The mature male prepares a small pad of silk on which a small amount ofsperm is deposited. This is taken up by the modified pedipalps, which are later inserted into the genital opening of the female to deposit the sperm. Males of each species have distinctly shaped palps, and a courtship thatresults in successful transfer ofsperm to the female. Depending on the species, males and females mate more than once. Females lay their eggs in silken sacs, and often carry or remain with the eggs until they hatch. The number of eggs laid is highly variable, but the majority of species lay about 100 eggs. Those that produce more than one egg-sac usually lay a small number of eggs in each, and large-bodied species tend to lay more eggs than small species. After hatching, the spiderlings remain together for several days without feeding, then molt and disperse. The number of molts during development is variable. In general, small spiders molt four or five times; medium-sized spiders molt seven or eight times; and large spiders molt more than 20 times. In the majority of species the life cycle varies from 8 months to 4 years. Spiders in temperate regions usually live 1 year, while those in tropical regions usually live 2-3 years.

There have been several attempts at establishing a practical and perhaps phylogenetic classification of these spiders. However, with at least 30 000 described species and differing views on whatinternal or external characters should be used to show relationships, no system has sustained wide acceptance. Only family and species names are used here, and the alphabetical arrangement removes any phylogenetic implication.

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