Claim: The ganglia of spiders are clumped together into one mass.

Spiders -LRB- order Araneae -RRB- are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom . They are the largest order of arachnids and rank seventh in total species diversity among all other orders of organisms . Spiders are found worldwide on every continent except for Antarctica , and have become established in nearly every habitat with the exceptions of air and sea colonization . , at least 45,700 spider species , and 113 families have been recorded by taxonomists . However , there has been dissension within the scientific community as to how all these families should be classified , as evidenced by the over 20 different classifications that have been proposed since 1900 .   Anatomically , spiders differ from other arthropods in that the usual body segments are fused into two tagmata , the cephalothorax and abdomen , and joined by a small , cylindrical pedicel . Unlike insects , spiders do not have antennae . 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 glands . Spider webs vary widely in size , shape and the amount of sticky thread used . It now appears that the spiral orb web may be one of the earliest forms , and spiders that produce tangled cobwebs are more abundant and diverse than orb-web spiders . Spider-like arachnids with silk-producing spigots appeared in the Devonian period about , but these animals apparently lacked spinnerets . True spiders have been found in Carboniferous rocks from , and are very similar to the most primitive surviving suborder , the Mesothelae . The main groups of modern spiders , Mygalomorphae and Araneomorphae , first appeared in the Triassic period , before .   A herbivorous species , Bagheera kiplingi , was described in 2008 , but all other known species are predators , mostly preying on insects and on other spiders , although a few large species also take birds and lizards . It is estimated that the world 's 25 million tons of spiders kill 400 -- 800 million tons of prey per year . Spiders use a wide range of strategies to capture prey : trapping it in sticky webs , lassoing it with sticky bolas , mimicking the prey to avoid detection , or running it down . Most detect prey mainly by sensing vibrations , but the active hunters have acute vision , and hunters of the genus Portia show signs of intelligence in their choice of tactics and ability to develop new ones . Spiders ' guts are too narrow to take solids , and they liquefy their food by flooding it with digestive enzymes . They also grind food with the bases of their pedipalps , as arachnids do not have the mandibles that crustaceans and insects have .   Male spiders identify themselves by a variety of complex courtship rituals to avoid being eaten by the females . Males of most species survive a few matings , limited mainly by their short life spans . Females weave silk egg-cases , each of which may contain hundreds of eggs . Females of many species care for their young , for example by carrying them around or by sharing food with them . A minority of species are social , building communal webs that may house anywhere from a few to 50,000 individuals . Social behavior ranges from precarious toleration , as in the widow spiders , to co-operative hunting and food-sharing . Although most spiders live for at most two years , tarantulas and other mygalomorph spiders can live up to 25 years in captivity .   While the venom of a few species is dangerous to humans , scientists are now researching the use of spider venom in medicine and as non-polluting pesticides . 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 . An abnormal fear of spiders is called arachnophobia .