Spiders (
order Araneae) 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.
[2] 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. As of November 2015, at least 45,700 spider
species, and 113
families have been recorded by
taxonomists.
[1]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.
[3]
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 websvary 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
386 million years ago, but these animals apparently lacked spinnerets. True spiders have been found in
Carboniferous rocks from
318 to 299 million years ago, 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
200 million years ago.
A
herbivorous species,
Bagheera kiplingi, was described in 2008,
[4] 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.
[5] 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.