Spiders are air-breathing arthropods that have eight legs and fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exception of air and sea colonization. As of 2008, approximately 40,000 spider species have been recorded
Although most spiders live for at most two years, tarantulas 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 pesticide. Spider silk provides a combination of lightness, strength and elasticity that is superior to that of man-made materials.
Females lay up to 3,000 eggs in one or more silk egg sacs, which maintain a fairly constant humidity level. In some species the females die afterwards, but females of other species protect the sacs by attaching them to their webs, hiding them in nests, attaching them to the spinnerets and dragging them along.
Baby spiders pass all their larval stages inside the egg and hatch as spiderlings, very small and sexually immature but similar in shape to adults. Some spiders care for their young, for example a wolf spider's brood cling to rough bristles on the mother's back.
Like other arthropods, spiders have to molt to grow as their cuticle ("skin") cannot stretch. In some species males mate with newly molted females, which are too weak to be dangerous to the males.
Spiders occur in a large range of sizes. The smallest are less than 0.015 inch in body length. The largest and heaviest spiders occur among tarantulas, which can have body lengths up to 3.5 inches and leg spans up to 10 inches.
The best-known method of prey capture is by means of sticky webs. Varying placement of webs allows different species of spider to trap different insects in the same area. Web-building spiders have poor vision, but are extremely sensitive to vibrations
The primitive trapdoor spiders and many tarantulas are ambush predators that lurk in burrows, often closed by trapdoors and often surrounded by silk threads that alert these spiders to the presence of prey. Wolf spiders and jumping spiders capture prey by chasing it, and rely mainly on vision to locate prey.
Most spiders will only bite humans in self-defense, and few produce worse effects than a mosquito bite or bee-sting. Most of those with medically serious bites, such as recluse spiders and widow spiders, are shy and bite only when they feel threatened, although this can happen accidentally.