Spiders are invertebrate animals that belong to the class Arachnida. There are about 40,000 species of spider, which live on all the continents except Antarctica. All spiders can produce silk, the strongest known natural fiber. Spiders are grouped by the types of webs they make.
Spiders have two main body parts, the cephalothorax and the abdomen. The spider's eight legs, the eyes, the fangs and the mouth parts or palps are all part of or attached to the cephalothorax. The silk-making spinnerets are part of the abdomen.
Spiders can produce up to seven kinds of silk, which is used for many functions besides making webs. Spiders line their burrows with silk, encase their eggs and their prey with it, make shelters with it and even use it to glide on the wind, a behavior known as ballooning. Humans also use spider silk to make optical communications equipment and body armor.
All spiders are predators that bite to kill their prey, and all but one family, the Uloboridae, are venomous. However, most spiders are not dangerous to human beings. In spite of this, arachnophobia, or the fear of spiders, affects up to 50 percent of women and 16 percent of men.