Spider, an animal closely related to mites, ticks, and scorpions. There are about 35,000 known species, more than 3,000 of them in the United States. Spiders live almost everywhere—in soil, under rocks, on grasses, on tree branches, in caves, and on water. Spiders are not insects, although they belong to the same phylum of animals as insects. They differ from insects in that they and wings and have eight, instead of six, legs.
Spiders are known for the silk webs they spin to trap their prey and for their venomous bites. However, not all spiders spin webs and, although most species have poison glands, only a few species are poisonous to humans. In the United States, the most poisonous spiders are the female black widow spider and both sexes of the brown recluse spider. Most spiders are helpful to humans, killing insects harmful to animals and plants.Spiders are eight-legged, fanged arachnids that hunt or build webs to catch food.