Vary widely from 1-100mm in body length. Have eight legs and two body divisions: the cephalathorax (fused head and thorax) and abdomen. Differ from other arachnids by a medial constriction of the body. Biting or piercing jaws produce venom. Spinnerets at end of abdomen produce silk. Found in most habitats. Eat a variety of invertebrates. Some will eat frogs and birds. Predators include birds, lizards, scorpions and other spiders.