Venomous snakes have venom glands and specialized teeth for the injection of venom. Members of the families Elapidae, Viperidae, Hydrophiidae, and Atractaspididae (and some from Colubridae, as well) are major venomous snakes.
Venomous snakes use modified saliva, snake venom, usually delivered through highly specialized teeth, such as hollow fangs, for the purpose of prey immobilization and self-defense. In contrast, nonvenomous species either constrict their prey, or overpower it with their jaws.
Venomous snakes include several families of snakes and do not form a single taxonomic group. This has been interpreted to mean venom in snakes originated more than once as the result of convergent evolution. Evidence has recently been presented for the Toxicofera hypothesis; however, venom was present (in small amounts) in the ancestors of all snakes (as well as several lizard families) as ‘toxic saliva’ and evolved to extremes in those snake families normally classified as venomous by parallel evolution. The Toxicofera hypothesis further implies that ‘nonvenomous’ snake lineages have either lost the ability to produce venom (but may still have lingering venom pseudogenes), or actually do produce venom in small quantities, likely sufficient to assist in small prey capture, but cause no harm to humans if bitten.