Venom types. Among the major classes of toxin in venoms are: Venom is widely distributed taxonomically, being found in both invertebrates and vertebrates; in aquatic and terrestrial animals; and among both predators and prey. The major groups of venomous animals are described below. Venomous arthropods include spiders, which use fangs — part of their chelicerae — to inject venom; and centipedes, which use forcipules — modified legs — to deliver venom; along with scorpions and stinging insects, which inject venom with a sting. In insects such as bees and wasps, the stinger is a modified egg-laying device — the ovipositor. In
Evolution of snake venom molecular history of snake venom have contradicted this, instead finding evidence of rapid adaptive evolution in many different clades, including the carpet vipers, "Echis", the ground rattlesnakes, "Sistrurus", and the Malayan pit viper, as well as generally in the diversification of PLA2 proteins. There is phylogenetic evidence of positive selection and rapid rates of gene gain and loss in venom genes of "Sistrurus" taxa feeding on different prey. Several studies have found evidence that venom and resistance to venom in prey species have evolved in a coevolutionary arms race. For example, wood rats of the genus "Neotoma" have a high