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 degree of resistance to the venom of rattlesnakes, suggesting that the rats have evolved in response to the snake venom, thus renewing selection pressure upon the snakes. The selection pressure on snake venom is thought to be selecting for functional diversity within the proteins in venom, both within a given species, and across species. The genes that code for venom proteins in some snake genera have a proportion of synonymous mutations that is lower than would be expected if venom were evolving through neutral evolutionary processes. In addition, snake venom is metabolically costly for a snake to produce, which scientists