The heads of vipers tend to be broad and triangular, in order to accommodate large venom glands that are connected by ducts to the snakes’ hollow, needle-like fangs. For the most part, they are ground-dwelling species, although a few, such as members of the genus Atheris, have invaded trees and bushes to develop an arboreal lifestyle. Although there are a few very short forms, as well, most of the true vipers have adult lengths of 2 to 4 feet. Vipers tend to be relatively short, stout snakes, with very few species reaching a maximum length of 6 feet. The true vipers are sometimes referred to as Old World vipers because their range is limited to Africa, Europe and Asia, whereas pit vipers are also found in the New World (the Americas of Earth’s Western Hemisphere). The true vipers are members of the subfamily Viperinae within the family Viperidae, which also includes the subfamilies Causinae (night adders), Azemiopinae (Fea’s viper) and Crotalinae (pit vipers).
The first authentic viper fossils known are from the early Miocene of Europe and Asia, although it is believed that vipers probably first appeared in Africa. At 2 inches in length, the fangs of the Gaboon viper ( Bitis gabonica) are the longest of any venomous snake.
