Vipers

The pitvipers are easily identified by the loreal pit, a heat-sensitive receptor that lies between the nostril and the eye on either side of the face. With this receptor the snakes can detect and accurately strike a warm-blooded prey animal in absolute darkness, guided by the infra-red (heat) rays that the prey animal produces. The Viperinae lack this heat-sensitive pit.

The pitvipers range from eastern Europe to the East Indies and Japan, and from Canada to Argentina in the Americas. Although the 145 species of New World pit vipers are much more diverse than the Old World vipers, most are terrestrial, with a few (usually green) arboreal species. The best-known pit vipers are the rattlesnakes (Crotalus and Sistrurus) which are found only in the Americas and whose modified tail skin vibrates to produce a rattling, warning sound. Most rattlesnakes are North American, ranging from southern Canada to Panama, with the greatest number inhabiting the dry regions of the southwestern United States and northern Mexico. However, a small tropical group (Crotalus durissus and its relatives) ranges as far as southern Brazil.

There are about 60 species of tropical pitvipers (Bothrops and relatives) that range from coastal Mexico southward to the Patagonian plains of Argentina. Most of these are relatives of the large terrestrial South American "fer-de-lance" Bothrops atrox, but Central America has a more diverse assemblage composed of several genera, some of which are treevipers.

The largest of all vipers is the bushmaster (Lachesis muta) of northern South America and Panama, which attains a reported length of 12 ft (4 m). Like the large African vipers, it has very long fangs (2 in/5 cm or more) and a large supply of venom. Very few people are bitten by these snakes, however, because bushmasters live in forested areas and are active only at night.

Asia has a large contingent of pitvipers (including green, arboreal species), similar to the American Bothrops, that are placed in the genus Trimeresurus. Some of its members are so similar to some of the American species that there is some doubt that they should be in different genera.

The water moccasins (genus Agkistrodon) are found both in North America and in Asia. The copperhead (Agkistrodon contortrix) is common in the forests of the eastern United States, and is responsible for most of the venomous snakebites in that region. Fortunately, its venom is not highly toxic to humans, and almost no one dies from these bites.