any of more than 200 species of venomous snakes belonging to two groups: pit vipers (subfamily Crotalinae) and Old World vipers (subfamily Viperinae), which are considered separate families by some authorities. They eat small animals and hunt by striking and envenomating their prey. Vipers are characterized by a pair of long, hollow, venom-injecting fangs attached to movable bones of the upper jaw (the maxillaries) that are folded back in the mouth when not in use. Their eyes have vertical pupils, and their scales are keeled. Vipers range in length from less than 25 cm (10 inches) in the Namaqua dwarf viper (Bitis schneideri) of southern Africa to more than 3 metres (10 feet) in the bushmaster (Lachesis muta) of the Amazon (Amazon River) basin and Central America.

Pit vipers are distinguished by a temperature-sensitive pit organ located on each side of the head midway between each nostril and eye. This structure is sensitive to infrared radiation, which enables the snake to “see” heat images of warm-blooded prey. As a pair, they provide a form of binocular vision that helps the snake accurately aim its strike at warm-blooded prey. At least some Old World vipers have infrared receptors in the same area as the pit organs, although there is no external evidence of them. Some boas and pythons have similar infrared organs located in pits between the lip scales.