VIPER

any of about 200 species of venomous snakes of the family Viperidae. Vipers form two groups, or subfamilies, the Viperinae (Old World vipers) and the Crotalinae (pit vipers), considered separate families by some authorities. Vipers are characterized by a pair of long, hollow, venom-injecting fangs attached to movable bones of the upper jaw (the maxillaries) that can be folded back in the mouth when not in use. The pit vipers (copperheads, rattlesnakes, and others) are further distinguished by a heat-sensitive pit organ located midway between each nostril and eye. The pit organs help the snake accurately aim its strike at warm-blooded prey. Vipers range in length from less than 30 centimetres (12 inches) in Orsini's viper (Vipera ursini) to more than 3 metres (10 feet) in the bushmaster (Lachesis muta; see ). They eat small animals and hunt by striking, then trailing, their prey. Old World vipers are found in Europe, Asia, and Africa. They are typically slow, stocky, and broad-headed. Many are terrestrial, but tree vipers (Atheris) are slender, prehensile-tailed, and arboreal, while mole vipers (Atractaspis) are small-eyed burrowers. Most species bear live young. Pit vipers are found from deserts to rain forests, primarily in the New World. A genus found in Central and South America is Bothrops . They may be terrestrial or arboreal. Some, such as the moccasins (Agkistrodon species), are aquatic. Some species lay eggs; others produce live young.