Old World Vipers, Pitvipers
Vipers are snakes in the family Viperidae, a group of short-tailed, (usually) stout-bodied snakes with long fangs at the front of the mouth, sited on a short jawbone that can be rotated to bring the fangs from their resting position parallel with the palate to an erect position for striking.
This efficient venom delivery system allows vipers to eat large (and sometimes dangerous) animals without a struggle that might expose them to harm. Vipers make a swift strike in which the long hollow (hypodermic needle-like) fangs inject a strong venom deep into the prey's body. The snakes then wait until the animal dies, tracking it down if necessary, and then calmly swallowing it. The venom also has the effect of initiating digestion even before the prey is swallowed. Many vipers do not find it necessary to eat more than once a month. The venom of these snakes are diverse, being adapted to quickly kill the preferred prey animals of each species.
Vipers are an old and diverse group. They are generally divided into the Old World "true" vipers (Viperinae) and the pit vipers (Crotalinae), which are found in Asia and the Americas. One strange viper, (Azemiops) from
southern China and northern Myanmar (Burma) of unknown relationships, is placed in its own subfamily, Azemiopinae.