pit viper, poisonous snake of the family Crotalidae, primarily a New World family. Like the Old World true vipers (family Viperidae), pit vipers have long, hollow, erectile fangs that are folded back against the roof of the mouth except when the snake is striking. In addition, the pit vipers have developed special organs of heat reception that help them to sense warm-blooded animals, an ability that is especially useful at night, when many of them hunt. These organs consist of pits, for which the group is named, located just behind the nostrils and covered with a temperature-sensitive membrane. Some pit vipers may also use these organs to find cool refuges from inhospitable daytime temperatures.
The largest group of pit vipers is the rattlesnake genus Crotalus, found in North, Central, and N South America. Other New World forms are the fer-de-lance (genus Bothrops) and the bushmaster (genus Lachesis). The genus Ancistrodon includes the copperhead and water moccasin, as well as about a dozen Asian species. Pit vipers are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, family Crotalidae.