Pit Viper, a poisonous snake with a deep pit on each side of the head, between the eye and the nostril. The pits are sensitive to heat and help the snake find warm-blooded prey, such as birds and rodents. The fangs in the upper jaw are movable and swing forward before the snake bites its victim. Each fang contains venom. Pit vipers are found from southern Canada to Argentina, in Asia, and in eastern Europe.

There are three genera of pit vipers in North America: the moccasins (Agkistrodon), the pigmy rattlesnakes (Sistrurus), and the rattlesnakes (Crotalus). Pit vipers make up the subfamily Crotalinae of the viper family, Viperidae.