Rattlesnakes are members of the subfamily Crotalinae (pit vipers), named for the two infrared (heat-sensing) pits located between the eye and nostril. They include the rattlesnakes, massasaugas, water moccasins (cottonmouths), and copperheads. There are approximately 155 species of Crotalines in the Americas and 30 recognized rattlesnake species. Arizona tops the list at 13 and California is home to 7 species.
