Facial Pit of Rattlesnakes, Cottonmouths, and Copperheads.
Side view of the head
Front view of the head
Illustrations of side and front views of Rattlesnake head showing the eye and nostril (black arrows) and facial pit (red arrow).
The facial pit is a sense organ that detects infrared heat, such as body heat. This enables the snake to locate its warm blooded prey even in the dark. Rattlesnakes, Cottonmouths, Copperheads, and a variety of other venomous snakes that have this pit are grouped together in the pit-viper family, a name that commemorates this unique organ.