What heat-sensing organ gives pit vipers their common name?

The heat-sensing organ that gives pit vipers their common name is the pit organ.

Pit vipers are members of the snake subfamily Crotalinae, distinguished by a pair of pit organs on the face. Each organ sits in a depression between the eye and nostril, in the loreal region, which is why these snakes are called “pit” vipers.

A pit organ detects infrared radiation from warm objects. Its thin, highly vascularized membrane contains heat-sensitive nerve endings connected to the trigeminal nerve. The paired organs provide directional information, helping a snake locate warm prey even in darkness.

The pit organ is not the same as the nostril, Jacobson's organ, or the nictitating membrane. Jacobson's organ detects chemical cues, while the nictitating membrane is a transparent protective covering over the eye. Boas and pythons also have heat-sensitive facial pits, but their labial pits differ anatomically from the larger loreal pits of pit vipers.