What defining sensory organ gives the Crotalinae subfamily its common name, Pit Vipers?

Answer

A heat-sensing pit located between the nostril and the eye

The Crotalinae subfamily is defined by the presence of a loreal pit, a specialized sensory organ that detects infrared radiation, allowing them to thermal image warm-blooded prey.

What defining sensory organ gives the Crotalinae subfamily its common name, Pit Vipers?
classificationreptilesnakeviper