Loreal pit

The loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes (pitvipers). It is located behind the nostril and in front of the eye, but below the line that runs between the centers of each. It is the external opening to an extremely sensitive infrared detecting organ. The loreal pit is bordered by lacunal scales.The loreal pit is thermal regulating system. Pitvipers maintain their temperature of body through loreal pit.

...
Wikipedia