The Loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes

Crotalinae

The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...

 (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

Infrared sensing in snakes

The ability to sense infrared thermal radiation evolved independently in several different families of snakes. Essentially, it allows these animals to “see” radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the...

. The loreal pit is bordered by lacunal scales

Lacunal scales

Lacunal scales are those scales that form the inner border of the loreal pit in crotaline snakes. Usually, these scales are large and also curve outward to form the outer border of the pit. In most cases the pit opening is triangular with one apex pointing back towards the eye...

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