The Pit Organs of Two Different Snakes

A python (top) and rattlesnake illustrating the positions of the pit organs. Arrows pointing to the pit organs are red; a black arrow points to the nostril.

The ability to sense infrared thermal radiation  evolved independently in three different groups of snakes, consisting of the families of Boidae (boas), Pythonidae (pythons), and the subfamily Crotalinae (pit vipers). What is commonly called a pit organ allows these animals to essentially "see" radiant heat at wavelengths between 5 and 30 μm. The more advanced infrared sense of pit vipers allows these animals to strike prey accurately even in the absence of light, and detect warm objects from several meters away. It was previously thought that the organs evolved primarily as prey detectors, but recent evidence suggests that it may also be used in thermoregulation and predator detection, making it a more general-purpose sensory organ than was supposed.