Infrared sensing in snakes

The ability to sense infrared thermal radiation evolved independently in two different groups of snakes, one consisting of the families Boidae (boas) and Pythonidae (pythons), the other of the family 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. The facial pit underwent parallel evolution in pitvipers and some boas and pythons. It evolved once in pitvipers and multiple times in boas and pythons. The electrophysiology of the structure is similar between the two lineages, but they differ in gross structural anatomy. Most superficially, pitvipers possess one large pit organ on either side of the head, between the eye and the nostril (loreal pits), while boas and pythons have three or more comparatively smaller pits lining the upper and sometimes the lower lip, in or between the scales (labial pits). Those of the pitvipers are the more advanced, having a suspended sensory membrane as opposed to a simple pit structure. In pit vipers, the heat pit consists of a deep pocket in the rostrum with a membrane stretched across it. Behind the membrane, an air-filled chamber provides air contact on either side of the membrane. The pit membrane is highly vascular and heavily innervated with numerous heat-sensitive receptors formed from terminal masses of the trigeminal nerve (terminal nerve masses, or TNMs). The receptors are therefore not discrete cells, but a part of the trigeminal nerve itself.