The Crotalinae, commonly known as pit vipers, crotaline snakes (named for the Ancient Greek: κρόταλον krotalon castanet/rattle of a rattlesnake's tail), or pit adders, are a subfamily of venomous vipers found in Eurasia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on both sides of the head. Currently, 18 genera and 151 species are recognized: seven genera and 54 species in the Old World, against a greater diversity of 11 genera and 97 species in the New World. These are also the only viperids found in the Americas. The groups of snakes represented here include rattlesnakes, lanceheads, and Asian pit vipers. The type genus for this subfamily is Crotalus, of which the type species is the timber rattlesnake, C. horridus.
These snakes range in size from the diminutive hump-nosed viper, Hypnale hypnale, that grows to an average total length (including tail) of only 30–45 cm (12–18 in), to the bushmaster, Lachesis muta, a species known to reach a maximum total length of 3.65 m (12.0 ft) in length.
What makes this subfamily unique is that all member species share a common characteristic: a deep pit, or fossa, in the loreal area between the eye and the nostril on either side of the head. These loreal pits are the external openings to a pair of extremely sensitive infrared-detecting organs, which in effect give the snakes a sixth sense to help them find and perhaps even judge the size of the small, warm-blooded prey on which they feed. Osine triphosphate, monoamine oxidase, generalized esterases and acetylcholine esterase have also been found in it. When prey comes into range, infrared radiation falling onto the membrane allows the snake to determine its direction. Experiments have shown, when deprived of their senses of sight and smell, these snakes can strike accurately at moving objects less than 0.2 °C (0.36 °F) warmer than the background. The paired pit organs provide the snake with thermal rangefinder capabilities. These organs are of great value to a predator that hunts at night, as well as for avoiding the snake’s own predators.
Among vipers, these snakes are also unique in that they have a specialized muscle, called the muscularis pterigoidius glandulae, between the venom gland and the head of the ectopterygoid. Contraction of this muscle, together with that of the m. compressor glandulae, forces venom out of the gland.