Pit viper

The Crotalinae, commonly known as pit vipers, crotaline snakes, or pit adders, are a subfamily of venomous vipers found in Asia and theAmericas. 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. 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, 

Description 

These snakes range in size from the diminutive hump-nosed viper, Hypnale hypnale, that grows to an average only 30–45 cm (12–18 in), to the bushmaster, Lachesis muta; a species known to reach a maximum of 3.65 m (12.0 ft) in length—the longest venomous snake in the New World.

What makes this group unique is that they all 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.

The pit organ is complex in structure and is similar to the thermoreceptive labial pits found in boas andpythons. It is deep and located in a maxillary cavity. The membrane is like an eardrum that divides the pit into two sections of unequal size, with the larger of the two facing forwards and exposed to the environment. The two sections are connected via a narrow tube, or duct, that can be opened or closed by a group of surrounding muscles. By controlling this tube, the snake can balance the air pressure on either side of the membrane. The membrane has many nerve endings packed with mitochondria. Succinic dehydrogenase, lactic dehydrogenase, adenosine 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. Having one of these organs on either side of the head produces a stereo effect that indicates distance, as well as 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 warmer than the background. The paired pit organs would seem to provide the snake with thermal rangefinder capabilities. Clearly, 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