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The Crotalinae, commonly known as pit vipers, or pit adders, are a subfamily of vipers found in Eurasia and the Americas. Like all other vipers, they are venomous. 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, 23 genera and 155 species are recognized: 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 a typical 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.
This subfamily is unique in 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. The pit organ is complex in structure and is similar to the thermoreceptive labial pits found in boas and pythons. 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 (0.36 °F) warmer than the background. The paired pit organs 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 muscularis compressor glandulae, forces venom out of the gland.
The subfamily Crotalinae is found from Eastern Europe eastward through Asia to Japan, China, Indonesia, peninsular India, Nepal, and Sri Lanka. In the Americas, they range from southern Canada southward to Central America to southern South America.
Crotalines are a versatile subfamily, with members found in habitats ranging from parched desert (e.g., the sidewinder, Crotalus cerastes) to rainforests (e.g., the bushmaster, Lachesis muta). They may be either arboreal or terrestrial, and at least one species (the cottonmouth, Agkistrodon piscivorus) is semiaquatic. The altitude record is held jointly by Crotalus triseriatus in Mexico and Gloydius strauchi in China, both of which have been found above the treeline at over 4,000 m above sea level.
Although a few species of crotalines are highly active by day, such as Trimeresurus trigonocephalus, a bright green pit viper endemic to Sri Lanka, most are nocturnal, preferring to avoid high daytime temperatures and to hunt when their favored prey are also active. The snakes' heat-sensitive pits are also thought to aid in locating cooler areas in which to rest.
As ambush predators, crotalines typically wait patiently somewhere for unsuspecting prey to wander by. At least one species, the arboreal Gloydius shedaoensis of China, is known to select a specific ambush site and return to it every year in time for the spring migration of birds. Studies have indicated these snakes learn to improve their strike accuracy over time.
Many temperate species of pit vipers (e.g. most rattlesnakes) congregate in sheltered areas or "dens" to overwinter (brumate, see hibernation), the snakes benefiting from the combined heat. In cool temperatures and while pregnant, pit vipers also bask on sunny ledges. Some species do not mass together in this way, for example the copperhead, Agkistrodon contortrix, or the Mojave rattlesnake, Crotalus scutulatus.
Like most snakes, crotalines keep to themselves and strike only if cornered or threatened. Smaller snakes are less likely to stand their ground than larger specimens. Pollution and the destruction of rainforests have caused many pit viper populations to decline. Humans also threaten pit vipers, as many are hunted for their skins or killed by cars when they wander onto roads.
With few exceptions, crotalines are ovoviviparous, meaning that the embryos develop within eggs that remain inside the mother's body until the offspring are ready to hatch, when the hatchlings emerge as functionally free-living young. In such species, the eggshells are reduced to soft membranes that the young shed, either within the reproductive tract, or immediately after emerging.
Brood sizes range from two for very small species, to as many as 86 for the fer-de-lance, Bothrops atrox, which is among the most prolific of all live-bearing snakes.
Many young crotalines have brightly coloured tails that contrast dramatically with the rest of their bodies. These tails are known to be used by a number of species in a behavior known as caudal luring; the young snakes make worm-like movements with their tails to lure unsuspecting prey within striking distance. 
In the past, the pit vipers were usually classed as a separate family: the Crotalidae. Today, however, the monophyly of the viperines and the crotalines as a whole is undisputed, which is why they are treated here as a subfamily of the Viperidae.
|Genus[ref 1]||Taxon author[ref 1]||Species[ref 1]||Common name||Geographic range[ref 2]|
|Agkistrodon||Palisot de Beauvois, 1799||6||Moccasins||North America from the northeastern and central USA southward through peninsular Florida and southwestern Texas. In Central America on the Atlantic versant from Tamaulipas and Nuevo León southward to the Yucatán Peninsula, Belize and Guatemala. Along the Pacific coastal plain and lower foothills from Sonora south through Guatemala, El Salvador, Honduras and Nicaragua to northwestern Costa Rica.|
|Atropoides||Werman, 1992||1||Picado's jumping pitviper||Costa Rica and western Panama|
|Bothriechis||Peters, 1859||11||Palm-pitvipers||Southern Mexico (southeastern Oaxaca and the northern highlands of Chiapas), through Central America to northern South America (Colombia, western Venezuela, Ecuador and northern Peru|
|Bothrocophias||Gutberlet & Campbell, 2001||9||Toadheaded pit vipers||Northern South America|
|Bothrops||Wagler, 1824||48||Lanceheads||Northeastern Mexico (Tamaulipas) southward through Central and South America to Argentina; Saint Lucia and Martinique in the Lesser Antilles; Ilha da Queimada Grande off the coast of Brazil|
|Calloselasma||Cope, 1860||1||Malayan pitviper||Southeast Asia from Thailand to northern Malaysia and Java, Indonesia|
|Cerrophidion||Campbell & Lamar, 1992||5||Montane pitvipers||Southern Mexico (highlands of Guerrero and southeastern Oaxaca), southward through the highlands of Central America (Guatemala, El Salvador, Honduras, northern Nicaragua, Costa Rica) to western Panama|
|Craspedocephalus||Kuhl & van Hasselt, 1822||15||Pit viper||India to Thailand to northern Malaysia and Indonesia|
|CrotalusT||Linnaeus, 1758||51||Rattlesnakes||The Americas, from southern Canada to northern Argentina|
|Deinagkistrodon||Gloyd, 1979||1||Hundred-pace pitviper||Southeast Asia|
|Garthius||Malhotra & Thorpe, 2004||1||Mount Kinabalu pit viper, Chasen's mountain pit viper||Borneo|
|Gloydius||Hoge & Romano-Hoge, 1981||22||Asian moccasins||Russia, east of the Ural Mountains through Siberia, Iran, the Himalayas from Pakistan, India, Nepal and China, Korea, Japan and the Ryukyu Islands|
|Hypnale||Fitzinger, 1843||3||Hump-nosed pit vipers||Sri Lanka and India|
|Lachesis||Daudin, 1803||4||Bushmasters||Central and South America|
|Metlapilcoatlus||Campbell, Frost, & Castoe, 2019||6||Jumping pitvipers||The mountains of eastern Mexico southeastward on the Atlantic versant and lowlands though Central America to central Panama. On the Pacific versant, they occur in isolated populations in east-central and southern Mexico, Guatemala, El Salvador, Costa Rica, and Panama.|
|Mixcoatlus||Jadin, H.M.Smith & Campbell, 2011||3||Mexican pit vipers||Mexico|
|Ophryacus||Cope, 1887||3||Mexican horned pitvipers||Mexico|
|Ovophis||Burger, 1981||7||Mountain pit vipers||Nepal and Seven Sisters (Assam) of India eastward through Myanmar, Cambodia, Thailand, Laos, Vietnam, West Malaysia, Taiwan, Japan (Okinawa) and Indonesia (Sumatra)|
|Porthidium||Cope, 1871||9||Hognose pit vipers||Mexico (Colima, Oaxaca and Chiapas on the Pacific side, the Yucatán Peninsula on the Atlantic side) southward through Central America to northern South America (Ecuador in the Pacific lowlands, northern Venezuela in the Atlantic lowlands)|
|Protobothrops||Hoge & Romano-Hoge, 1983||14||Pit vipers||Asia|
|Sistrurus||Garman, 1883||3||Ground rattlesnakes||Southeastern Canada, eastern, central and northwestern USA, isolated populations in northern and central Mexico|
|Trimeresurus||Lacépède, 1804||43||Asian lanceheads||Southeast Asia from India to southern China and Japan, and the Malay Archipelago to Timor|
|Tropidolaemus||Wagler, 1830||5||Temple vipers||Southern India and Southeast Asia|
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