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The Crotalinae, commonly known as pit vipers,Mehrtens JM (1987). Living Snakes of the World in Color. New York: Sterling Publishers. 480 pp. .Sometimes spelled "pitvipers" – Campbell & Lamar, 2004 or pit adders, are a subfamily of Viperidae found in Asia and the Americas. Like all other vipers, they are venomous snake. 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 genus and 155 species are recognized: These are also the only Viperidae found in the Americas. The groups of snakes represented here include , Bothrops, and Trimeresurus. 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 , to the bushmaster, Lachesis muta, a species known to reach a maximum total length of 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 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.Campbell JA, Lamar WW (2004). The Venomous Reptiles of the Western Hemisphere. Ithaca and London: Comstock Publishing Associates. 870 pp., 1,500 plates. . The pit organ is complex in structure and is similar to the thermoreceptive labial pits found in Boidae and Pythonidae. 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 , 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 warmer than the background.Parker HW, Grandison AGC. 1977. Snakes -- a natural history. Second Edition. British Museum (Natural History) and Cornell University Press. 108 pp. 16 plates. LCCCN 76-54625. (cloth), (paper). The paired pit organs provide the snake with thermal rangefinder capabilities.Bullock, T. H. and Diecke, F. P. J. (1956). Properties of an infrared receptor. Journal of Physiology 134, 47-87. These organs are of great value to a predator that hunts at night, as well as for avoiding the snake’s own predators.Stidworthy J. 1974. Snakes of the World. Grosset & Dunlap Inc. 160 pp. .
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.
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.Richard Shine, Sun L, Kearney M, Fitzgerald M (2002). "Why do Juvenile Chinese Pit-Vipers ( Gloydius shedoaensis) Select Arboreal Ambush Sites?" Ethology 108: 897–910. ISSN 0179-1613. PDF at University of Sydney School of Biological Sciences. Accessed 26 October 2006.
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.
Among the oviparous (egg-laying) pit vipers are Lachesis, Calloselasma, and some Trimeresurus species. All egg-laying crotalines are believed to guard their eggs.
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.
| 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 pit viper | Costa Rica and western Panama |
| Bothriechis | Wilhelm Peters, 1859 | 19 | Palm-pit vipers | 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 pit viper | Southeast Asia from Thailand to northern Malaysia and Java, Indonesia |
| Cerrophidion | Campbell & Lamar, 1992 | 5 | Montane pit vipers | 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 | Heinrich Kuhl & van Hasselt, 1822 | 15 | Pit viper | India to Thailand to northern Malaysia and Indonesia |
| Crotalus | Carl Linnaeus, 1758 | 51 | Rattlesnakes | The Americas, from southern Canada to northern Argentina |
| Deinagkistrodon | Gloyd, 1979 | 1 | Hundred-pace pit viper | 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 pit vipers | 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 pit vipers | 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 | Samuel 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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