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Pit viper

Pit viper
Temporal range:
Timber rattlesnake (Crotalus horridus), showing the characteristic infrared pit below and between the eye and nostril
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Suborder: Serpentes
Family: Viperidae
Subfamily: Crotalinae
Oppel, 1811
Synonyms
List
  • Crotalini Oppel, 1811
  • Crotales Cuvier, 1817
  • Crotalidae Gay, 1825
  • Crotaloidae Fitzinger, 1826
  • Cophiadae Boie, 1827
  • Crotaloidei Eichwald, 1831
  • Crotalina Bonaparte, 1831
  • Bothrophes Fitzinger, 1843
  • Crotalinae Cope, 1860
  • Teleuraspides Cope, 1871
  • Crotalida Strauch, 1873
  • Bothrophera Garman, 1884
  • Cophiinae Cope, 1895
  • Lachesinae Cope, 1900
  • Lachesinii Smith, Smith & Sawin, 1977
  • Agkistrodontinii Hoge & Romano-Hoge, 1981
  • Agkistrodontini Hoge & Romano-Hoge, 1983[1]

The Crotalinae, commonly known as pit vipers,[2][3] or pit adders, are a subfamily of vipers found in Asia 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:[4] 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.[citation needed]

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.[5] 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.[2] 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.[5] When prey comes into range, infrared radiation falling onto the membrane allows the snake to determine its direction.[2] 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.[6] The paired pit organs provide the snake with thermal rangefinder capabilities.[7] Clearly, these organs are of great value to a predator that hunts at night, as well as for avoiding the snake’s own predators.[8]

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.[5]

  1. ^ McDiarmid RW, Campbell JA, Touré T (1999). Snake Species of the World: A Taxonomic and Geographic Reference, Volume 1. Washington, District of Columbia: Herpetologists' League. 511 pp. ISBN 1-893777-00-6 (series). ISBN 1-893777-01-4 (volume).
  2. ^ a b c Mehrtens JM (1987). Living Snakes of the World in Color. New York: Sterling Publishers. 480 pp. ISBN 0-8069-6460-X.
  3. ^ Sometimes spelled "pitvipers" – Campbell & Lamar, 2004 [page needed]
  4. ^ "Crotalinae". Integrated Taxonomic Information System. Retrieved 26 October 2006.
  5. ^ a b c Campbell JA, Lamar WW (2004). The Venomous Reptiles of the Western Hemisphere. Ithaca and London: Comstock Publishing Associates. 870 pp., 1,500 plates. ISBN 0-8014-4141-2.
  6. ^ 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. ISBN 0-8014-1095-9 (cloth), ISBN 0-8014-9164-9 (paper).
  7. ^ Bullock, T. H. and Diecke, F. P. J. (1956). Properties of an infrared receptor. Journal of Physiology 134, 47-87.
  8. ^ Stidworthy J. 1974. Snakes of the World. Grosset & Dunlap Inc. 160 pp. ISBN 0-448-11856-4.

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