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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. ISBN 0-8069-6460-X.〕〔Sometimes spelled "pitvipers" -- Campbell & Lamar, 2004 〕 crotaline snakes, 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 either side of the head. Currently, 18 genera and 151 species are recognized: 7 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''. ==Description== These snakes range in size from the diminutive hump-nosed viper, ''Hypnale hypnale'', that grows to an average only , to the bushmaster, ''Lachesis muta''; a species known to reach a maximum of in length. 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.〔Campbell JA, Lamar WW. 2004. The Venomous Reptiles of the Western Hemisphere. Comstock Publishing Associates, Ithaca and London. 870 pp. 1500 plates. ISBN 0-8014-4141-2.〕 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 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. ISBN 0-8014-1095-9 (cloth), ISBN 0-8014-9164-9 (paper).〕 The paired pit organs would seem to 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.〕 Clearly, 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. ISBN 0-448-11856-4.〕 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pit viper」の詳細全文を読む スポンサード リンク
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