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A pit viper is one of several species of venomous snakes. Its venom is considered dangerous and has been known to cause human fatalities. It can be distinguished from other vipers by the pit-like openings on its head that act as a thermal sensory organs.
The presence of the pit organ necessitates a larger head, which is usually triangular compared to most other vipers. The pit organ lies between the nostrils and the eye on each side of the head. It is a small slit-like opening that is usually visible on pit viper species.
The pit organ in a pit viper is a complicated piece of anatomy. It functions as a sensory organ that affords the snake an additional sense. Beyond the skin there is a membrane that divides the opening into two areas and allows the snake to detect even minimal differences in temperature between an object and the background.
LIke all vipers the pit viper species has front-mounted, hollow fangs that inject venom into prey. Unlike some other snake families, these fangs are hinged on a rotating bone on the roof of the viper's mouth that allows them to be retracted. A pit viper is further distinguished by its ability to inject a desired amount of venom, as opposed to an uncontrolled amount like most other snakes. This is the result of a muscle in the head that is located next to the venom gland.
Much of the world contains some form of pit viper. They are especially prevalent in the Americas and Asia, and the size of the pit vipers within these regions can vary. Some average only a foot in length (30 cm), while others, like the South American bushmaster, reach up to 12 feet in length (about 3.7 meters). They are quite adaptable and can be found in almost any climate type, including desert and tropical.
Most of the pit viper species are nocturnal, avoiding high temperatures during daytime and hunting when their pit organs are of the most use to them. They do not actively search for their prey, opting instead to wait for something to pass by and striking when it is close. Hunting at night allows the pit organ to more clearly detect a hot-blooded body against a cooler background.
One of the most well known members of the pit viper subfamily is the rattlesnake found in the Americas. The lanceheads are another common group of pit vipers. They are considered to be some of the most easily agitated snakes. In the Americas, these snakes account for the majority of human deaths that result from a snake bite. In Asia the genus Trimeresurus, commonly called Asian pit vipers, represents the pit viper population.
Of the venomous snakes in the U.S., only the coral snake is not a pit viper. It's an elapid, and bites are, fortunately, pretty rare.
The U.S. pit vipers are the rattlesnake, copperhead and cottonmouth (water moccasin). Some are more aggressive than others, but even the copperhead -- smallest of the trio -- can deliver a fatal bite if the venom load is high enough and medical treatment isn't given.
The good thing is that many U.S. pit viper bites are "dry" bites, since venom is for hunting, not defense. Still, all snakebite cases should get medical attention as soon as possible.
The other good thing is the only really big snake among the U.S. pit vipers is the Eastern diamondback
-- which has been known to be upwards of 7 feet long. Because of their size, they are capable of delivering a lot of venom, which can be fatal. Antivenin is available, but it's best to avoid a bite altogether by leaving this animal (and all snakes) strictly alone.
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