SNAKE!

Snake (reptile) , common name for any reptile belonging to the suborder Serpentes. The body is greatly elongated and most often cylindrical, but the sea snakes and many tree-dwelling snakes have bodies that are flattened laterally. Snakes lack external limbs, although a few still retain tiny remnants of hind legs, the tips of which are visible in large boas and pythons at the base of the tail. The body is covered with scales of a horny epidermal material, arranged in regular rows and usually overlapping, like tiles on a roof. The scales located on the back and sides are characteristically smaller than those on the top of the head and along the ventral (under) surface. The scales on the dorsal (upper) surface of the snake may be smooth or may bear a long raised ridge or keel; other scale modifications may include small knobs and tiny pits. The skin and outer covering of the horny scales are shed periodically and usually in one piece, including the hard, transparent covering of the eye known as the spectacle; snakes lack movable eyelids, and the spectacle protects the constantly open eyes. The frequency of shedding varies with different species and within a species, according to the size and age of the individual. Young, rapidly growing snakes shed their skins more frequently than the slow-growing adults. In some species the skin is shed approximately every 20 days; in others it is shed only once a year.

One surprising characteristic of snakes is their ability to move rapidly without legs. Four quite different types of locomotion are used by snakes. The most frequently used method is the simple, undulating crawl, which appropriately is called the serpentine method. In this type of locomotion, the snake pushes against the ground on the back side of each curve or undulation and flows smoothly forwards. Another method is called caterpillar locomotion, and is used only by the heavier-bodied snakes. The skin of the ventral surface is moved forwards and backwards by strong muscles, and the broad belly scales grip the ground, moving the snake forwards in a straight line. This method has given rise to the erroneous statement that snakes walk on their ribs; actually the ribs do not move forwards and backwards in any of the four types of movement. Several desert-dwelling species use a special type of locomotion, called sidewinding, to move on loose sand. In this method the snake throws its body sideways along the ground in a looping motion. The fourth method is known as concertina locomotion, because the body is alternately stretched out and pulled together as the snake moves from one anchor point to another. The concertina is used in crossing smooth surfaces and in climbing. The most common of the four, and the one that enables all snakes to achieve maximum speed, is the serpentine method. Not all snakes can use each of the other methods. The fastest recorded speed achieved by any snake is about 13 km/h (8 mph), slower than a human adult can run, but few can move that fast. In climbing, any of the methods except sidewinding may be used, but snakes swim only by means of the serpentine method. Some species of snakes of the family Colubridae (garters, kings, and allies) in East Asia and New Guinea are described as being able to fly. They do not actually fly, but they can drop or hurl themselves from fairly high trees and fall or even partly glide to the ground without injury. The long, slender body of the snake contains a large number of vertebrae, never fewer than 100 and sometimes more than 300, each with a pair of ribs except the first two, which connect with the head.

The skeleton is light in structure and is modified to provide greater freedom of movement. The skull in particular is loosely built and can be stretched in several directions, permitting the snake to swallow large prey in relation to the size of the head and body. This mobility can be seen especially in the two bones of the lower jaw, which are attached to the skull by a short, movable bone and which are united at the front end merely by an elastic ligament. Both jaws have a large number of sharp, needle-like teeth , all curved towards the rear of the mouth. The teeth are arranged in six rows paralleling the long axis of the head that is, two rows on each side of the upper jaw and one on each side of the lower jaw. Except in venomous species, the teeth are solid and are replaced periodically. When the snake catches its prey, the recurved teeth enable it to retain a firm hold. The prey is killed quickly and swallowed by alternate movements of the rows of teeth, which work to pull the food into the mouth. As the food passes through the mouth, it is covered with saliva; contrary to popular belief, snakes do not cover their food with saliva before taking it into the mouth. Most meals are swallowed easily and quickly, but a truly large meal may require several hours. The big pythons can consume animals that weigh up to about 68 kg (150 lb), but swallowing such a meal is a laborious process. Venomous snakes have two hollow teeth, somewhat resembling hypodermic needles, and known as the fangs, in the front of the upper jaw. Like the other teeth, they are replaced periodically, and the new fang moves into place before the old one is lost. Thus, for a short time, the snake may have two fangs on each side of the jaw. In the solenoglyphs, a large group of snakes including the rattlesnakes and vipers , the fang-bearing bone is attached to the skull in such a manner that the fangs can be moved back up against the roof of the mouth when not in use. The cobras and coral snakes represent another large group of snakes, the proteroglyphs, in which the fangs are not movable but are constantly erect. The fangs are connected by means of the venom duct to the two venom glands, which are modified salivary glands situated on each side of the head behind the eye. The snake must bite to inject its venom; no snake has a sting in its tail.

Snakes can bite at any time and from any position, including underwater. They normally strike out from a defensive coil, projecting the head and front part of the body at the victim. As soon as the head leaves the coil, the mouth is wide open. The solenoglyphs stab the erect fangs into the target and usually withdraw them immediately, either with or without a biting movement. The proteroglyphs usually bite, and the bite may be retained for several seconds. An interesting modification found in a few species of snake enables them to spit or eject the venom in a fine spray, which is aimed at the eyes of an enemy and projected for distances up to 2.4 m (8 ft). If the venom gets into the eyes, it may cause blindness. The spitting is used only in defence and never to acquire food. In common with other reptiles, snakes are cold blooded and dependent on their environment to maintain their body temperature. They bask in the sun to increase their temperature but seek shade when the temperature rises too high. In the tropics certain snakes aestivate (become dormant) during the hottest season. In regions that have cold winters, snakes hibernate. Venom is a complex mixture of proteins that destroy various tissues of the body, immobilizing or killing the prey. Venoms are frequently classified in two major categories: neurotoxic, or nerve-affecting, and haemotoxic, or blood-affecting. Actually all venoms contain elements of both types, but one type usually predominates. In general, the neurotoxic type is more effective, usually acting on the central nervous system to produce respiratory failure or suppression of the heart action. Haemotoxins destroy blood cells, the lining of the blood vessels, and other tissue. Because of the local destruction of circulatory cells, haemotoxic venoms spread more slowly and have a slower action than the neurotoxins. Snakes have a well-developed nervous system and an intelligence intermediate between that of fish and that of mammals. They show a marked adaptability to captivity, and many become quite tame. The members of many species are nervous and excitable when first caught but usually respond quickly to gentle care. When disturbed in the wild, some snakes go through elaborate bluffing performances; harmless snakes can even appear more dangerous than venomous ones. Although snakes lack a true voice, they often hiss loudly. The bull snakes and their relatives have a small flap in front of the opening of the windpipe that enables them to produce an extremely loud hiss. Many snakes vibrate their tails back and forth when excited, and if they happen to be in dry grass or leaves, an audible whirring noise is produced. The extreme of this condition is seen in the rattlesnake's horny rattles, which are vibrated at an average rate of 50 times per second. The rattling sound, like the hiss, is a warning to enemies; it is not a lure or a signal to other rattlesnakes. A new segment or rattle is added every time the snake sheds its skin, which occurs from one to four times a year, and many segments are broken off from time to timetherefore, contrary to popular opinion, the number of rattles does not indicate the age of the snake. Vision is well developed in most snakes, but many burrowing snakes are virtually blind. The sense of smell is acute and is relied on to a large extent in hunting food. Snakes have an auxiliary chemical sense that is widely employed during most activities. The tongue flicks out, picking up odours and carrying them to the roof of the mouth, into contact with a sensory receptor called Jacobson's organ. The tongue also receives touch stimuli. Snakes are deaf to airborne sounds. Thus a rattlesnake does not hear the sound of another snake's rattle, nor does the cobra hear the snake-charmer's flute. They can perceive vibrations through the ground or whatever they are resting on, however.

The pit vipers , as well as some boas and pythons, have another kind of sense organ, a heat receptor that can perceive small differences in temperature. In the pit vipers the heat receptor is located in the facial pit, between the eye and nostril, that gives the family its name; in the boas and pythons the receptor is located in the labial (lip) pits. By means of these receptors the snakes can locate and seize warm-blooded prey at night.

All snakes are carnivorous, eating a variety of animal life from insects, spiders, and snails to frogs, mice, and rats. Oddly, a number of snakes eat only other kinds of snakes. An alternative to biting prey or injecting them with venom is suffocation. Both the boas and the pythons squeeze their victims to death. Snakes can survive a long time without food. Many may go without food for months and certain large snakes regularly fast for more than a year. Snakes reproduce either by laying eggs, which the female usually abandons in some secluded spot, or by bearing live young, which are also abandoned. Eggs and young may number as many as 100 at one time, but much smaller clutches are usual. Parental care is entirely absent as far as the young are concerned, but a few species guard the eggs during incubation, and the female python actually broods them. Growth is fairly rapid, and the young reach maturity in one to five years. The greatest age known to have been attained by any snake is just under 30 years for both the anaconda and the black-lipped cobra. Snakes are found in all the warmer parts of the world, except on a number of oceanic islands. They occur as far north as the Arctic Circle in Europe, but the number of varieties decreases rapidly from the Tropics towards the poles. About 2,500 different species are known. They are classified in eight to ten families, depending on the system of classification employed for some of the specialized burrowing snakes and arboreal (tree-dwelling) snakes. The family Colubridae is the largest and includes the most common snakes found on all continents except Australia. It includes many harmless snakes such as the garter snakes and the rat snakes. In Australia snakes of the Elapidae family outnumber all others. It includes some of the deadliest snakes in the world, such as the mambas, cobras, and coral snakes. Two other important groups of venomous snakes are the pit vipers, including the rattlesnakes, the bushmaster, and the fer-de-lance, of the family Crotalidae, and the true vipers, including the adders, of the family Viperidae, into which both groups are sometimes placed. The largest snakes in the world are members of the family Boidae, which includes the boas and pythons. Some members of this family never attain a length of more than 0.6 m (2 ft); the giants of the family occasionally reach a size of more than 9 m (30 ft). Snakes, through their carnivorous habits, often play an important part in preserving the balance of life, particularly in the control of such pests as rodents, which tend to multiply at a rapid rate. Snakes are consequently of great service to agriculture in keeping down the numbers of injurious species that afflict the farmer.

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