Physical Characteristics

                        The body of the snake is long and usually cylindrical, although the bodies of sea
                        snakes and many tree-dwelling snakes are flattened on the sides. The body is
                        covered with horny scales arranged in regular rows and usually overlapping, like tiles
                        on a roof. The scales on the back and sides are generally smaller than those on the
                        top of the head and along the under, or ventral, surface. The scales on the upper, or
                        dorsal, surface of the snake may be smooth or may have a long raised ridge or keel;
                        the scales may also have 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 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 about
                        every 20 days; in others, only once a year. 

                        Structure and Function

                        The long, slender body of the snake contains no fewer than 100 vertebrae and
                        sometimes more than 300. Except for the first two, which connect with the head,
                        each vertebra is connected to a pair of ribs. The skeleton is light and provides great
                        freedom of movement. The skull in particular is loosely built and can be stretched in
                        several directions, permitting the snake to swallow large meals 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, needlelike teeth, all curved toward the rear of the mouth. The teeth
                        are arranged in six rows, 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 movement
                        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 large meal may take several hours. The big
                        pythons can eat animals that weigh up to about 68 kg (150 lb), but swallowing such
                        a meal is a difficult process. 

                        Venomous snakes have two hollow, hypodermiclike teeth, known as fangs, in the
                        front of the upper jaw that, like the other teeth, are replaced periodically. The new
                        fang moves into place before the old one is lost, so that, for a short time, the snake
                        may have two fangs on each side of the jaw. In the solenoglyphs, a large group of
                        snakes that includes the vipers, the fang-bearing bone is attached to the skull in
                        such a way 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 of both groups are connected by 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 stinger 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. Three species of snake can
                        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 defense and never to obtain food. 

                        Snake venom damages various tissues of the body, immobilizing or killing the
                        snake?s prey. Venoms are usually classified in two major categories, neurotoxic, or
                        nerve-affecting, and hemotoxic, or blood-affecting. Because of local damage to
                        circulatory cells, hemotoxic venom spreads more slowly and has a slower action
                        than the neurotoxins. 

                        Although snakes lack a true voice, they often hiss loudly. The bull snakes and their
                        relatives, for example, have a small flap in front of the opening of the windpipe that
                        enables them to produce an extremely loud hiss. 

                        Vision is well developed in most snakes, but many burrowing snakes are virtually
                        blind. Snakes have a strong sense of smell, which is relied on to a large extent in
                        hunting food. They also have an extra chemical sense that is widely used during
                        most activities. The tongue flicks out, picking up odors 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, as it is believed, the snake-charmer?s flute. They can, however, feel vibrations
                        through the ground or whatever they are resting on. 

                        The pit vipers, as well as some boas and pythons, have another kind of sense
                        organ, a heat receptor that can detect small differences in temperature. In the pit
                        vipers the heat receptor is located in facial pits, between the eye and nostril on each
                        side of the face; in the boas and pythons the receptor is located in the labial pits.
                        By means of these receptors the snakes can locate and seize warm-blooded prey
                        at night. 

                        Locomotion

                        Most snakes lack external limbs, although a few have vestigial hind legs, the tips of
                        which are visible at the base of the tail in large boas and pythons. A surprising
                        characteristic of snakes is their ability to move rapidly without legs. Four different
                        types of locomotion are used by snakes in getting from place to place, although not
                        all snakes can use all four methods. The most commonly used method is the
                        simple, undulating crawl, which 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 forward. All snakes use the serpentine method for
                        maximum speed. Snakes move slower than an adult human can run; the fastest
                        recorded speed achieved by any snake is about 13 km/hr (8 mph), but few can go
                        that fast. In another method called caterpillar, used only by the heavier-bodied
                        snakes, the skin of the ventral surface is moved forward and backward by strong
                        muscles, and the broad belly scales grip the ground, moving the snake forward in a
                        straight line. This method has led to the mistaken belief that snakes walk on their
                        ribs; actually the ribs do not move forward and backward 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 rolls its body
                        sidewise along the ground in a looping motion. The fourth method is known as
                        concertina, 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. Except for sidewinding, any of the methods may
                        be used in climbing, but snakes swim only by means of the serpentine method.
                        Some species of the Colubridae family, such as garters, and king snakes, in East
                        Asia and New Guinea reportedly fly. These snakes 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. It was once mistakenly believed that the mud snake could
                        take its tail in its mouth and roll along like a hoop, which gave rise to its popular
                        name, the hoop snake. 

                        Behavior

                        Snakes have a well-developed nervous system and an intelligence intermediate
                        between that of fish and mammals. They adapt well 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, such as the hognose, go through elaborate bluffing performances; harmless
                        snakes can even appear more dangerous than venomous ones. 

                        Many snakes vibrate their tails back and forth when excited, and if they happen to
                        be in dry grass or leaves, a whirring noise may be heard. The rattlesnake?s horny
                        rattles, as an extreme example, 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 mating sound or a signal to other rattlesnakes. Contrary to popular opinion, the
                        number of rattles does not indicate the age of the snake. A new segment or rattle is
                        added every time the snake sheds its skin, which occurs from one to five times a
                        year, and many segments are broken off from time to time. 

                        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. 

                        Reproduction

                        Snakes reproduce either by laying eggs, which the female usually abandons in a
                        secluded spot, or by bringing forth living young from eggs hatched within the body,
                        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 so 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 from one to five years. The greatest age known for any snake is just
                        under 30 years, attained by both the anaconda and the black-lipped cobra. 

                        Habitat

                        Snakes are found in all the warmer parts of the world, except Ireland, New Zealand,
                        and a number of oceanic islands. They occur as far north as the Arctic Circle, but
                        the variety decreases from the Tropics poleward. 

                        Importance

                        Snakes are the most recent group of reptiles to appear, probably evolving from
                        lizards or lizardlike animals about 100 million years ago, toward the end of the
                        Mesozoic era. Human encroachment upon their habitats, as well as the belief that
                        they are dangerous and must be exterminated, threatens to diminish their numbers.

                        Through their carnivorous food habits, however, snakes often play an important part
                        in preserving an ecological balance, particularly in controlling such pests as
                        rodents, which tend to multiply at a rapid rate. They also help to keep down a
                        number of species injurious to agriculture. 

                        The snake has been held in mythical or religious awe by various cultures throughout
                        history. The ancient Egyptians worshipped the asp as a symbol of fertility; the
                        Greeks and Romans associated the Aesculapian snake with the god of healing and
                        medicine; and the serpent figures prominently in the Bible. Among some American
                        Indian tribes, the snake has been associated with rain and fertility. The cobra has
                        been highly regarded in India, and snakes play a role in the practice of voodoo in the
                        West Indies, as well as in snake-handling cults in the U.S. The serpent also
                        appears, usually as a symbol of evil, throughout art and literature.