In vipers or adders (Viperinae) and pitvipers (Crotalinae), the head is roughly triangular and is distinct from the rest of the body. The head bears nine symmetrical plates (as in Agk-istrodon and Gloydius), fragmented head plates (as in many Vipera), or numerous small scales (as in Bitis and Ophryacus). The pupils are usually vertical and elliptical. All the face bones are movable. Each of the two shortened, retractile upper jawbones (maxillae) bears only the tubular venom fang (which can be activated only for a short period of time) and often one to several significantly smaller reserve teeth of various sizes, none being a firmly positioned poison fang. The tail is short, and the male copulatory organ is bifid (forked).
Viperines are generally compact, sturdy snakes, and their length ranges from 11.8 in (30 cm: dwarf puff adder) to 5.9 ft (180 cm: Gaboon adder). Coloration is usually drab, and in the genus Vipera it often includes a dark zigzag pattern or a rhomboid band along the back. Desert species are sand yellow, whereas jungle vipers often have a colorful carpet marking.
Pitvipers exhibit great variety of size, shape, and color, but any pitviper can be recognized easily by the deep, heat-sensitive pits, one on each side of the head, between the eye and the nostril. Color variation within the Crotalinae reflects the diversity of habitats occupied by these snakes: invariably, pitvipers are cryptic in their native haunts. In general, pitvipers are relatively stout, although some arboreal species may be more slender. Tail length varies from quite short in rattlesnakes to relatively long in most arboreal species. The tail is prehensile in species that are adapted fully to an arboreal existence. The longest viperids are pitvipers of the genus Lachesis, some specimens of which are known to reach 11.8 ft (3.6 m).
One of the most well-known and interesting features of pitvipers is the rattle, which is unique to the genera Crotalus and Sistrurus. A neonatal rattlesnake has a keratinized "button" at the tip of its tail; each time the snake sheds, a rattle segment is added. Segments of the rattle fit loosely together, so that a clearly audible sound is produced when the tail is vibrated. Interspecific variation in rattle size has considerable effect on the sound, with large species producing particularly loud and effective warnings. Because most rattlesnakes shed more than once a year and because the ends of rattles tend to break off in wild snakes, the number of rattle segments cannot be used as a direct indication of age.
Sometimes known as night adders, species of Causus are relatively small, terrestrial snakes with a stout body covered by weakly keeled scales. Nine plates cover the top of the head. The pupils are round, and the fangs are relatively short. Despite the short fangs, some species have very long venom glands. The sole living species of Azemiopinae is characterized by smooth scales and nine large plates on top of the head.
The most important distinguishing characteristic in all viperids is the venom apparatus. Their poison fangs have no sign of grooves; they actually have enclosed canals within the fangs that transmit venom out of the body, very much like a hypodermic needle. The two upper jawbones (maxillae), which bear the fangs, are very short. Each maxilla has a special joint that permits this bone, along with the fang anchored firmly within it, to rotate 90°. When the viper closes its mouth, the fangs lay back, tip inward, and are covered by a fold in the mucous membrane. When the mouth is opened, a lifting mechanism is activated, putting the fangs into a vertical position by means of the rod-shaped ectopterygoid bones and the pterygoid. The fangs are then in position to bite and inject venom. The fangs (or, more precisely, the maxillae) are laid back with the same action as when a pocketknife is snapped together. The adaptation of folding back the fangs (referred to as solenoglyphous dentition) permits them to be extremely long, far exceeding the length of those in such snakes as cobras, which bear fangs that are fixed in the down, or vertical, position. The fangs of the giant king cobra are not much longer than those of the rather small adder. The long fangs enable vipers to bite deeply into the tissues and cause the victim to suffer severe necrosis. The fangs fold back into the mouth after they are withdrawn from the victim.
Viper venom contains primarily hematoxic material (i.e., substances injurious to the blood and the blood vessels). Thus, a viper bite typically has a very different effect from a cobra or mamba bite (their venom being primarily neurotoxic, that is, injurious to the nervous system). Viper bites are accompanied by prominent local irritation and symptoms of severe blood poisoning, with burning pain, inflamed swellings, pronounced discoloration, sudden drop in blood pressure, internal bleeding, degeneration of the tissues, and the formation of an abscess. Death ensues because the heart stops, not as the result of respiratory arrest, as in cobra bites. Some vipers, whose venom contains neurotoxic as well as hematoxic substances, are especially dangerous.
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