VIPERS, snakes of the family Viperidae, which is character ised by the presence of poison fangs on a movable upper jaw. The fangs are simply enlarged teeth perforated longitudinally for the passage of the venom and, like those of all other snakes, they are fused to the supporting bones. In this family, however, there are no other teeth on the upper jaw and the bone itself is movable so that the fangs are folded down parallel with the roof of the mouth when not in use.
Venom is secreted by a pair of glands, situated behind the angle of the mouth, and is carried to the fangs by a short duct which opens close to their base inside a fold of skin (the vagina dentis) which surrounds them; within this fold of skin there is also a series of reserve fangs in different stages of development and, should one of the functional fangs be broken, the largeSt of these reserve teeth moves into its place and becomes fused to the jaw. Vipers, as a rule, are stout sluggish creatures with a broad, flat tened head, and lack the large head shields so characteristic of the majority of other poisonous snakes ; most of them are ter restrial though there are aquatic, arboreal and burrowing species. A few lay eggs but the majority produce fully developed young.
All the viperidae are very poi sonous and the bite of most of them is dangerous to man; the toxicity of the venom varies with each species and the virulence of any bite depends, not only on the species of snake responsible for it, but also on the amount of venom injected, the position of the bite and the physical condition of the snake. In composition viperine venom resembles that of the back-fanged Colubrids, rather than that of the cobras and their allies, and its action con sists largely in the destruction of the blood corpuscles and vessels.
The family is subdivisible into two well-defined groups: () Viperinae.—The true vipers or adders, confined to the Old World and characterised by the absence of a pit between the nostril and the eye.
The majority of the snakes of this series are terrestrial, though Atractaspis of Tropical and South Africa is a genus of small burrowing creatures with enlarged shields on the head ; the Night Adders (Causus), of the same region, have the head similarly covered. Arboreal forms are represented by Atheris, of the forests of tropical Africa, which is equipped with a prehensile tail and is usually green or olive in colour to harmonise with its surround ings. The colours of the terrestrial species, on the other hand, are more frequently shades of grey, brown or black to harmonise with the rocky or sandy localities which they frequent. The Puff Adder (Bitis arietans), which occurs throughout the drier areas of Africa, is a sluggish, heavily-built creature which may grow to a length of four or five feet; it is usually pale brown with a series of regular, dark, chevron-shaped cross-bars along the back, a colouring which harmonises so well with sandy soil that many accidents occur through people failing to notice the animal until they actually tread upon it. The Gaboon Viper
(Bitis gabonica), unlike its relative, is an inhabitant of the forested regions of Africa and exhibits a geometrical "camouflage" colour pattern of blues, reds and yellows. Many desert species show the feature so characteristic of desert-dwelling animals and plants, the development of spines ; Bitis nasicornis has a pair of horn like scales on the tip of the snout and Cerastes cornutus, the Horned Viper of Egypt and northern Africa, has a prominent spine above each eye. Cerastes and Echis, the latter found through northern Africa and southern Asia, including India, exhibit a specialisation for desert life not found elsewhere ; the scales of the sides are small and have pronounced, serrated ridges which act, through lateral shovelling movements of the body, as scoops to dig up loose sand and throw it onto the creatures' backs, and so enable them to bury themselves completely. In Europe the family is represented by the genus Vipera of which the Adder (V. berus) is the best known species; it is an inhabitant of the northern countries and is the only venomous serpent in Britain.
In southern Europe an allied species V . aspis, is more common, characterised by a "snub-nose" and this feature is even more pronounced in V. latastei of the Iberian Peninsula, whilst in V. ammodytes of S.E. Europe the tip of the snout is prolonged upwards into a definite scaly appendage. In India the commonest and most dangerous viper is the Daboia or Tic Polonga (V. russelli) which reaches a length of 5 feet.
(2) Crotalinae.—Pit vipers and rattlesnakes, centred in Amer ica but extending into southern Asia and distinguished by the presence of a deep pit on the side of the head between the eye and the nostril.
This pit is lined by scales, similar to those of the rest of the head, and has a rich nerve supply but its function is quite unknown. Two American genera of this series are equipped with a "rattle" on the end of the tail and are more fully described in the article on Rattlesnake (q.v.). Of the remainder Agkistrodon is distinguished by the possession of large shields on the head and is found through S. Asia and Central and N. America; the American species are semi-aquatic, the Cotton Mouth (A. pis civorus) rather more so than the Copperhead (A. mokasen), but the Asiatic species (including A. hypnale of S. India and Ceylon and A. halys of the Caspian region) are terrestrial. Also occur ring in S.E. Asia are a number of arboreal species of the genus Trimeresurus, the commonest and most widely distributed being T. gramineus, a bright green creature with a yellow stripe along the flanks and with the tip of the prehensile tail red. In America some of the most dangerous poisonous snakes belong to this family; they include the dreaded Bushmaster (Lachesis mutes) which sometimes attains a length of 12 feet, the Fer-de-Lance (Bothrops atrox), the Jararaca (B. jararaca) and the Jararacussu (B. jararacussu), large forms which inhabit the tropical parts of Central and South America.