RATTLESNAKE, any snake of the genera Sisturus and Crotalus, American pit-vipers with the tail terminating in a rattle.
The "pit" characteristic of this sub-family of the vipers is a deep depression on each side of the snout between eye and nostril.
The rattle is developed as a modification of the single scale which covers the tip of the tail; instead of being a simple, conical sheath, as in ordinary snakes, it has two ring-like constrictions so that it resembles three hollow bulbs, gradually diminishing in size and each one opening into its neighbour ; the largest is also open at its free end, where the tail enters. All snakes shed the horny, epidermal covering of their scales periodically, a new one forming beneath the old one before the latter is sloughed; when however, the new covering of the end of the tail is developed in the rattlesnakes, the middle "globe" develops within the largest of the old sheath and, though the rest of the old skin is sloughed, the old tail sheath remains loosely fitting over the new one but prevented from slipping off by its shape. Thus, newly-born rattlesnakes have no rattle but with each succeeding moult a joint is added; the older joints, however, gradually wear away and are lost so that the number of joints on the rattle is not necessarily any criterion of a snake's age.
The characteristic noise is produced by rapid vibration of the whole tail, when the loosely fitting horny shells produce a shrill noise which may be audible 20 yards away. The majority of the species are dwellers of the plains and open spaces and it has been suggested that the development of the rattle was to give a warning audible at a distance.
The venom of the rattlesnakes is of the same general type as other viperine venoms and all species are large enough to be able to inject sufficient to kill a man. They are, however, not easily provoked and will usually endeavour to escape or, if cornered, to frighten the aggressor by rattling and puffing themselves out as much as possible. In all species the eggs are retained within the body of the mother until the young are ready to emerge.
Sisturus, with only two species and confined to the southern United States and northern Mexico, may be distinguished from Crotalus by the presence of large, regularly arranged shields on the top of the head; both species are small, not more than about 21 ft. long. Crotalus, with small irregular scales on the top of the head, contains about 20 species, most of which occur in North and Central America. The largest and most dangerous species is the diamond back (C. adamanteus) which reaches a length of about 8 ft.; it is confined to the south-eastern United States. Other well known species are the prairie rattler (C. confluentus) which occurs all over the Great Plains from Canada to Texas, the timber rattlesnake (C. horridus) of the eastern United States and C. terrificus which ranges from Mexico to the Argentine and is the only representative of the genus in South America. The sidewinder (C. cerastes) is a small, desert-dwelling form from south-western North America and is characterized by the develop ment of a blunt, horn-like process above each eye. (H. W. P.)