The vipers form a well defined and advanced family of snakes found throughout much of the world but absent from Madagascar and Australia . Their most distinctive characteristic is a pair of shortened maxillae to each of which is attached a single long fang . Each maxilla is hinged so that the fangs can be folded back when not in use . The fangs have an enclosed canal through which venom is forced .
Vipers are typically short and stocky with broad heads .
They are mainly terrestrial or arboreal but there are semi-aquatic . The Crotalinae are unique among snakes in possessing a pair of large heat-sensitive pits between the eye and the nostril . The rattle on the tail of rattlesnakes , genera Crotalus and Sistrurus , is formed from the successive remains of the scale covering the extreme tip of the tail . In most snakes , this scale is conical and the epidermis covering it comes away with the rest of the shed skin . In rattlesnakes , however , the terminal scale is shaped like an hour-glass , with a constriction somewhere around its centre line .