Again, generalizations are difficult in light of the extraordinary diversity encompassed within this family. Compared with basal families and with boas and pythons, colubrids are characterized by the loss or simplification of many features. Notably, all vestiges of the hind limbs are absent in colubrids, and the coronoid bones of the lower jaws have been lost. Externally, most colubrids have relatively wider ventral scales and fewer rows of dorsal scales than boas and pythons. In addition, most colubrids possess a standard complement of enlarged scales on the head, including nine scales on the top of the head. That feature, like other external attributes, is shared with members of the Atractaspididae and Elapidae and even with some Viperidae, and at the same time is not seen in all members of the Colubridae. The vertebrae of colubrids are, in general, more slender and lightly built than those of basal snakes, boas, and pythons. In addition, the interconnecting chains of muscles that effect locomotion in snakes are longer in colubrids than in those other groups.
One important feature that distinguishes most colubrids is the presence of Duvernoy's glands, a pair of glands located on either side of the head behind the eye. These are the homologues (the evolutionary counterparts) of the venom glands of vipers, elapids, and Atractaspis. Although they are often described as modified salivary glands, they in fact are very different in both their tissue characteristics and their origin. The Duvernoy's glands are associated with the rear teeth of the maxillary bones, the major toothed bones of the upper jaws, and they secrete a complex mix of chemicals whose composition is still poorly understood. In some colubrids the secretion is known to serve as a slow-acting venom or as a digestive adjunct. The rear pair of maxillary teeth are often modified to assist with the delivery of secretion from Duvernoy's glands, and may be either enlarged, grooved, or both. That is generally known as the rear-fanged, or ophisthoglyphous, condition, although a variety of terms has been applied to specific conditions of the rear teeth. In most cases the secretion of Duvernoy's gland has no apparent effect on humans, but in some species it can cause local swelling. A few taxa, including the boomslang (Dispholidus) and twigsnake (Theletornis) of Africa and the Ya-makagashi (Rhabdophis) of Asia, are capable of delivering a lethal bite to humans. The xenodontine genera Apostolepis and Elapo-morphus resemble some aparallactine atractaspidids in approaching the front-fanged condition of elapid snakes, with large Duvernoy's glands and few teeth preceding the greatly enlarged rear fangs. Although Duvernoy's gland is found, to some degree, in most colubrids, it is not universally present. In some colubrids it may have been lost as a consequence of the evolution of constricting behavior, which constitutes an alternative mechanism for immobilizing prey. In other colubrids the enlarged rear maxillary teeth may serve a purely mechanical function. In some toad-eating snakes, such as hog-nosed snakes (Heterodon), those teeth may be used to deflate the prey, which in others, such as kukrisnakes (Oligodon), they are used to slice open the eggs of lizards and snakes.
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