The differentiated serous-secreting dental glands and associated teeth of advanced snakes are diverse in form despite their developmental homology. This variation makes the elucidation of their evolutionary history a complex task. Over the past decade and a half, molecular systematics and toxinology have deepened our understanding of the evolution of these fascinating and occasionally enigmatic structures. Appropriate use of terminology, especially that pertaining to homologous structures (including the controversial “venom gland” vs “Duvernoy’s gland” debate), is essential.
Research on snake oral glands has been adversely affected by terminological confusion. This confusion has primarily resulted from the extreme lability of the glands in structure and arrangement amongst species, which confounded many early attempts to establish the homology of particular glands and resulted in numerous authors coining their own names for the same structures in different snakes.
Considerable controversy remains regarding appropriate nomenclature for the post-orbital toxin-secreting dental glands of the advanced snakes. Taub in 1966 coined the name “Duvernoy’s gland” for this gland in non-front-fanged snakes to replace the term “parotid gland”, which invited confusion with the mammalian parotid gland, a non-homologous structure.
That the “Duvernoy’s gland” of non-front-fanged snakes and the “venom gland” of front-fanged snakes are developmentally homologous had previously been established although Taub did not comment on this. HRather, his discussion reflects a number of common confusions regarding the concept of homology and he mentions the then prevalent (now refuted here and here for details) idea that the venom glands might form an “evolutionary series” in which the venom glands of the Elapidae evolved into the “more advanced” venom glands of the Viperidae. The term “Duvernoy’s gland” may have been erected in order to avoid confusion generated by the suggestion of homology with mammalian parotid gland, but apparently Taub did not consider the confusion its coinage might engender via the implication of non-homology with the venom glands of front-fanged snakes.
Taub ends his discussion of venom glands by commenting, “there is now some evidence that the two types [of venom gland] may not be homologous.” Indeed, they cannot be referred to as “homologous” without including the “Duvernoy’s gland” of non-front-fanged snakes under the same classification, because they are polyphyletic with respect to the latter and their relatively superficial similarities (essentially limited to their “high-pressure” delivery of a venom bolus and their association with hollow fangs) are the result of convergent evolution. For this reason, the high-pressure venom systems of front-fanged snakes (Elapidae, Viperidae and Atractaspidinae), might be described as homoplastic (convergent in form and function) with one another and homologous with the differentiated dental glands of non-front-fanged snakes.
In his discussion of the “Duvernoy’s gland”, Taub noted that “the idea has often been expressed that these glands produce venom,” citing several studies going back to Phisalix in 1922, and noted that because of their predominantly serous secretory cells and their single duct associated with the posterior maxillary teeth, this idea is well supported and the glands should be considered an entity separate to the supralabial glands, which are typically mucous-secreting and polystomatic. In fact it had long been reported that non-front-fanged snakes use their toxic oral secretions to subdue their prey (Alcock and Rogers in 1902) and this observation has since been corroborated by a large number of experiments and observations of many different species of non-front-fanged snakes. A toxic oral secretion produced in a specialised gland and delivered by a bite in order to facilitate the subjugation of prey is “venom” by any modern definition.
Venom is a functional trait and “venom glands” constitute a function category. Other obvious function categories include hearts, wings and eyes. The traits within a function category are not necessarily homologous, rather they are homoplastic – grouped together as a result of their shared functional role. The venom glands of hymenopteran insects, for example, are not homologous with those of viperid snakes, but they share a function: the production of venom. Function categories may be defined stipulatively, descriptively, or theoretically. The members of a stipulative function category are grouped together for the purposes of analysis – we may not have a theory level understanding of their functional similarities, but we know that they form a natural group that should be considered together. Stipulative function categories are especially useful for biologists studying the evolution of a specific trait, because traits often exist in a variety of forms, points on a continuum, which we may interpret as different branches of the same evolutionary tree.
The venom glands of advanced snakes are homologous but diverse in form, particularly amongst non-front-fanged species. Despite this diversity of form, they may all be considered members of the same function category: those with a well characterised venomous function (i.e. those that we are confident contribute to prey subjugation or defence against predators) are descriptively “venom glands”, those that are merely homologous structures (e.g. may have lost their venomous function following the evolution of effective constriction) are stipulatively so. Splitting them into myriad subcategories will not help us understand their evolution – such categories could never be clearly defined because discernible boundaries between the forms are not likely to exist.
In order to avoid creating further confusion regarding the development, evolution and function of ophidian oral glands, it is vitally important that each homologous structure be given a single, appropriate name. The term “Duvernoy’s gland” is inappropriate — its original purpose was to avoid the implication of homology between ophidian and mammalian "parotid” glands, but it generates the equally spurious implication of non-homology between the venom glands of front-fanged and non-front-fanged snakes. Our modern appreciation of the homology of the toxin-secreting oral glands of non-front-fanged snakes with those of front-fanged snakes, coupled with the large number of experiments demonstrating the use of the products of these glands in prey subjugation and a sound application of the biological function concept, leaves no doubt that the most sensible name for these glands is “venom glands”.