Duvernoy's Gland

 

The three largest snake families that contain “Venomous” species are;

-Elapidae (containing species such as Cobras, Mambas and Taipans. Sea snakes are also tentatively placed in this family; Cogger 2014)

-Viperidae (including Rattlesnakes, Puff adders and Saw-scaled vipers)

-Colubridae (including Rat snakes, Tree snakes and Garter snakes)

Pictures: Top right: Ringed Brownsnake (Elapidae)
Opposite: Bornean Keeled Green Pit Viper (Viperidae)
Below: Brown Tree Snake (Colubridae)

The Colubridae family actually consists of several independent clades awaiting further reclassification (Pyron et al. 2011).This large group consisting of nearly 2000 species includes species of opistoglyphs (rear fanged) and aglyphs (no fangs) (Kardong et al. 2009). Not all species produce toxins and of those that do, most are not considered medically significant. There are some notable exceptions including the Dispholidus(Boomslang), Philodryas, Rhabdophis (Asian Keelbacks) and Thelotornis(Twig snake) genra. Unlike the members of Elapidae and Viperidae, the opistoglyphs don’t possess true “venom glands”, but a pair of Duvernoy’s glands(Figure 1.). These snake deliver their toxins, produced in this gland, through a grooved tooth in the back of the mouth (Kardong 2002).

Figure 1. Oral glands of Colubrids.(Note, not all species have each gland; Kardong 2002)

Opistoglyphs are often described as having an “inefficient delivery apparatus”, referring to the often symptomless bites recorded in humans, however, the Duvernoy’s gland is an homologous structure (sharing common structure but a different function) with venom glands (Kardong 2002). The toxins excreted by this oral gland likely serve other purposes as very few species toxins display rapid prey death (the foremost biological purpose of elapid and viper venom),other roles of the excretions may play include; defence, post-strike prey tracking, digestion, lubrication and immobilisation of prey (Kardong 2002).

The Duvernoy’s gland is different to “true venom glands” in that it doesn’t contain a large storage area for the toxin. “True Venom glands” expel the venom through a series of ducts under pressure. This allows the immediate transfer of venom into the target upon penetration (Kardong et al. 2009). The Duvernoy’s glands excretions however use capillary action to be moved to the fang, therefore require a prolonged bite or chewing action to envenomate (Kardong et al. 2009).

References

Cogger H.G., 2014, "Reptiles and amphibians of Australia, 7th edn, CSIRO Publishing, Collingwood, VIC.

Kardong V.K., 2002, Colubrid Snakes and Duvernoy’s “Venom” Glands.", Journal of Toxicology: Toxin Reviews, Vol: 21 No: 1-2, pp: 1-19.

Kardong, V.K., Weinstein, S.A., Smith, T.L. and Mackessy, S.P., 2009. "Reptile venom glands: form, function, and future.", Handbook of venoms and toxins of reptiles, pp:65-91.

Pyron R.A, Burbrink F.T., Colli G.R., De Oca A.N.M., Vitt L.J., Kuczynski C.A. and Wiens J.J.,  2011, "The phylogeny of advanced snakes (Colubroidea), with discovery of a new subfamily and comparison of support methods for likelhood trees.", Molecular Phylogenetics and Evolution, Vol: 58, No: 2, pp: 329–342.
Images by Nick Weigner