Melanin deposits in the heads of both true vipers (Viperinae) and pit vipers (Crotalinae) are concentrated over the dorsal and dorsolateral aspects of the venom glands. This pigment may occur in any or all of six sites which include the epidermis, dermis, tissues covering the venom glands, and the interior of the glands themselves. The extreme localization of these melanin deposits suggests that they shield the venom glands from light. Calculations indicate that without such shielding the light energy penetrating the venom glands in the visible and ultraviolet portions of the solar spectrum would damage the venomsynthesizing apparatus and detoxify stored venom.Elapid and hydrophiid snakes have less dense pigment over the venom gland than vipers. Literature reports indicate that elapid venom is less sensitive to photodetoxification than is venom from vipers. Most colubrid snakes, including several with protein-secreting Duvernoy's glands, have little or no melanin associated with the glands. Venomous colubrids in the genera Ahaetulla, Dryophis, Leptophis, and Oxybelis have pigment over the glands as dense as that seen in vipers.Iridophores probably also shield venom glands from radiation. In puff adders and Gaboon vipers (Bitis) there appears t o be an ontogenetic change in the shielding of the venom glands from melanocytes in young individuals to iridophores in adults.Among the multitude of effects that visible and ultraviolet light have upon biological systems, a number depend upon penetration of light energy through the skin into deeper tissues (Urbach, '69). Melanin deposits in the skin andlor peritoneum of vertebrates have been plausibly interpreted as shields to block potentially damaging visible and ultraviolet light (Porter, '67; see Daniels, '69, for a review of the optical properties of skin). I t has long been known that snake venom absorbs strongly in the ultraviolet portion of the spectrum (Massol, '11) and deterioration of stored venom in vitro has been ascribed partially to this cause. More recent in vitro studies have indicated that photodetoxification of snake venom can be produced by short exposures to either ultraviolet or visible light (Kocholaty, '66; Tejasen and Ottolenghi, '70).Our curiosity about the possibility of in vivo photodetoxification of snake venom was stimulated by the observation that the dark eyestripe of many species of rattlesnakes (Crotalus) extends over part of the venom gland. Consequently we undertook a survey of the position of the eyestripe and other deposits of melanin in the heads of snakes in relation to protein-secreting glands.
MATERIALS AND METHODSWe dissected the heads of 1 to 10 individuals of 34 species of viperid snakes, 10 elapids, 3 hydrophiids, 18 colubrids, and 1 boid. Most of these snakes had been preserved with 3% formaldehyde and stored in either 50% isopropanol or 70% ethanol. Some fresh material was utilized. We noted the position of melanin pigment in the skin, connective tissue, and muscle covering the head in general and the venom gland in p...
