Snake scale

Snake scales are of different shapes and sizes. Snake scales may be granular, have a smooth surface or have a longitudinal ridge or keel on it. Often, snake scales have pits, tubercles and other fine structures which may be visible to the naked eye or under a microscope. Snake scales may be modified to form fringes, as in the case of the eyelash bush viper, Atheris ceratophora, or rattles as in the case of the rattlesnakes of North America.[8]

Certain primitive snakes such as boas, pythons and certain advanced snakes such as vipers have small scales arranged irregularly on the head. Other more advanced snakes have special large symmetrical scales on the head called shields or plates.[8]

Cycloid scales on Leptotyphlops humilis and other blind snake species are fluorescent, as a result when they are put under low frequency ultraviolet light (black light) they glow.

Snake scales occur in variety of shapes. They may be cycloid as in family Typhlopidae,[11] long and pointed with pointed tips, as in the case of the green vine snake Ahaetulla nasuta,[12] broad and leaf-like, as in the case of green pit vipers Trimeresurus spp.[12] or as broad as they are long, for example, as in rat snake Ptyas mucosus.[12] In some cases, scales may be keeled weakly or strongly as in the case of the buff-striped keelback Amphiesma stolatum.[12] They may have bidentate tips as in some spp of Natrix.[12] Some snakes, such as the short seasnake Lapemis curtus, may have spinelike and juxtaposed scales[8] while others may have large and non-overlapping knobs as in the case of the Javan mudsnake Xenodermus javanicus.[8]

Another example of differentiation of snake scales is a transparent scale called the brille or spectacle which covers the eye of the snake. The brille is often referred to as a fused eyelid. It is shed as part of the old skin during moulting.[2]

Modified tail scales form a rattle on a western diamondback rattlesnake (Crotalus atrox).

The most distinctive modification of the snake scale is the rattle of rattlesnakes, such as those of the genera Crotalus and Sistrurus. The rattle is made up of a series of loosely linked, interlocking chambers that when shaken, vibrate against one another to create the warning signal of a rattlesnake. Only the bottom is firmly attached to the tip of the tail.[13]

At birth, a rattlesnake hatchling has only a small button or 'primordial rattle' which is firmly attached to the tip of the tail.[13] The first segment is added when the hatchling sheds its skin for the first time.[14] A new section is added each time the skin is shed until a rattle is formed. The rattle grows as the snake ages but segments are also prone to breaking off and hence the length of a rattle is not a reliable indicator of the age of a snake.[15]

Scales mostly consist of hard beta keratins which are basically transparent. The colours of the scale are due to pigments in the inner layers of the skin and not due to the scale material itself. Scales are hued for all colours in this manner except for blue and green. Blue is caused by the ultrastructure of the scales. By itself, such a scale surface diffracts light and gives a blue hue, while, in combination with yellow from the inner skin it gives a beautiful iridescent green.

Some snakes have the ability to change the hue of their scales slowly. This is typically seen in cases where the snake becomes lighter or darker with change in season. In some cases, this change may take place between day and night.[9]

Empty skin (exuvia) of a grass snake (Natrix natrix) whose length exceeds one metre.

The shedding of scales is called ecdysis, or, in normal usage moulting or sloughing. In the case of snakes, the complete outer layer of skin is shed in one layer.[16] Snake scales are not discrete but extensions of the epidermis hence they are not shed separately, but are ejected as a complete contiguous outer layer of skin during each moult, akin to a sock being turned inside out.[9]

Moulting serves a number of functions – firstly, the old and worn skin is replaced, secondly, it helps get rid of parasites such as mites and ticks. Renewal of the skin by moulting is supposed to allow growth in some animals such as insects, however this view has been disputed in the case of snakes.[9][17]

Moulting is repeated periodically throughout a snake's life. Before a moult, the snake stops eating and often hides or moves to a safe place. Just before shedding, the skin becomes dull and dry looking and the eyes become cloudy or blue-colored. The inner surface of the old outer skin liquefies. This causes the old outer skin to separate from the new inner skin. After a few days, the eyes clear and the snake "crawls" out of its old skin. The old skin breaks near the mouth and the snake wriggles out aided by rubbing against rough surfaces. In many cases the cast skin peels backward over the body from head to tail, in one piece like an old sock. A new, larger, and brighter layer of skin has formed underneath.[9][18]

An older snake may shed its skin only once or twice a year, but a younger, still-growing snake, may shed up to four times a year.[18] The discarded skin gives a perfect imprint of the scale pattern and it is usually possible to identify the snake if this discard is reasonably complete and intact.[9]

Nomenclature of scales (underside view of head)

Identification of cephalic scales is most conveniently begun with reference to the nostril, which is easily identified on a snake. There are two scales enclosing the nostril which are called the nasals. In colubrids, the nostril lies between the nasals, while in vipers it lies in the centre of a single nasal scale.[22] The outer nasal (near the snout) is called the prenasal while the inner nasal (near the eye) is called the postnasal. Along the top of the snout connecting the nasals on both sides of the head are scales called internasals. Between the two prenasals is a scale at the tip of the snout called the rostral scale.[22]

The scales around the eye are called circumorbital scales and are named as ocular scales but with appropriate prefixes. The ocular scale proper is a transparent scale covering the eye which is called the spectacle, brille or eyecap.[9][23] The circumorbital scales towards the snout or the front are called preocular scales, those towards the rear are called postocular scales, and those towards the upper or dorsal side are called supraocular scales. Circumorbital scales towards the ventral or lower side, if any, are called subocular scales. Between the preocular and the postnasal scales are one or two scales called loreal scales.[22] Loreal scales are absent in elapids.

The scales along the lips of the snake are called labials. Those on the upper lip are called supralabials or upper labials, while those on the lower lip are called infralabials or lower labials. On top of the head, between the eyes, adjacent to the supraoculars is the frontal scale. The prefrontal scales are the scales connected to the frontal towards the tip of the snout which are in contact with the internasals. They may have a scale in between them.[22] The back of the top of the head has scales connected to the frontal scale called the parietal scales. At the sides of the back of the head between the parietals above and the supralabials below are scales called temporal scales.[22]

Enlarged vertebral scales of Banded Krait Bungarus fasciatus

On the underside of the head, a snake has an anterior scale called the mental[lower-alpha 1] scale. Connected to the mental scale and all along the lower lips are the infralabials or lower labials. Along the chin connected to the infralabials is a pair of shields called the anterior chin shields. Next to the anterior chin shields, further back along the chin is another pair of shields called the posterior chin shields. In some texts the chinshields are referred to as submaxillary scales.[22]

Scales in the central or throat region, which are in contact with the first ventral scales of a snake's body and are flanked by the chin shields, are called gular scales. The mental groove is a longitudinal groove on the underside of the head between the large, paired chin shields and continuing between the smaller gular scales.

The scales on the body of the snake are called the dorsal or costal scales. Sometimes there is a special row of large scales along the top of the back of the snake, i.e., the uppermost row, called the vertebral scales. The enlarged scales on the belly of the snake are called ventral scales or gastrosteges. The number of ventral scales can be a guide to the species.[22] In "advanced" (Caenophidian) snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection.

Nomenclature of scales (underside view of body),

At the end of the ventral scales of the snake is a cloacal plate that protects the opening to the cloaca (a shared opening for waste and reproductive material to pass) on the underside near the tail. This scale has also been the anal scale, which is a misnomer since it does not cover an anus but a cloaca. This cloacal scale may be single or paired. Most authors have differentiated between single and divided cloacal scales. However, based on the origin of scales during development, a scale does not spontaneously divide, but it originates as paired structures that subsequently overlap. The part of the body beyond the cloacal scale is considered to be the tail.[13]

Sometimes snakes have enlarged scales, either single or paired, under the tail; these are called subcaudals or urosteges.[22] These subcaudals may be smooth or keeled as in Bitis arietans somalica. The end of the tail may simply taper into a tip (as in the case of most snakes), it may form a spine (as in Acanthophis), end in a bony spur (as in Lachesis), a rattle (as in Crotalus), or a rudder as seen in many sea snakes.

Sources. Details for this section have been sourced from scale diagrams in Malcolm Smith.[24] Details of scales of Buff-striped Keelback have been taken from Daniels.[25]

• Canthus, or Canthus rostralis, the angle between the supraocular scale and the rostral scale.
• Mental groove.

Banded Krait (Bungarus fasciatus), an Elapid, with no loreal scale between nasal and pre-ocular scales.

There is no simple way of differentiating a venomous snake from a non-venomous one merely by using a scale character. Finding out whether a snake is venomous or not is correctly done by identification of the species of a snake with the help of experts,[31]^: 190 or in their absence, close examination of the snake and using authoritative references on the snakes of the particular geographical region to identify it. Scale patterns help to indicate the species and from the references, it can be verified if the snake species is known to be venomous or not.

Species identification using scales requires a fair degree of knowledge about snakes, their taxonomy, snake-scale nomenclature as well as familiarity with and access to scientific literature. Distinguishing by using scale diagrams whether a snake is venomous or not in the field cannot be done in the case of uncaught specimens. It is not advisable to catch a snake to check whether it is venomous or not using scale diagrams.[31]^: 190 Most books or websites provide an array of traits of the local herpetofauna, other than scale diagrams, which help to distinguish whether a snake in the field is venomous or not.[26][32]^: 52

In certain regions, presence or absence of certain scales may be a quick way to distinguish non-venomous and venomous snakes, but used with care and knowledge of exceptions. For example, in Myanmar, the presence or absence of loreal scales can be used to distinguish between relatively harmless Colubrids and lethally venomous Elapids.[29] The rule of hand for this region is that the absence of a loreal scale between the nasal scale and pre-ocular scale indicates that the snake is an Elapid and hence lethal.[29] This rule-of-thumb cannot be used without care as it cannot be applied to vipers, which have a large number of small scales on the head. A careful check would also be needed to exclude known poisonous members of the Colubrid family such as Rhabdophis.[29]

In South Asia, it is advisable to take the snake which has bitten a person, if it has been killed, and carry it along to the hospital for possible identification by medical staff using scale diagrams so that an informed decision can be taken them as to whether and which anti-venom is to be administered. However, attempts to catch it or kill the venomous snake are not advised as the snake may bite more people.[33]