 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
|
|
|
Scales
Elaborately shaped scales on the head of a Vine snake Ahaetulla nasuta
Scales are important for snakes - they are deemed to be reptiles by the presence of scales, amongst other things. Snakes are entirely covered with scales or scutes. These scales vary in shape and size. Scales in their myriad variety cover the skin, protect the body of the snake, allow moisture to be retained within and give simple or complex colouration patterns which help in camouflage and antipredator display. Modifications of the scales serve other functions such as 'eyelash' fringes, rattles, and protective covers for the eyes.
Morphology of scales
Keeled scales of Buff-striped keelback Amphiesma stolata, a Colubrid.
Snake scales are formed from the epidermis. Each scale has an outer surface and an inner surface. The skin from the inner surface hinges back and forms a free area which overlaps the base of the next scale which emerges below this scale. [2] A snake is born with a fixed number of scales. These are not added to or lost as it matures. The scales however grow larger in size and may change shape with each moult.[3] Snake skin and scales help retain moisture in the animal's body. [4] Snakes can also 'hear' by sensing vibrations with their lower jaw and belly scales.
Surface and shape
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 Etharis ceratophorus, or rattles as in the case of the rattlesnakes of North America.
Certain snakes such as boas, pythons and certain 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.
Modified tail scales form a rattle on a Western Diamondback Rattlesnake Crotalus atrox.
Snakes have smaller scales around the mouth and sides of the body which allow expansion so that a snake can consume prey of much larger width than itself. The ventral scales or belly scales are large and oblong. They protect the soft underside of the snake and also grip surfaces allowing the snake to move. The large scales (called 'shields') on the snake's head play a similar role.
Snake scales are cool and dry and not slimy. That is because scales are made of keratin, the same material that nails and fingernails are made of. Scales, more specifically, consist of mostly hard ß keratins which are 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 iridiscent 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. Snake scales occur in variety of shapes. They may be :-
Cycloid scales on Leptotyphlops humilis.
Another example of differentiation of snake scales is a transparent scale called the brille or spectacle which covers the eye of the snake. The snake has no eyelids and the brille protects the eye. It is shed as part of the old skin during moulting.
Moulting
Cascavel (Crotalus durrisus), a rattlesnake, seen moulting.
The shedding of scales is called moulting or sloughing. In the case of snakes, the complete outer layer of skin is shed in one layer. 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.
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.
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 prior to 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.
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.[12] 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
From Wikipedia
|
|
