Scale (zoology)

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Keeled scales of a colubrid snake

In zoology, scale (Greek lepid, Latin squama) generally refers to a small, platelike outgrowth of the skin of an animal that affords protection. These dermal or epidermal structures form the integument of reptiles, most fish, and some mammals, and similar structures are found on insects, such as butterflies and moths.

While modern amphibians generally lack scaled integuments of any kind, other vertebrate classes generally have bony or horny scales of some sort. Most extant fish species are characterized by an external covering of bony scales, which are dermal ossifications (hardening of soft tissue into bonelike material). Reptiles are typified by scaled integuments with horny scales or scutes. These tend to develop embryologically as an outgrowth of epidermal tissues and sometimes include dermal ossifications. Horny scales are found on the feet and sometimes legs of birds and these resemble closely modern crocodilian scales. Among mammals, the mouse and rat, for example, have scales on the tail and the armadillo has large horny scales as the primary integument.

In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration.

Scales are generally classified as part of an organism's integumentary system. Scales are quite common and have evolved multiple times with varying structure and function. There are various types of scales according to shape and to class of animal.

Fish scales

Scales of Rohu - Labeo rohita

Cosmoid scales

True cosmoid scales can only be found on the extinct Crossopterygians. The inner layer of the scale is made of lamellar bone. On top of this lies a layer of spongy or vascular bone and then a layer of dentinelike material called cosmine. The upper surface is keratin. The coelacanth has modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales.

Ganoid scales

Ganoid scales can be found on gars (family Lepisosteidae) and bichirs and reedfishes (family Polypteridae). Ganoid scales are similar to cosmoid scales, but a layer of ganoin lies over the cosmine layer and under the enamel. They are diamond-shaped, shiny, and hard

Placoid scales

Placoid scales are found on cartilaginous fish and sharks. These scales, also called denticles, are similar in structure to teeth.

Leptoid scales

Leptoid scales are found on the higher bony fishes and come in two forms, ctenoid and cycloid scales.

Cycloid scales

Cycloid scales have a smooth outer edge, and are most common on more primitive fish with soft fin rays, such as salmon and carp.

Ctenoid scales

Ctenoid scales have a toothed outer edge, and are usually found on more derived fishes with spiny fin rays, such as bass and crappie.

As they grow, cycloid and ctenoid scales add concentric layers. The scales of bony fishes are laid so as to overlap in a head-to-tail direction, a little like roof tiles, allowing a smoother flow of water over the body and therefore reducing drag.

Reptilian scales

Reptile scale types include: cycloid, granular (which appear bumpy), and keeled (which have a center ridge).

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.[1] Snakes are entirely covered with scales or scutes of various shapes and sizes. Scales protect the body of the snake, aid it in locomotion, allow moisture to be retained within and give simple or complex colouration patterns which help in camouflage and anti-predator display.

Scales have been modified over time to serve other functions such as 'eyelash' fringes, and protective covers for the eyes[2] with the most distinctive modification being the rattle of the North American rattlesnakes.

Snakes periodically moult their scaly skins and acquire new ones. This permits replacement of old worn out skin, disposal of parasites and is thought to allow the snake to grow. The arrangement of scales is used to identify snake species.

Snakes have been part and parcel of culture and religion. Vivid scale patterns have been thought to have influenced early art. The use of snake-skin in manufacture of purses, apparel and other articles led to large-scale killing of snakes, giving rise to advocacy for use of artificial snake-skin. Snake scales are also to be found as motifs in fiction, video games and films.

Functions of snake scales

The dorsal (or back) scales of a snake protect it from damage due to friction as it moves.[3] The ventral (or belly) scales, which are large and oblong, 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.[4]

Snake skin and scales help retain moisture in the animal's body.[5]

Snakes pick up vibrations from both the air and the ground, and can differentiate the two, using a complex system of internal resonances (perhaps involving the scales) [1].

Morphology of snake scales

Keeled scales of Buff-striped keelback Amphiesma stolata, a Colubrid.

Snake scales are formed by the differentiation of the snake's underlying skin or epidermis.[2] 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.[6]

A snake is born with a fixed number of scales. The scales do not increase in number as the snake matures nor are do they reduce in number over time. The scales however grow larger in size and may change shape with each moult.[4]

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.

Snake scales are made of keratin, the same material that hair and fingernails are made of.[4] They are cool and dry to touch.[7]

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, 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.[6]

Snake scales occur in variety of shapes. They may be :

File:Leptotyphlops humilis - head.jpg
Cycloid scales on Leptotyphlops humilis.
  • cycloid as in Family Typhlopidae.[9]
  • long and pointed with pointed tips, as in the case of the Green Vine Snake Ahaetulla nasuta.[10]
  • broad and leaf-like, as in the case of green pit vipers Trimeresurus spp.[10]
  • as broad as they are long, for example, as in Rat snake Ptyas mucosus.[10]
  • keeled weakly or strongly as in the case of the Buff-striped keelback Amphiesma stolatum.[10]
  • with bidentate tips as in some spp of Natrix.[10]
  • spinelike, juxtaposed as in the Short Seasnake Lapemis curtus.[6]
  • large, non-overlapping knobs as in the case of the Javan Mudsnake Xenodermis javanicus.[6]

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]

Rattles

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 rattlesnake tail 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 button is firmly attached to the tip of the tail.[11]

Arthropod scales

Butterfly and moth species of the order Lepidoptera (Greek "scale-winged") have membranous wings covered in delicate, powdery scales. Each scale consists of a series of tiny stacked platelets of organic material. Because the thickness of the platelets is on the same order as the wavelength of visible light the plates lead to structural coloration and iridescence through the physical phenomenon described as thin-film optics.

Bibliography

References
ISBN links support NWE through referral fees

  1. Boulenger, George A. 1890 The Fauna of British India. page 1
  2. 2.0 2.1 The Snakes of Indiana at The Centre for Reptile and Amphibian Conservation and Management, Indiana. Accessed 14 August 2006.
  3. - Evolution, Snakes And Humans - Appearance and behavior of Encyclopaedia volume 5.
  4. 4.0 4.1 4.2 Are snakes slimy? at Singapore Zoological Garden's Docent. Accessed 14 August 2006.
  5. Kentucky Snake Publication (pdf). University of Kentucky
  6. 6.0 6.1 6.2 6.3 Greene, Harry W. Snakes - The Evolution of Mystery in Nature, page 22
  7. Herpetology FAQ at San Diego Museum of Natural History. Accessed 14 August 2006.
  8. Greene, Harry W. Snakes - The Evolution of Mystery in Nature, page 23
  9. Boulenger, George A. The Fauna of British India... page 234
  10. 10.0 10.1 10.2 10.3 10.4 Smith, Malcolm A. Fauna of British India...Vol III - Serpentes, page 6
  11. Reptiles - Snake facts. Columbus Zoo & Aquarium.

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