Scale (zoology)

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Scale (anatomy)
)
colubrid snake (banded water snake
; Nerodia fasciata)

In

moths), scales are plates on the surface of the insect wing, and provide coloration. Scales are quite common and have evolved multiple times through convergent evolution
, with varying structure and function.

Scales are generally classified as part of an organism's integumentary system. There are various types of scales according to shape and to class of animal.

Fish scales

Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically. Genetically, the same genes involved in tooth and hair development in mammals are also involved in scale development.[1]

Cosmoid scales

True

dentine-like 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

ganoin lies over the cosmine layer and under the enamel[clarification needed
]. Ganoin scales are diamond shaped, shiny, and hard. Within the ganoin are guanine compounds, iridescent derivatives of guanine found in a DNA molecule.[2] The iridescent property of these chemicals provide the ganoin its shine.

Placoid scales

Placoid scales are found on cartilaginous fish including sharks and stingrays. These scales, also called denticles, are similar in structure to teeth, and have one median spine and two lateral spines. The modern jawed fish ancestors, the jawless ostracoderms and later jawed placoderms
, may have had scales with the properties of both placoid and ganoid scales.

Leptoid scales

Leptoid scales are found on higher-order bony fish. As they grow they add concentric layers. They are arranged so as to overlap in a head-to-tail direction, like roof tiles, allowing a smoother flow of water over the body and therefore reducing drag.[3]
They come in two forms:

Reptilian scales

Brightly colored scales on a gold dust day gecko

Reptile scale types include: cycloid, granular (which appear bumpy), and keeled (which have a center ridge). Scales usually vary in size, the stouter, larger scales cover parts that are often exposed to physical stress (usually the feet, tail and head), while scales are small around the joints for flexibility. Most snakes have extra broad scales on the belly, each scale covering the belly from side to side.

The scales of all reptiles have an epidermal component (what one sees on the surface), but many reptiles, such as

crocodilians and turtles, have osteoderms underlying the epidermal scale. Such scales are more properly termed scutes. Snakes, tuataras
and many lizards lack osteoderms. All reptilian scales have a dermal papilla underlying the epidermal part, and it is there that the osteoderms, if present, would be formed.

Avian scales

Scales on talons of a Steller's sea eagle (Haliaeetus pelagicus)

Birds' scales are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. The scales and scutes of birds were thought to be homologous to those of reptiles,[4] but are now agreed to have evolved independently, being degenerate feathers.[5][6]

Mammalian scales

Scale-covered Indian pangolin

An example of a scaled mammal is the

therapsids
lost scales), except that they use a similar gene.

On the other hand, the musky rat-kangaroo has scales on its feet and tail.[7] The precise nature of its purported scales has not been studied in detail, but they appear to be structurally different from pangolin scales.

Anomalures also have scales on their tail undersides.[8]

Foot pad epidermal tissues in most mammal species have been compared to the scales of other vertebrates. They are likely derived from cornification processes or stunted fur much like avian reticulae are derived from stunted feathers.[9]

Arthropod scales

Scales on a luna moth (Actias luna)
Generalized structure of a keeled, lanceolate scale from a jumping spider

visible light the plates lead to structural coloration and iridescence through the physical phenomenon described as thin-film optics. The most common color produced in this fashion is blue, such as in the Morpho
butterflies.

Some types of spiders also have scales. Spider scales are flattened setae that overlay the surface of the cuticle. They come in a wide variety of shapes, sizes, and colors. At least 13 different spider families are known to possess cuticular scales, although they have only been well described for jumping spiders (Salticidae) and lynx spiders (Oxyopidae).[10][11]

Some crustaceans such as Glyptonotus antarcticus have knobbly scales.[12] Some crayfish have been shown to use antennal scales that are activated in rapid response movements.[13]

See also

  • Armour (zoology)
  • Psoriasis: a long-lasting autoimmune disease characterized by patches of thin pieces of hard skin like scale.

References

  1. S2CID 18868124
    .
  2. .
  3. .
  4. ^ Lucas AM (1972). Avian Anatomy - integument. East Lansing, Michigan, USA: USDA Avian Anatomy Project, Michigan State University. pp. 67, 344, 394–601.
  5. S2CID 206544531
    .
  6. .
  7. ^ "Musky Rat Kangaroo". Rainforest-Australia.com. Archived from the original on 2013-10-30. Retrieved 2014-03-08.
  8. .
  9. .
  10. .
  11. ^ Hill, David E. (22 October 2022). "Jumping spider scales (Araneae: Salticidae)" (PDF). Peckhamia. 279 (1): 1–83.
  12. S2CID 9216365
    .
  13. . Retrieved 5 January 2024.

Further reading