Everything about Scale Zoology totally explained
In most
biological nomenclature, a
scale (
Greek lepid,
Latin squama) is a small rigid plate that grows out of an
animal's
skin to provide protection. In
lepidopteran (
butterfly and
moth) species, scales are plates on the surface of the
insect wing, and provide coloration. Scales are quite common and have
evolved multiple times 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.
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 higher order bony fish and come in two forms, ctenoid and cycloid scales.
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.
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.
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Reptilian scales
Reptile scale types include: cycloid, granular (which appear bumpy), and keeled (which have a center ridge).
The scales of all reptiles have an epidermal component (what one sees on the surface), but many lizards have osteoderms underlying the epidermal scale, as do crocodilians and turtles. Snakes,
tuataras and many lizards lack osteoderms. All reptilian scales have a dermal papilla underlying the epidermal part, and it's there that the osteoderms, if present, would be formed.
Arthropod scales
Butterflies and
moths - the order
Lepidoptera (
Greek "scale-winged") - have membranous wings covered in delicate, powdery scales, which are modified
setae. Each scale consists of a series of tiny stacked platelets of organic material, and butterflies tend to have the scales broad and flattened, while moths tend to have the scales narrower and more hair-like. Scales are frequently
pigmented, but some types of scales are metallic, or iridescent, without pigments; 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. The most common color produced in this fashion is
blue, such as in the
Morpho butterflies. Other colors can be seen on the
Sunset moth.
Bibliography
Further Information
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