Scale (zoology)

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 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.

There are various types of scales according to shape and to class of animal.

Structure
Scales are generally classified as part of an organism's integumentary system.

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).

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.