Fish armor and moth dust are both called scales
In zoology a scale is a small rigid keratin plate growing from vertebrate skin for protection; butterfly and moth wings wear a different, powdery kind. Fish scales arise from dermal mesoderm, unlike reptile scales, and the same gene networks that build teeth and hair help pattern them.
Zoologists use one word, from the Greek lepis and Latin squama, for surprisingly different structures. In vertebrates a scale is a small stiff plate of keratin growing from the skin as armour, part of the integumentary system. Fish scales arise from the dermis, specifically the mesoderm, which separates them from reptile scales in the fossil record. At the genetic level, the genes that build mammalian teeth and hair also help make scales.
Fish show the widest range. Cosmoid scales, found only in lobe-finned fish, layer lamellar bone, spongy bone, a dentine-like cosmine and a keratin surface; the coelacanth's version is thinner and has lost the cosmine. Gars and bichirs wear hard, shiny, diamond-shaped ganoid scales whose gleam comes from iridescent guanine compounds. Sharks and rays are covered in placoid scales, or denticles, built much like teeth with one central and two side spines. Most modern bony fish have thin overlapping scales laid like roof tiles to cut drag: smooth-edged cycloid scales on salmon and carp, and toothed ctenoid scales on spiny-finned bass and crappie.
Reptile scales may be smooth, granular or keeled, bigger where the body takes knocks such as the head, feet and tail, and smaller at joints. Crocodilians and turtles back their scales with bony osteoderms, properly called scutes, which snakes, tuataras and many lizards lack. The green iguana's big neck scales, and skin impressions of dinosaurs like Carnotaurus, show large feature scales without bony support. Birds' leg scales, long assumed to be inherited from reptiles, are now thought to have evolved separately as reduced feathers.
Among mammals, the pangolin's keratin scales evolved independently, since the therapsid ancestors of mammals had lost theirs, though a similar gene is involved; the musky rat-kangaroo and anomalures carry scales on feet or tails. Butterflies and moths, whose order name Lepidoptera means scale-winged, dust their wings with chitin scales that are modified bristles, broad in butterflies and slimmer in moths. At least 13 spider families also bear flattened scale-like bristles.
Source: Scale (zoology)
