Scales on Cecropia Moth Wing
The wings of butterflies and moths are covered with scales. Scales are modified hairs. A hair is formed by a hair cell that secretes a linear strand of cuticle. Scales form when secreted cuticle instead of growing as a linear hair, forms a flat plate.
Scales are tiny but visible under a light microscope. A typical scale would measure 100 microns long by 50 microns across. A square millimeter of butterfly wing typically contains several hundred scales.
Source: Living With Insects Blog