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.
A single cell in the butterfly wing produces a single scale. When the wings are fully formed, the cells die. If a scale is lost, there is no cell to replace it.
Handling butterflies by the wings will dislodge some of the scales and leave a powder of scales on your fingers. Scales are useful to butterflies and moths that fly into spider webs. The web will stick to the scales but not the wing itself. This allows moths to escape the web leaving a few scales stuck to the web and little worse for the wear.