Why do butterflies and moths have "powder" on their wings?

Butterfly Wings - The "powder" on the wings of moths and butterflies is really a layer of tiny, coloured scales which overlap each other almost like the tiles of a roof. If you touch the wing with a finger the "powder" is rubbed off, leaving the wing more or less transparent and colourless. The scales is generally like the shape of a hand tapered off at the wrist, and the whole surface is often grooved or cross grooved. They are really hollow bags growing from tiny cup joints formed in the outer skin of the wing membranes. They are either filled with colouring materials, or so minutely grooved and surfaced that they refract light to give off an irridescent colour, even though they contain no pigment. The brown, red, yellow, white or black scales are pigmented. The blues and greens are irridescent. Many male butterflies and moths have specially shaped "scentscales" (androconia). These are long and feather like or broad and bat-shaped. they contain glands for making scents which attract the females.