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.
 

When does a caterpillar become a butterfly?

Butterfly - a caterpillar starts to become a butterfly as soon as it enters the chrysalis stage by wrapping itself in a cocoon. During this phase known as pupation the insect gradually develops all the butterfly characteristics including wings. For some butterflies the transformation may be complete in one or two weeks. But others need many months before they are ready to leave the cocoon and fly away. From time it is hatched to the time it starts the pupation stage a caterpillar or larva does little more eat grow larger and moult several times. This larval stage varies according to the species. Small caterpillars will complete their development in a week but some large varieties will take up to two years or more. When this period is over the caterpillars spins a cocoon under leaf or even underground and enters it to begin the transformation. Many cocoon are of silk.