jec1958/science

From the page: "Remarkable light show

Butterfly wings consist of intricate layers of chitin cuticle separated by air spaces. These can be seen using the magnifying power of an electron microscope.

"A solid block of chitin is almost transparent, but when it is arranged in specific geometrical layers, it can reflect light in a remarkable way," said Dr Pete Vukusic of Exeter University, who has been studying the scales.

"The diversity and complexity of these chitin arrangements place them among the most complicated extra-cellular structures anywhere in the living world," he added.

Two excellent examples of iridescence of butterflies and moths are the wing scales of most of the Morpho family of butterflies and the Urania family of moths. These were among the first to be examined because of their brilliant iridescence.
Layers in Christmas tree patterns interfere with the light
Each Morpho scale has a series of cuticle ridges arranged in a "Christmas tree" formation. Each layer reflects light in the same way as a thin film of oil on water. The effect of several layers produces multiple light reflections.

The well-known effect of light interference causes some colours to be cancelled out while others are reinforced. The result is the striking colours we see.

The colours depend upon the exact structure of the wing and also the angle at which it is observed. Morpho can be bright blue when seen from above but looking from an angle makes it appear more violet.

As well as using an electron microscope the Exeter research team have also examined butterflies wings using lasers. They have found that some wings can reflect as much as 75% of blue light, a remarkable value for a natural substance."