Butterflies that flit throughout tropical forests often have vividly coloured wings that iridescent in the sun. But it’s not pigments that create those eye-catching shadows. It’s microscopic structures on the insects’ wings that reflect the light. Researchers at the University of Cambridge studied an Indonesian butterfly known as the peacock or swallowtail. Scaling on the wings are made up of tiny structures that researchers say resemble the inside of an egg carton, with alternative layers of circle and air. The light bounces off the structures so that the scales appear to us as a shimmering green. But using optical equipment that can polarize light, those scales appear bright blue.