The Delias durai butterfly
Structural scales on the wings of butterflies reflect light of certain wavelength and lend the insect its colour. WWF

In a chrysalis of a different kind, a brown-winged butterfly Bicyclus anynana changed colour to sprout purple wings, thanks to selective breeding.

The research appears in the Proceedings of the National Academy of Sciences.

The experience of colour comes from a chemical or structural origin. Pigments in plants and animals absorb certain wavelengths of colours and reflect the rest. A flower looks red because it absorbs all other wavelengths except red.

Organisms can also structurally adjust the reflection of certain light. This is how butterfly wings exhibit colour, using scales on the backs of their wings.

The scientists undertook the study to see if by selectively breeding the wings of B. anynana could be turned from brown to purple.

They measured the wavelengths of light reflected from the wings of B. anynana specimens and selected and bred those butterflies whose wings reflected light closest to the violet spectrum.

They performed this process six times in eight consecutive generations.

As the breeding process continued, reflected wavelengths moved further toward the violet spectrum. By the sixth generation, the butterflies had purple wings.

The thickness of some of the scales of the wings that had changed colour had changed. This had altered the wavelengths of light they reflected.

The changed scales were also different in type from those in the wings of B. sambulos and B. medontias, which naturally have purple wings.

The experiment shows that butterfly wing colours can evolve very quickly and that natural selection could play an important role in the development of wing colour.

The team says engineers could use these findings when designing devices that tune colour, trap light or steer light beams.