Scientists Uncover Secret of Butterflies' Color-Changing Abilities

Butterflies are living jewels, their wings changing in the light depending on how they're positioned. As they flit through rainforests or delicately land on flowers, their chameleon-like colors morph. Now, scientists have discovered a little bit more about how the subtle differences in the tiny crystals of butterfly wings create stunningly varied patterns of color even among closely related species. The findings could lead to new ways to express color in manufactured materials.

In order to examine the unique properties of butterfly wings, the researchers turned to three tropical butterflies that all display iridescence, a property of materials that causes them to change colors depending on the viewing angle. The Ulysses butterfly appears a bluish green when seen from above. Its cousin, Papilio peranthus, looks more of a yellowish green. The third butterfly, called the green swallowtail, is a deep green. Yet all three switch to deep blue when seen from a sharp angle.

The scientists examined a cross-section of each species' wing under a scanning electron microscope. This allowed them to see that the wings possessed specialized architectures in which solid flat layers known as cuticles alternate with thin "air" layers known as laminae. The laminae also contain tiny pillars of the cuticle material, which gives the wing a repeating crystal-like structure.

They weren't done yet, though. The scientists then measured the light spectrum reflected from the wing at different angles, using a technique called angle-resolved spectroscopy. In the end, they found that the varying colors on the different butterflies arise from slightly differences in the crystal parameters. For example, the Ulysses butterfy had seven cuticle layers while P. peranthus had eight.

"It was very exciting to see how nature can create a nanostructure that's not easy to replicate by humans," said Kok Wai Cheah, a physicist at Hong Kong Baptist University, in a news release. "It all comes from the fact the wing structure has subtle differences between the three types of butterflies."

The findings could eventually lead to materials that wouldn't need to be painted or dyed one specific color. Instead, the same material could change colors depending on light sources or other factors. Currently, the scientists are planning to investigate color-changing mechanisms in other insect bodies to learn more about how they might be used.

The findings are published in the journal Optical Materials Express.