How the Butterfly Got Its Spots

Look, I'm gonna give it to you straight: butterfly wings are cool af.

In the first place, they're covered in scales that give the butterfly its distinctive pattern and help the butterfly escape from spider webs—kind of like a technicolor suit of armor. They also function like a butterfly's personal solar panels, soaking up the rays to help the insect regulate its body temperature. What's more, butterfly wing design has taught humans a lot, like how to design better screens for our cell phones.

But hands down the best thing about butterfly wings is that they're just plain beautiful to look at, an evolutionary trait that helps butterflies do everything from attract mates to warn predators not to mess with them. Although lepidopterists have known for a century that butterfly wings get their color from complex nanostructures called photonic crystals, how these colors came to be so bright wasn't understood until now.

Recently, a team of physicists from the University of California San Diego and Yale explained this mysterious phenomenon, by bombarding a butterfly's wing with x-ray lasers.

As the team detailed in a paper for Science Advances, they used the Advanced Photon Source at the Argonne National Laboratory to scan the wing of an Emperor of India butterfly with x-rays. The imaging technique the researchers used, called ptychography, allowed them to examine the nanostructures that make up the butterfly's wing scales without having to dissect the wing, as was previously the case.

This allowed the team to see that the wings of the Emperor of India were made up of "highly oriented" photonic crystals, nanostructures that are arranged in such a way that they affect the movement of photons so that they can only reflect one particular color.

The team's big discovery however, was the presence of small irregularities—the shift of just one row of atoms—within the otherwise strictly ordered crystalline lattice. According to Andrej Singer, a postdoctoral researcher at UC San Diego, these irregularities "enhance light scattering properties" which make the butterfly wings appear brighter, an effect known as light localization.

The imaging of the Emperor of India butterfly wing is a big first step toward understanding how nature engineers photonic crystals and incorporates defects by design. According to the team, such revelations could prove to be "invaluable" for developing photonic devices (anything from lasers to solar cells) for humans in the future—but first researchers would need to figure out how to mimic growth conditions for the "controlled manufacturing of defects."

"Defects may have a negative connotation, but they are actually very useful in improving materials," said Singer. "In the evolution of butterfly wings it appears nature learned how to engineer these defects on purpose."