Understanding butterfly wing color

Mutualism—an ecological interaction between different organisms that benefits both—takes many variations in nature. Evolutionary ecologist Dan Janzen has spent a lifetime studying such interactions, but recently, he’s embarked on an unlikely collaboration of his own, one that promises to unlock the origins, evolution, and applications of butterfly wing color.

Janzen, a professor in Penn’s Department of Biology in the School of Arts & Sciences, has spent the last two years in collaboration with Shu Yang, a professor in the School of Engineering and Applied Science’s Department of Materials Science and Engineering. The two share a common interest: The exceptional beauty and diversity of butterfly wings. Their partnership has led to fundamental insights in two fields that rarely cross-pollinate.

“Dan asks why butterfly wing colors are this way, while Shu asks how colors are this way,” says John Tresch of Penn’s Department of History and Sociology of Science, who moderated a recent panel discussion in which the two researchers talked about their mutually evolving work on butterflies. The event was part of the 2014-14 Penn Humanites Forum on Color.

Butterflies, famous for their exquisite colors, are also unusual in how those colors are produced. Their wings are not only colored by pigments, but also by the scattering of light within micro- and nanoscale lattice structures. In addition, the molecular architecture behind this “structural color” creates surface roughness that causes the wings to be extremely hydrophobic, or water-repellent.

Yang, a prolific innovator with more than 20 patents, draws inspiration from wide-ranging sources, including giant clams and origami. Yang and Janzen met two years ago, shortly after Yang’s group used a holographic lithography technique to create a material with iridescence and hydrophobicity of butterfly wings. Yang and collaborators are now exploring the use of such materials as responsive building skins.

“After that study came out, I got an email from Dan that said, ‘You know, I actually study butterflies,’” says Yang. “‘Perhaps we can help each other out.’”

Janzen is a world expert on tropical insects, including butterflies. He’s spent more than 50 years cataloging the biodiversity of Guanacaste, a Philadelphia-sized region of Costa Rican rainforest that’s home to more than 375,000 species. Over the decades, his team has collected thousands of butterfly specimens, which are housed in the Smithsonian Museum of Natural History.

“When I first heard about Shu’s work, I realized I have raw materials that could be useful to her,” says Janzen. “And that perhaps she could help me answer some questions.”

Among these questions is why some butterflies have ephemeral wing spots that appear white at certain angles, but disappear at others. Using scanning electron microscopy, Yang discovered these wing spots were composed of  fan-like scales arranged vertically, tilted or parallel to the wings, causing them to scatter light differently at different angles.

“Shu has helped me start looking at things in a way I’ve never looked at them before,” says Janzen.

Janzen is hopeful that Yang’s work might clarify how many architectural changes are needed to switch a wing’s color. For instance, the famously iridescent blue morpho butterflies have a lesser-known, snow-white cousin.

Understanding how many structural changes are needed to go from blue to white could have profound implications for butterfly evolution.

“Shu’s got the machinery, enthusiasm, and speed to immediately explain how much engineering it takes to flip from one color to another,” says Janzen.

For Yang, collaborating with Janzen has also been eye-opening.

“What we see in the natural world is far more complex than what we can create in the lab,” says Yang. “Dan is helping us to understand the ecological benefits of the many structures found in nature. Then we ask how we can re-engineer that structure efficiently with enhanced functions.”

The impacts of Yang and Janzen’s collaboration may reach beyond either researcher’s discipline. The two, along with Randall Kamien of the Department of Physics & Astronomy and Kathleen Stebe of the Department of Chemical and Biomolecular Engineering, recently submitted a National Science Foundation proposal for a new, interdisciplinary center at Penn that will bring together scientists, engineers, designers, architects, and policymakers to create biologically inspired structures and solutions for human ecosystems.

Yang already has some experience in that regard; her team has developed a bioinspired transparent, superhydrophobic, and oil-repelling spray coating, which could keep solar panels and buildings drier, cleaner, and more efficient.

“We want to continue pushing forward the idea that human environments are an extension of natural environments,” says Yang. “We are very excited that Penn provides the opportunity and environment to foster such collaborations.”

Butterfly wings