While many butterflies are characterized by the colorful patterns on their wings, the wings of the Greta oto species are almost opposite that of their cousins—they are transparent.
“It's interesting because there just aren't that many transparent organisms on land," said Aaron Pomerantz, a Ph.D. candidate at the University of California, Berkeley, and lead author of a new paper on the topic. "So, we asked the question, what is the actual developmental basis of how they create their transparent wings?"
To find the answer, Pomerantz and his fellow students used confocal and scanning electron microscopy to construct a developmental time scale of how transparency emerges in Greta oto, also called glasswing butterfly. Comparing the wings of Greta oto to butterfly species with opaque wings, the researchers noticed that the bristles and scales developed differently.
Shortly after pupation, Greta oto cells fall into an orderly, gird-like arrangement. This spacing of the scale cells farther apart—which reduced the overall density of the scales—is the initial step to generating transparent wings. Then, while maturing, glasswing butterflies develop thin, bristle-like scales in the areas that will later turn transparent. Conversely, butterfly species with opaque wings develop flat, round scales.
“Ultimately, what Greta oto does is to make fewer scales and to make them in these very different, bristle-like shapes," said Nipam Patel study co-author and Pomerantz’ advisor. "But, getting the scales out of the way is only part of the problem of creating transparency.”
The second part involves waxy nanostructures that sit atop the wings of the glasswing butterfly. The nanostructures themselves are not unique, as many terrestrial arthropods have wax lipids on the surface of their cuticle to prevent them from drying out or desiccating. However, glasswing butterflies have a double layer of waxy nanostructures that serve the dual purpose of reducing both water loss and glare.
“When light hits these little arrays of nanostructures, it doesn't reflect—it goes straight through,” said Patel.
“Our experiments demonstrated that the [upper layer of nanostructures] is very important for helping reduce that glare,” added Pomerantz.
In their paper published in the Journal of Experimental Biology, the researchers found Greta oto even have a biochemical advantage that aids their transparency efforts. Unlike cicadas, whose nanopillars include fatty acids and hydrocarbons, or dragonflies, whose secretion comprises long-chain methyl ketones, the irregularly sized nanopillars in Greta oto comprise mainly n-alkanes. Two specific n-alkanes, pentacosane (C25) and heptacosane (C27), have been proved to help attain transparency.
"As humans, we think we're so brilliant because we figured out how to put anti-glare coating on glass, but butterflies basically figured that out tens of millions of years ago," said Patel.
The researchers say future work should investigate the role of alkanes, as well as the 2D surface growth geometry, in generating 3D anti-reflective nanostructures.
“If we can learn more about how nature creates new types of nanostructures, that can be very informative for human applications,” concluded Pomerantz.
Photo: A glasswing butterfly on a leaf. Credit: Aaron Pomerantz