In a small tent in the middle of Panama’s rainforest, Aaron Pomerantz assembled a makeshift field lab, filled with microscopes, chemical reagents, and delicate lab equipment. At times, it was so hot that Pomerantz, an integrative biologist at the University of California, Berkeley, struggled to keep his own sweat from contaminating his delicate lepidopteran samples. He was looking for something nearly invisible—transparent butterflies known as glasswings.
The rare butterflies “are like ghosts in the rainforest,” says Nipam Patel, Pomerantz’s Ph.D. adviser. Now, Pomerantz and Patel have done more than just find the butterflies—they’ve also solved an enduring mystery: how their wings are transparent in the first place.
The glasswing butterfly (Greta oto), a baseball-size flier that lives throughout Central and South America, is one of hundreds of butterfly species with transparent wings. This rare adaptation helps it evade potential predators. Compared with other see-through species, such as dragonflies, glasswings are even more adept at fluttering through the rainforest unnoticed because their wings don’t shine or glimmer in sunlight.
Patel, who normally studies arthropod evolution, has a lifelong interest in glasswings—and a collection of tens of thousands that he has assembled since the age of 8. To understand what makes the critters so stealthy, Patel, now director of the Marine Biological Laboratory in Woods Hole, Massachusetts, asked a group of graduate students to take microscopic images of the wings of a dozen or so species of transparent butterflies.
His students found that “every way you can think of being transparent, some butterfly or moth has figured out,” Patel says. A butterfly’s wings consist of a thin, membranous layer of a natural polymer called chitin, which is typically covered with tiny scales that resemble interlocking tiles. Species with transparent wings have found ways to move light around these scales, producing fewer of them, turning them vertically, or simply getting rid of them.
The group found that glasswings not only produce fewer scales, but they also convert many of those scales into bristles, allowing light to pass through the wings more easily. Using a scanning electron microscope, Pomerantz also discovered that tiny mounds between the bristles, known as nanopillars, are coated in a layer of wax.
The nanopillars seem to help reduce glare, Pomerantz says. Glare happens when light hits a surface and bounces off at the same angle, as if striking a mirror. The nanopillars “rough up” the surface of the wings and cause the light to bounce off at multiple angles, diffusing the reflection, the researchers wrote last month in the Journal of Experimental Biology. “Because they’re so small, they act kind of like little bitty speed bumps,” Pomerantz says.
In addition, the waxy coating slows down light that passes through the wings because it is more dense than air—like forcing someone to swim through molasses. That reduction in speed softens the impact of light hitting the scales, further reducing glare. Stripping the glasswings of their waxy coating and nanopillars resulted in wings that were shiny, Pomerantz says.
Although many transparent species, including the hand-size hyperiid, have developed these microscopic speed bumps, the wax coating is a new and somewhat puzzling find, says Sonke Johnsen, a biologist at Duke University. That’s because butterflies’ chitin covering is strong—and the addition of the wax layer weakens it. “Why forgo those amazing advantages that you get with chitin to replace it with this wax?” Johnsen asks. “I bet there’s more to the story that they’re going to find out.”
Understanding these antireflective properties could one day help researchers efficiently funnel light into solar panels and create cheaper antiglare lenses for cameras or glasses. But for now, Pomerantz and Patel want to focus on how glasswings evolved from nontransparent ancestors, using genomics to identify the key genes.
“It’s just fascinating to know how nature solves really interesting problems like this,” Patel said. “You can pay extra for glasses that have an antireflective coating on them. But, of course, essentially, butterflies figured that out maybe tens of millions of years ago.”
sciencemag.org, 22 June 2021