Researchers studying glasswing butterflies have created a developmental timeline for the growth of their clear wings, tracking the formation of detailed nanostructures on the surface that could provide bioinspiration for new reflective materials.

Transparency has evolved numerous times throughout the history of life, but because it is more common in aquatic organisms, its origins and mechanisms in land animals are relatively unknown. The study, published May 10 in the Journal of Experimental Biology, takes a closer look at how this mysterious property works in butterflies.  

"Transparency is very rare on land [because] it's hard to make your body just not reflect light," said first author Aaron Pomerantz, a Ph.D. candidate in integrative biology at University of California, Berkeley. "That's why one of the innovations that clearwings [evolved] is that they make nanostructures, and these nanostructures help to reduce that contrast between light and air and what they're made out of."

Though they are currently under threat from climate change and other human activities, butterflies are still among the most diverse groups of animals on Earth, even in seemingly inhospitable environments. And while most butterfly wings are covered with millions of tiny scales that give the wings their colorful patterns, some butterflies, such as the glasswing, have nearly invisible wings.

The glasswing butterfly makes its home in the tropical jungles of the Americas, where it uses its wings to stage a disappearing act. When flying into the sun, the butterflies flash brightly, visible "from 50 yards away," according to senior author and Pomerantz's advisor Nipam Patel, currently a biology professor at University of Chicago and director of its Marine Biology Lab, though he completed this research while still affiliated with UC Berkeley.

Despite these flashes, the butterflies become invisible in the understory.

"One hypothesis is that there's two different predators, one of which will find them unpalatable, that they can advertise to when they're flying up above the trees, and then something else that's going after them that finds them perfectly tasty, so they're trying to hide," Patel said.  

The jury is still out on precisely why the butterflies perform this disappearing act. But the researchers were able to determine precisely how the transparent wings form by studying butterfly pupae at different ages using electron microscopy, a technique for getting images of biological specimens at extremely high resolution, and later by analyzing the biochemicals found in the wings.

The researchers found that during pupation, the scales in transparent areas of the wing become widely spaced apart, resembling hairs. This allows light to hit the surface of the wing. But this didn't adequately explain how the butterfly can disappear so easily, because bare insect wings are usually shiny.

"Now you have light hitting the membrane, and you're still going to get a surface reflection," Pomerantz said. "So the second thing that needs to happen is that you add nanostructures to the surface of the membrane."

These structures, which are "smaller than a wavelength of light," according to Pomerantz, add texture to the surface of the wing that makes it less shiny in the light, like a matte screen protector.

Once the researchers were able to achieve the right magnification using different electron microscopy techniques, they found that the wings produce two types of nanostructures during development. The first type resembles structures found in the eyes of insects, which could contribute to the wings' optical properties.

However, the researchers attribute much of the wings' anti-glare capabilities to the second type of structure that was found, a series of wax-based columns that the researchers termed "nanopillars." These nanopillars are like a series of millions of tiny skyscrapers on the surface of the wing, and they act like a natural anti-glare coating by scattering any light that hits the surface.

"It seems like there are multiple layers to these nanostructures, but that actually has important implications for their anti-glare properties," Pomerantz said. "Since these little waxy-type nanostructures are very important for actually helping to make them less reflective."

The researchers are now working on using their comprehensive knowledge of glasswing butterfly wings as a source of bioinspiration to develop new anti-glare technologies. And the team has a good track record for bioinspiration. In 2017, study co-author Radwanul H. Siddique worked on a project using butterfly-inspired scales in solar cells, though those were based on a different species.

"I've always thought that it's really cool when physicists or engineers kind of take inspiration from nature and then try to apply that to technologies," Pomerantz said. "For some of these transparent insects, anti-glare coatings on solar panels is one application that's been under development."

The study, "Developmental, cellular, and biochemical basis of transparency in clearwing butterflies," published May 10 in the Journal of Experimental Biology, was authored by Aaron F. Pomerantz and Nipam Patel, University of California, Berkeley and Marine Biological Laboratory; Radwanul H. Siddique, Samsung Semiconductor, Inc. and California Institute of Technology; Elizabeth I. Cash, University of California, Berkeley; Yuriko Kishi, University of California, Berkeley and California Institute of Technology; Charline Pinna and Marianne Elias, Sorbonne Université; Kasia Hammar, Marine Biological Laboratory; and Doris Gomez, Université de Montpellier.