The wings of most butterflies have vibrant, eye-catching patterns. However, some species, such as the glasswing butterfly, use mostly transparent wings to remain hidden in plain sight.
Researchers examined the wings of glasswing butterflies (Greta oto) to learn how these Central American butterflies hide. Researchers report in the May issue of the Journal of Experimental Biology that sparse, spindly scales overlaid on a see-through wing membrane with antireflective properties help these insects be so stealthy.
According to James Barnett, a behavioral ecologist at McMaster University in Hamilton, Canada, who was not involved in the research, transparency is the ultimate form of camouflage. Transparent animals can easily blend into any environment. “It’s extremely difficult,” Barnett admits. “You must modify your entire body to reduce light scattering and reflection.”
While conducting research in Peru, Aaron Pomerantz, a biologist at the University of California, Berkeley, became fascinated by butterflies with transparent wings. “They were really interesting and mysterious, like these little, invisible jets that glide around in the rainforest,” he recalls.
Pomerantz and colleagues discovered that the black rims of G. oto’s wings were densely packed with flat, leaflike scales using confocal and electron microscopes. The transparent areas, on the other hand, had narrower, bristle-like scales that were spaced further apart. As a result, only about 2% of the underlying clear wing membrane was visible in black areas, but approximately 80% of the membrane was visible in transparent areas.
“You’d think the simplest solution would be to simply not have any scales,” says coauthor Nipam Patel, a biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts. However, butterflies require some scales in the transparent parts of their wings: He claims that the water-repellant scales keep the wings from sticking together when it rains.
The texture of G. oto’s wing membrane also aids in reducing glare from transparent areas of the wing. If the membrane surface were flat, the abrupt change in optical properties between the air and the wing would cause light traveling through the air to bounce off the surface of the wing, reducing transparency, explains Patel. However, an array of tiny wax bumps coats the membrane’s surface, causing a more gradual shift between the optical qualities of the air and wing. This allows more light to pass through the wing rather than reflecting off of it, softening the glare.
The transparent regions of glasswing butterfly wings naturally reflect only about 2% of light, according to the researchers. After removing the waxy layer, the wings reflected about 2.5 times as much light as they normally do.
According to Pomerantz, these findings may not only help biologists better understand how these butterflies hide from predators, but they may also inspire new antireflective coatings for camera lenses, solar panels, and other devices.