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The new images shed light on how crystal butterflies make their wings transparent

Most butterflies feature colorful and attractive patterns on the wings. But some species, such as the crystal butterfly, use mostly transparent wings to hide from view.

To find out how these Central American butterflies go incognito, the researchers put the wings of the butterflies (Greta oto) under a microscope. Sparse, round scales that line a transparent membrane with antireflective properties help make these insects so stealthy, researchers report in the May issue of the Journal of Experimental Biology.

Transparency is the ultimate form of camouflage, says James Barnett, a behavioral ecologist at McMaster University in Hamilton, Canada, who did not participate in the work. Transparent animals can be mixed instantly on any background (SN: 05/05/19). “It’s very hard to do,” Barnett says. "You have to modify the whole body to minimize the scattering or reflection of light."

Aaron Pomerantz, a biologist at the University of California, Berkeley, became fascinated by butterflies with transparent wings while researching in Peru. "They were really interesting and mysterious," he recalls, "like these little invisible jets gliding in the rainforest."

Using confocal and electron microscopes, Pomerantz and colleagues found that the black edges of G. oto's wings were densely packed with flat, leafy scales. But the transparent areas wore narrow, bristle-like scales spaced. As a result, only about 2 percent of the membrane of the underlying transparent wings was visible in the black regions, but about 80 percent of the membrane was exposed in transparent areas.

The boundary between the clear, opaque regions of a crystal butterfly wing (enlarged image on the left) reveals two types of scales. The transparent region contains sparse, thin scales that have simple or bifurcated bristles (shown in false color in the center). The black region contains overlapping scales, like leaves (shown in false color on the right).A. Pomerantz et al / JEB 2021

“I would think the simplest solution would be to have no scales,” says co-author Nipam Patel, a biologist at the Marine Biology Laboratory in Woods Hole, Mass. But butterflies need at least some scales on the transparent parts of their wings. : Water-repellent scales help prevent the wings from sticking when it rains, he says.

The membrane texture of the wings of G. oto also helps to reduce the brightness coming out of the transparent regions 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 and decrease transparency, Patel explains. But a series of small wax protrusions line the surface of the membrane, creating a more gradual shift between the optical qualities of the air and the wing. This allows more light traveling through the air to pass through the wing instead of reflecting, softening the brightness.

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The researchers found that the transparent regions of glass-winged butterfly wings naturally reflect only 2 percent of the light. Removal of the wax layer caused the wings to reflect approximately 2.5 times more light than normal.

These results may not only help biologists better understand how these butterflies hide from predators, but also inspire new anti-reflective coatings for camera lenses, solar panels and other devices, Pomerantz says.

images of a wax-coated and non-wax-coated beer butterfly wingThe transparent regions of a butterfly wing with glass wings (top left) are coated with a layer of bumpy wax (microscope image, top right) that prevents glare from coming out of the wing. When the researchers removed the layer of wax, the smoothed wing (bottom right) reflected 2.5 times more light (bottom left).A. Pomerantz et al / JEB 2021

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