The GlassWing butterfly

The Incredible Glasswing Butterfly

A butterfly with transparent wings? Surely not. Yet there is a species that exhibits this trait. Take a close look at the incredible Glasswing, an enchanting species that confounds science.

Greta oto may sound like the name of a silent movie star from Eastern Europe but is in fact the scientific name for one of the most exquisite – and little known – species of butterfly on the planet. This butterfly’s claim to fame is that its wings, spanning up to six centimeters, are almost completely transparent. That’s right, you can see just about right through them.

The common English name for this remarkable butterfly is glasswing, which in itself speaks volumes about the appearance of this small but unusual insect. However, it takes the romance languages to step in and give the butterfly the name which, for many, suits it best. The Spanish name for the glasswing is ‘espejitos’. Literally translated, this means little mirrors. Just a glance at the insect in question and one can imagine the thrill of pleasure when the moment of inspiration that came to its Hispanic name giver.

A close look at Greta oto reveals that between the veins of its wings the tissue is virtually see through) or, properly, translucent). Most other butterflies have colored scales which pattern the wings, quite often to ward off predators. The glasswing has another way of doing this entirely, but over the millennia it has evolved these specific wings to hide itself from predators rather than to warn them off. The only way that you can tell that it has wings at all are the borders, which are of a dark hue, sometimes bordering on the orange. Were it not for these borders, the glasswing would be more or less invisible to the human eye.

The glasswing is part of a specific clade of butterfly. Now for the science, as Jennifer Aniston used to say. A clade is a ‘branch’ and is a term used in the taxonomy of species. When groups of species has a single common ancestor (which does not necessarily need to be extant) then it is known as monophyletic. The common ancestor of the glasswing is long extinct but the clade it belongs to is known as the clearwing clade.

Transparency in nature is not something that has been very well understood. In order to achieve transparency the tissue must not absorb light. Neither can it scatter light, as this is the major obstacle to being see-through. Humans, for example, will never be able to be transparent because they have chemical and biological compounds that all have different refraction.

The wings of the glasswing must, therefore, have the same refractive index all the way through them as otherwise this transparency could not possibly occur. It is thought (a postulation at the moment rather than sure fire fact) that the surface of the wing has a covering of protrusions that are so small they can be called submicroscopic. They have a single refractive index and so do not scatter light, so making the wings transparent.

As with most butterflies it is a delicate looking species, but those who breed it in captivity have found it to be quite resilient and its wings are no less strong than those of other species. Another relief is the fact that in its native habitat it is quite common. Unless you live in South America, however, the only chance you will get of seeing the glasswing alive is in a butterfly house or farm.

If you want to see ‘little mirrors’ in the wild, however, you will have to take a trip – anywhere from Mexico to Panama in Central America will do the trick. You will also have to locate the nearest rainforest as the understory of this environment is where the glasswing prospers. They feed off the nectar of a variety of rainforest flowers but when it comes to laying their eggs and ensuring the survival of the next generation, the glasswing has a fine trick up its (metaphorical) sleeve.

The glasswing, where possible, will lay its eggs on a plant of the genus ‘Cestrum’. Its common name, to you and I, is the nightshade and it is highly poisonous. The caterpillars, which are striped in bright purple and red to warn possible predators, are thus a snack that birds and other animals will not enjoy at all. The alkaloids, a chemical in the plants that occurs naturally and is full of nitrogen, stay in the bodies of the glasswing in to adulthood meaning that even then they will not be an attractive meal.

During mating, the males will convert some of these alkaloids in to pheromones which will attract the females to them. The glasswing is also noted for its long migrations and the fact that the males of the species, when about to meet, practice lekking. This is when a host of males gather together to show off their best features en masse – the females then choosing the most dominant and visually exciting.

The glasswing, while not rare as a species, is one of very few land based animals that have successfully mastered the act of transparency. Now you see it, now you don’t.

Most butterflies sport colorful, eye-catching patterns on their wings. But some species, like the glasswing butterfly, use mostly transparent wings to hide in plain sight.

To figure out how these Central American butterflies go incognito, researchers put the wings of glasswing butterflies (Greta oto) under the microscope. Sparse, spindly scales overlaying a see-through wing membrane with antireflective properties help make these insects so stealthy.

Transparency is the ultimate form of camouflage. Transparent animals can instantly blend into any background. “It’s really hard to do,” Barnett says. “You have to modify your entire body to minimize any scattering or reflection of light.”

Aaron Pomerantz, a biologist at the University of California, Berkeley, became fascinated by butterflies with transparent wings while doing research in Peru. “They were really interesting and mysterious,” he recalls, “like these little, invisible jets that glide around in the rainforest.”

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

microscope images of a glasswing butterfly's wing
The boundary between clear and opaque regions of a glasswing butterfly wing (magnified image at left) reveals two types of scales. The transparent region contains sparse, thin scales that have either single or forked bristles (shown in false color at center). The black region contains overlapping, leaflike scales (shown in false color at right).A. POMERANTZ ET AL/JEB 2021

“You’d think the simplest solution would be to just not have any scales,” says coauthor Nipam Patel, a biologist at the Marine Biological Laboratory in Woods Hole, Mass. But butterflies need at least some scales in the transparent parts of their wings: The water-repellant scales help prevent the wings from sticking together when it rains, he says.

The texture of G. oto’s wing membrane also helps reduce the glare coming off transparent regions of the wing. If the membrane surface were flat, the abrupt change in optical properties between the air and wing would cause light traveling through the air to bounce off the wing’s surface and reduce transparency, Patel explains. But an array of tiny wax bumps coats the surface of the membrane, creating a more gradual shift between the optical qualities of the air and wing. That allows more light traveling through the air to pass through the wing rather than reflect off of it, softening the glare.

The researchers found that the transparent regions of glasswing butterfly wings naturally reflect only about 2 percent of light. Stripping off the waxy layer caused the wings to reflect about 2.5 times as much light as they normally do.  

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

images of a glasswing butterfly's wing coated in wax and not coated in wax
The transparent regions of a glasswing butterfly wing (top left) are coated in a bumpy layer of wax (microscope image, top right) that prevents glare coming off of the wing. When researchers stripped off the waxy layer, the smoothed wing (bottom right) reflected 2.5 times as much light (bottom left).A. POMERANTZ ET AL/JEB 2021

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