The glasswing butterfly has a unique type of transparency

Some butterflies and moths have achieved the ultimate self-defense mechanism. Species like the glasswing butterfly, Greta oto, have evolved transparent wings that make them nearly invisible to predators.

A team of scientists led by the Marine Biological Laboratory (MBL) set out to investigate how transparent structures, which are far more common in marine animals, developed in the glasswing butterfly.

“It’s an interesting biological question because there just aren’t that many transparent organisms on land,” said study lead author Aaron Pomerantz, a PhD candidate at UC Berkeley. “So we asked the question, what is the actual developmental basis of how they create their transparent wings?”

The colorful patterns found on butterfly wings are created by tiny chitinous scales that reflect or absorb various wavelengths of light. 

Pomerantz explained that although scale coloration has been intensively studied, investigating the developmental origins of transparency in land-based butterflies hadn’t been done before. “Transparency is sort of the opposite of color.”

The current research was inspired by the work of students in MBL’s Embryology course, which is taught by study co-author MBL Director Nipam Patel.

“I decided to bring some of the transparent butterfly and moth species I had in my collection, which I never really looked at in detail, to the course and present it as a challenge for the students to look at how these wings were transparent,” said Patel.

“A group of students took that on by imaging the wings with various microscopes. And they realized that pretty much any way you could think to make the wing transparent, some butterfly or moth had figured out how to do it. That’s what got us looking in more detail at the development of transparency.”

The team used an advanced electron microscopy technique to construct a developmental time scale of how transparency emerged in the glasswing butterfly from the pupal stage to adulthood. 

The study revealed that the glasswing butterfly’s wings do not develop in the same way as other opaque species. 

“What Greta oto does is to make fewer scales and to make them in these very different, bristle-like shapes,” explained Patel. “But getting the scales out of the way is only part of the problem of creating transparency.” 

“Aaron also made a series of observations about nanostructures on the wing that prevent glare in bright sunlight. When light hits these little arrays of nanostructures, it doesn’t reflect off – it goes straight through. So that gives much better transparency.”

“As humans, we think we’re so brilliant because we figured out how to put anti-glare coating on glass, but butterflies basically figured that out tens of millions of years ago.”

Beyond their unusual wing scales, the experts identified a second layer of waxy hydrocarbon nanopillars over the wing surface that enhance the anti-reflective properties. The researchers removed this waxy layer to compare the reflectivity of the wings with and without it. 

“We measured the amount of light that reflected off the wing,” said Pomerantz. “Those experiments demonstrated that that upper layer was very important for helping to reduce that glare.” 

Further research is needed to explore how these remarkable transparent structures evolved. Pomerantz noted that learning more about how nature creates new types of nanostructures could be very informative for human applications.

The study is published in the Journal of Experimental Biology.

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By Chrissy Sexton, Earth.com Staff Writer