When I catch sight of a butterfly, I’m struck by its patterning, wing movement, and color—the beauty of it all. Take the coastal green hairstreak butterfly (Callophrys viridis) that emerges in the Bay Area every spring. I’ve always marveled at its vivid lime hues. Would you be surprised to learn that this coloration is not due to a really bright pigment? I was. It’s created by tiny, complex structures embedded in the scales of the wings. In other words, the wings aren’t actually green the way a dyed fabric might be green. Rather, the scales contain molecular nanostructures that bend and refract the light, giving the impression of greenness.
Bay Nature’s Spring 2019 magazine explores the much hyped “insect apocalypse” through the lens of local scientists, naturalists, and artists.
If you were to enter one of the hairstreak’s scales—which is roughly between 50 and 170 micrometers wide—you would find honeycomb-like shapes inside. Each “honeycomb,” technically known as a gyroid, contains an array of connected spiral-shaped cuticle cells. The geometry of these interconnected shapes bends white light, causing some wavelengths to amplify and others to cancel each other out. The result is the brilliant iridescent green that we see.
Dr. Nipam Patel, former co-chair of UC Berkeley’s Department of Molecular and Cell Biology and current director of University of Chicago’s marine biology laboratory in Woods Hole, explains this to me as he pulls out a drawer from a large cabinet full of butterfly specimens. The drawer contains a few dozen preserved green hairstreaks whose iridescent wings shimmer in the light.
The phenomenon he’s describing is called structural color. Our everyday sense of color is usually based on the absorption and removal of certain wavelengths of light. If your shirt appears blue, a dye or pigment is probably absorbing all the wavelengths except for the blue wavelength, which is reflected back to your eyes. Structural color employs minuscule physical landscapes—those gyroids—that catch and scatter light in a way that amplifies the strength of certain wavelengths while diminishing the power of others.
Just how big are a visible wavelength of light and the gyroids that interact with it? Take a ruler and look at a millimeter. Imagine expanding that millimeter until it seems like the size of a meter. Divide that by a thousand. Then divide one of these thousandths by half. That’s the size of the wavelength of green light. Just 18 times wider are the gyroids that refract the light on the hairstreak’s wing. They’re teensy but also wondrous. If you were a tiny person inside this structure, it might appear that you were standing in a spiral hall of mirrors. Incoming white light would bounce from curved wall to curved wall, with only bright green light exiting.
Some structural colors are produced simply by passing light through different substances, like air and water. The different qualities of these media cause specific wavelengths in white light to piggyback on one another, growing stronger, while other wavelengths get canceled out by being absorbed, or when a trough and crest of certain light waves meet up.
Patel and his lab member Rachel Thayer show me how this works in common buckeye butterflies (Junonia coenia), some of which have bright iridescent blue spots. Incoming light that’s traveling through air hits a thin film on the butterfly’s wing, they explain. The sudden shift in refractive material causes the light waves to bend at different angles and then “knock” into one another, creating a single strong, bright blue light wave.
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“If you think of light waves like waves in a pond and you see the ripples overlap, they are ‘in phase’ and the waves get higher,” says Patel. “If they don’t [overlap], they are ‘out of phase’ and cancel each other out.”
This kind of amplification, called constructive interference, produces some of the most brilliant colors in the natural world. Structural color is widespread in nature. It exists in bird feathers, scarab beetles, abalone, opals, bismuth, peacocks, blue skin on monkeys, blue and green human eyes, and of course, butterflies.
In the Bay Area, several butterfly species besides green hairstreaks exhibit structural color, including the shimmery blue Acmon blues (Plebejus acmon), the dark green pipevine swallowtails (Battus philenor), and the common buckeye with blue spots of iridescence.
Patel wants to know how, genetically speaking, animals make the nanostructures that produce these bright colors. “It’s just a fascinating question,” he says. “If you think about making structures that are smaller than the wavelength of light, it’s kind of like an engineering problem for the cell.” If scientists figure out the molecular process that produces these tiny prisms and mirrors, they may be able to affordably manufacture nanostructure-based pigments for human use. Though Patel’s interest is based just on the remarkable nature of these cells themselves.