Behind the Cover: September 2023

Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.

Years ago, a friend and I visited a beach in Long Island, New York, that was a bit off the beaten path, not yet discovered by the throngs of beachgoers who packed the popular seaside spots. We would have had the beach to ourselves if not for the hundreds of monarch butterflies that were floating gracefully in swarms above the sand. I remember thinking how elegant and effortless their flying appeared.

Yet as Amy Lang of the University of Alabama points out in the cover story of Physics Today‘s September issue , the wing shape of monarchs and other butterflies is not particularly conducive to efficient flight. Unlike birds, bats, and even other insects, butterflies have “short, broad, and large wings relative to their body,” she writes. The substantial surface area of the wings seemingly exposes the insects to stifling drag forces.

The secret weapon for butterflies, Lang explains, is the array of hundreds of thousands of microscopic scales that cover the insects’ wings like roof shingles. Along with repelling water and providing color to the wings, the scales play a crucial role in reducing aerodynamic drag. Recent research on monarchs by Lang and others reveals that the scales create a buffer between the wings and their surroundings, allowing air to quickly glide past the insect. The efficiency boost provided by the scales is essential for the species’ seasonal migration, which can extend 4000 kilometers.

To help illustrate monarch flight on the magazine’s cover, Physics Today art director Freddie Pagani turned to Cynthia Cummings, the magazine’s longtime assistant editor and photographer. Cummings pored through her sizable photo archive and found hundreds of butterfly pictures, but none featured an airborne monarch. She turned to plan B and visited two butterfly conservatories near her home outside Chicago. Neither housed monarchs.

Fortunately, the grounds of the Chicago Botanic Garden that surround its indoor butterfly exhibit attract plenty of native pollinators. Cummings spotted three monarchs fluttering around a milkweed plant, aimed her camera, and took a burst of exposures. Among the dozens of shots, one captured a pair of butterflies that were in flight—and, just as important, in focus. “This was a shot of luck, pure luck,” Cummings says. As a bonus, “the butterflies put themselves in just the right spot for the cover text.”

The photo reminded Pagani of a hazy summer evening, imagery she thought was appropriate for the last issue before the start of Northern Hemisphere autumn. She adjusted the photo to enhance the colors of the butterflies and de-emphasize the bright pink milkweed flowers in the background. To complement the creatures’ graceful motion, Pagani chose a thin, extended typeface called Adorn Garland for the word flight in the main cover line.