Shedding Light on the Mysteries of Fireflies

Everyone loves fireflies. But Sara Lewis understands them—and she’s determined to make sure they survive

In one of the more than 100 research papers Sara Lewis has published, you’ll come across this line: “Fireflies . . . rank among the most charismatic beetles.” It might make you wonder which other beetles are in the running. Maybe ladybugs would make the cut. But really, do any other insects inspire people—from children to poets to artists—as much as fireflies?

“You can admire a butterfly, definitely,” said Lewis, a professor of biology at Tufts University’s School of Arts and Sciences. “But fireflies, they amaze. It’s a total sensory experience—you step out into the mysterious night, then these tiny, silent sparks of light appear. Even in New York City, the fireflies can transform Central Park into a luminous landscape.”

Lewis has studied fireflies for more than three decades. She and her students have uncovered fascinating details about how they glow and why, expanding knowledge about the ways animals mate and pass on their traits. Along the way, she has tapped into the charisma of the firefly, using it as a gateway bug to help people understand the key role that insects play in nature.

Through a book and a TED Talk (with more than a million views), through tweets and conferences, she has made fireflies into tiny ambassadors for the science of evolution and the importance of conservation and biodiversity. And somehow, she has managed to keep a sense of wonder for these incandescent creatures, like a kid traipsing through the yard with a Mason jar.

four fireflies take flight

Fireflies are threatened by loss of habitat, light pollution, and pesticides, but efforts to protect them are underway in the United States and beyond. Photo: Terry Priest

Watching Fish Dance

Lewis grew up in Woodbridge, Connecticut, where she spent a lot of time climbing trees and playing in the forest. “I don’t remember watching much television,” she said. After high school, she hoped to attend Hampshire College, the just-opened experimental school, but didn’t get in. So she settled for her second choice, Harvard, where she majored in organismal biology.

During graduate school at Duke University, she studied the ecology of coral reefs. She conducted research from a tiny Smithsonian Institution marine station off the coast of Belize, on an island barely big enough for one house, six people, and 18 palm trees.

Among other discoveries, she was able to show how crucial parrotfish are to the health of coral reefs. Their constant grazing on the algae and seaweed that grows on the reef helps to keep the slower-growing coral alive. The finding illustrated the importance of biodiversity, as losing just one kind of fish could have devastating effects on an ecosystem.

During four years spent mostly underwater, Lewis became intrigued by the mating dances she saw fish performing around her. She decided to turn her attention toward the evolutionary process of sexual selection, whereby animals pass down traits to their offspring that increase their chances of attracting a mate. (Think of the stunning plumage on a peacock or the rack of antlers on a male elk.)

But studying fish courtship would be tricky: Keeping track of individual fish in the ocean is no easy task. Besides, “I was pretty tired of being wet all the time,” she said.

Sara Lewis with a headlamp at night in a field

Sara Lewis, a professor of biology at Tufts, has uncovered details about how fireflies glow and why. Here, she searches for elusive females in Lincoln, Massachusetts. Photo: Don Perlman

As she was writing her dissertation back in North Carolina, Lewis became captivated by the fireflies she watched lighting up her yard at night. A quick library stop told her that the flying males were signaling to females below, but why were the males so active with their flashes and the females so coy?

She had a million questions—and she’d discovered her next research path. It was a bonus that the path was on dry land and as close as her own backyard.

Flashy Males Make Good Providers

Scientists believe that about 130 million years ago, a firefly’s glow warned predators that it carried toxins and just plain tasted bad. It still serves that purpose. (Lewis tried a drop of firefly juice once. Never again.) But over time, fireflies evolved to use their glow to also communicate with each other.

Each species has a pattern of flashes that females of the same species recognize and, if the males are lucky, respond to. Yet the females are picky and don’t always flash back.

Lewis, who joined Tufts as an assistant professor in 1991, wanted to know why they were playing so hard to get. In a clever experiment, a Ph.D. student in Lewis’ lab, Christopher Cratsley, AG00, used carefully timed LED lights to simulate male flashes. He found that females were more likely to respond to male flashes that lasted longer; other researchers found the females also liked faster pulse rates.

The differences in the flashes were so tiny they couldn’t be detected by the human eye, but the females knew all the same.

two fireflies mating

This composite photo shows two fireflies mating and a magnified image of the elegant gift that Lewis discovered male fireflies offer to females. Photo: Sara Lewis

But why do they care? The answer lay in another of Lewis’ discoveries. Many animals give tokens of their esteem to aspiring mates during courtship: a dead vole for your songbird sweetie; a fly wrapped in webbing for the attractive she-spider on the corner. Biologists call it a nuptial gift.

Lewis was the first to discover that fireflies give gifts, too, but their gifts are more subtle. Working under a microscope, she found that during mating, male fireflies transfer more than simply sperm: They also provide a gooey package of nutrients called a spermatophore.

Through a series of experiments, Lewis and her students discovered the importance of this nutritional dowry. Female fireflies can absorb nutrients from these male gifts and transfer protein into their eggs. And they get something out of it too, as females that receive larger gifts can live longer.

There is something about the chemical makeup of the gift that benefits the males, as well. Through DNA paternity tests, Lewis’ lab found that even though a female may mate with several males during the mating season, a male that gives a larger nuptial gift is more likely to have his genes end up in the eggs the female produces.

All these discoveries connected back to the flash dance. In at least one species, the males that have the more desirable flashes turned out, in many cases, to have the larger nuptial gifts

“That was just a whole different take on what male flashes were all about,” said Darryl Gwynne, professor emeritus at the University of Toronto, who has done extensive research on nuptial gifts in other insects. Previous firefly flash research had focused mainly on male competition and male/female deception. (Notable fact: One species of firefly mimics the flashes of other species to attract them—not as a mate, but as a meal.) “But Sara’s perspective was very different in showing that the flashes could well advertise how good a male is, in terms of being a provider.”

Lewis’ lab had deciphered the Morse code of the lightshow that so many people have admired on a summer’s evening. It’s a shining example of sexual selection and its powerful influence on evolution, as those genes that give the fireflies mating success get passed on from generation to generation.

Evolving as a Teacher

As a teacher, Lewis has had her own influence on the next generation. Cratsley, Lewis’ former Ph.D. student, was a TA for several of her classes and saw her constantly figuring out how to do a better job, revising her courses if she found a clearer way to demonstrate something, such as a biostatistics concept.

When a student makes a presentation, he said, “she is always right there in the front row, nodding her head, making eye contact, and is always amongst the first to ask a thoughtful, probing, and always kind question.”

She’s the opposite of the detached scientist who is more focused on what’s under her microscope than on the humans around her. “She’s interested in you as a person—how you’re doing, how things are really going in your life—and that’s important and something I try to emulate,” said Cratsley, who is now a professor of biology and chemistry at Fitchburg State University in Massachusetts and still very active in firefly research.

Lewis also passes on her passion for science communication. Whether Lewis is putting together a TED Talk for the public or a research paper that only committed scientists will read, “there’s a care to every single word, and a real focus on that as a craft in and of itself,” Cratsley said. “How do you communicate effectively?” Because all the scientific discovery in the world is pointless if no one listens.

Harnessing Awe

Every summer, people travel to the Smoky Mountains to see the synchronous fireflies there. Wandering among the tourists late at night, Lewis would hear grown-ups explaining to their kids that “God put fireflies on Earth so humans could feel awe.”

Being awestruck is great, but even more important, Lewis wants people to understand how nature has produced this mesmerizing phenomenon and maybe even appreciate the beauty and magic of evolution.

Especially in recent years when science deniers have had a bully pulpit, “fireflies seemed like a good ambassador for explaining the creative improvisation that we call evolution,” she said. “How did we get from some totally mundane beetle to this amazing firefly?”

In 2016, that inspired her to write a book, Silent Sparks: The Wondrous World of Fireflies, a fact-filled page-turner replete with courtship, sex, and deception, all grounded in scientific discovery.

Writing the book was something of a coming-out for Lewis: In the scientific world, she wrote in the preface, “to acknowledge wonder is tantamount to unreason, and therefore treason.” But Lewis says she remains enthralled by fireflies, and she hopes that the emotional connection to fireflies felt by so many people may be the beetles’ salvation.

Lewis describes the typical reaction she gets when she tells someone what she does for a living. “The first thing they say is, ‘Oh gosh, I love fireflies!’” Lewis said, conveying the joy in their voices. “The second is, ‘Why aren’t there as many as when I was a kid?’”

Part of the answer might be that people now spend more time in front of TV and computer screens rather than outside on hot summer nights.

“A famous firefly biologist used to say that if air conditioning hadn’t been invented, people would think there are more fireflies around,” Lewis said.

Sadly, however, it seems that many firefly species really are disappearing. Globally, the top threats to fireflies are loss of habitat; light pollution, which interferes with the signals used to find mates; and pesticides that are used to boost agricultural production.

In a study focused on the 170 firefly species found in the United States and published in November of 2021, Lewis and others identified some 20 species facing medium-to-high extinction risks, while about one-third seem to be doing just fine. In other parts of the world, declining populations have been documented in some of Malaysia’s synchronous fireflies and in the common European glowworm.

Fireflies Forever

Lewis continues to have her eye on firefly ecology, but in recent years she has increasingly focused her efforts on conservation. In 2017, she founded the firefly specialist group of the International Union for the Conservation of Nature. Its 45 members are working to understand why certain fireflies are declining in some parts of the world, “and then we’re taking action to protect them locally, nationally, and globally,” Lewis said.

“That has been hugely instrumental in shifting people’s focus to conservation of fireflies,” said Candace Fallon, senior conservation biologist for the Xerces Society for Invertebrate Conservation, who said she admires Lewis’ ability to bring together researchers, land managers, and other stakeholders across disciplines. “It’s been incredible to watch her build this force of folks who are really excited about fireflies and firefly conservation. I don’t think we’d be where we are now without having Sara at the helm of that.”

A single firefly
Photo: Terry Priest

Report Your Sightings

You can help conservation efforts by reporting firefly sightings in your yard or anywhere else in North America. To do so, join Firefly Watch, a citizen science project that Sara Lewis and Christopher Cratsley helped found and have been involved with since 2008. (Photo: Terry Priest)

Lewis says that this kind of coalition building is new to her. “I haven’t been trained to do any of this stuff, so I’m learning something new pretty much every day.” But she finds it exciting and acknowledges that her deep knowledge of firefly biology, and the connections she has made over the years, make her the “right person at the right time in the right place.”

Lewis has continued to evolve as a teacher. Last semester, she had a wonderful time leading, for the first time, a seminar in science communication, which drew in eager students from physics, psychology, neuroscience, child development, and other disciplines. But in December of 2022, Lewis will retire from teaching to devote more time to her conservation efforts while still conducting research as a professor emeritus.

Why work so hard to save the fireflies? Beyond their being part of the intricate web of life, Lewis believes that losing these insects would be a blow to the human psyche. If all the fireflies blink out, it would be like never again feeling the wonder of seeing a perfect spider’s web or a rainbow in the sky.

“And I think that’s essential to the continued survival of the human race,” she said, “remembering that we are connected to nature in that way.”

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