The Original Flash Dancers

Against the night sky, fireflies advertise their availability with a whole lot of burning love. How do they do it?

04-01-2004 // Cynthia Berger

STATE COLLEGE, Pennsylvania, is famous for its Fourth of July fireworks. But my new hometown has another light show, one I like even better—and I don’t have to fight the crowds for a good view. I just drag a patio chair into my yard at dusk, when twinkling fireflies light up the lawn.

Entranced by those lights, poets through the ages have linked fireflies to love. But if the "fairy lights" strike a spark in the human soul, their real raison d’être is romance among fireflies themselves. Typically, males flash while flying, as if to say "Notice me!" Females watch the show while perched and, if they like what they see, flash back.

Also known as lightning bugs, fireflies are neither flies nor bugs but soft-winged beetles. There are about 170 species in the continental United States; most range east of the Mississippi. The vast majority flash to attract mates, with each species flashing in a distinct way.

Many members of the firefly genus Photurus, for example, glow green, while Photinus flash yellow. Photuris versicolor is distinctive because it flashes four times, fast: bright, dim, dim, dim. Males of Photinus pyralis, the common eastern firefly, light up for a solid half second while flying in an unmistakable pattern—picture a glowing letter "J" sketched in the night air, starting from the hook and moving upwards.

The firefly’s light comes from a controlled chemical reaction that takes place inside the light-producing organs on the underside of the abdomen. Light without heat is the by-product.

Once you’ve studied the luminous language spoken by your local fireflies, try some cross-species communication. With a small penlight, you can mimic the female’s response and lure a male right to your fingertips. Or, here’s a tip from Richard Moore, a professor of rural sociology and environmental sciences who sometimes leads summer firefly walks at the Ohio State University arboretum: If you see fireflies dancing in a meadow or woodland edge, point your car headlights in that direction, then flash like a male. To this supersized signal, males and females will wink back en masse.

My backyard light show fades by late summer, which raises the question: What are fireflies doing the rest of the year? Females lay eggs right after mating and the larvae hatch after three or four weeks. The little grubs are ferocious predators. Some live at the edges of farm ponds and lumber into the water after snails; others tunnel through leaf litter or loose soil after small insects, or even pursue earthworms in their burrows.

Life histories vary with species, but typically larvae spend winter buried in soil, then feed in spring before pupating and then transforming into beetles. As winged adults, most fireflies don’t feed at all; although, a few species sip nectar, and female tiger fireflies mimic the flashes of other species to lure in males they make a meal of, rather than a mate.

Fireflies might seem less common than they used to be. The truth may be that, with the advent of air conditioning and television, fewer people are out on their porches to see fireflies put on their show. Or the widespread use of lawn and farm chemicals could be a factor in their decline, along with excess outdoor lighting, which interferes with the fireflies’ luminous courtship rituals. "But there hasn’t been any real study that I know of," says Marc Branham, a University of Florida firefly expert. "You do seem to find fewer fireflies on well-manicured lawns."

Fireflies couldn’t survive without their fairy lights—so essential for courtship and reproduction. And now it seems that the firefly’s glow may help keep humans alive as well. Scientists have found a way to genetically engineer firefly light genes into other living cells. Possible applications include tests for tainted meat, wastewater treatment and even, perhaps, a cure for cancer. For me, knowing that firefly light may have this kind of power makes watching fireflies at dusk even more magical.

Cynthia Berger last wrote about Arctic owls in the February/March 2004 issue.


Create a Firefly-Friendly Backyard

To encourage fireflies to visit your property:

  • Limit or eliminate the use of pesticides in your garden and on your lawn—they kill harmless bugs along with those you consider pests.

  • Turn off outside lights from mid-June to the end of July. Fireflies can’t communicate with one another against a bright background.


All Together Now

On a nighttime water taxi ride along a brackish river in Thailand or Malaysia, you can see what looks like the world’s largest display of Christmas lights: fireflies perched on the branches of mangrove trees, flashing in perfect unison.

North American fireflies were thought to lack this skill—until 1992, when Jonathan Copeland of Georgia Southern University and Andrew Moiseff of the University of Connecticut checked reports of coordinated flashing in Great Smoky Mountains National Park and confirmed that Photinus carolinus engages in showy group displays. (The Georgia coastal plain firefly, Photuris frontalis, and the Photinis pyralis in Ohio are other in-sync flashers.)

How do the bugs coordinate their flashes? Moiseff and Copeland think fireflies notice others flashing nearby and speed up or slow down just a bit to match their neighbors. A similar process occurs at concerts when random clapping converts to synchronized applause. Why coordinate? Perhaps to send a clear signal to females. "If a whole lot of males are flying," says Moiseff, "and each one flashes on his own, the female may see so many flashes, she can’t actually see the species-specific signal. Synchrony avoids the issue; even though there are a lot of males flashing, the pattern is recognizable."

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