Sara Lewis is impersonating a firefly. She stands in waist-deep grass and brush, the hood of her jacket pulled tight around her ears to ward off mosquitoes, and clicks her penlight into the darkness. Frogs chirp. A dog barks. Lewis clicks again. Still nothing. She turns and flashes it in another direction. Off in the tall grass, a lone firefly lights up. Then another. Lewis has convinced the bugs that she too is a firefly, and they are flirting with her.
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Lewis wanders toward one of the flashers and locates a female on the tip of a tall blade of grass. Nearby, a male hovers, interested. The male flashes, waits, and the female flashes back. "It's just like talking on the telephone," says Lewis.
Their libidinous flashes call out, "Hey, glitter bug. Why don't you come over and see me?" That's one reason why Lewis and other biologists love fireflies: the flashes literally shed light on how the insects communicate, mate, reproduce and pass along their genes to the next generation. More than just glittery magic on a summer evening, firefly flashes are part of a fierce competition for evolutionary survival.
During firefly season, Lewis and her students spend nearly every night at this dark field just west of Boston. They spy on courting fireflies and catch some to study in their entomology lab at Tufts University at Medford-Somerville.
Most of the fireflies that Lewis studies—like those chased down by children in backyards throughout the eastern United States—belong to the genus Photinus. They live underground as larvae for about two years before emerging for a two-week, flat-out sprint through adulthood. They spend their adult lives courting and mating—they don't even stop to eat. "They're very single-minded," Lewis says.
Around 2,000 species of fireflies—which are not flies at all, but beetles—have been identified worldwide, and scientists are still finding new species. (Fireflies west of the Rocky Mountains do not flash. They emit chemicals called pheromones to elicit a potential mate's interest.) In the eastern United States, fireflies from three genera—Photinus, Photuris and Pyractomena—punctuate the dusk each summer with a billion bursts of yellow, orange or green light.
A Photinus firefly's light organ, called a lantern, is in its abdomen. In males, two segments of the abdomen light up, and in females, just part of one. A firefly flips its light on when two chemicals—the devilishly named luciferin and luciferase—react inside the lantern in the presence of oxygen. Telling one species from another is difficult. In her lab, Lewis pulls out a box containing about a dozen species of pinned fireflies that look pretty much the same. Species identification depends partly on minute distinctions in male genitalia and partly on differences in flash behavior.
Every firefly species that flashes produces a unique pattern while courting. Males of some taciturn species flash just once; other kinds of males blink twice or several times. Males fly around advertising their identity to females in the grass below. A female recognizes her species' code and flashes back if she wants to mate. Still, even within a species, not all male flashes are exactly alike, and biologists are eager to learn more about what attracts a female to one flash more than another.
"For a group that is so well known by nonscientists and appreciated by people sitting outside on a warm summer evening, there's still so much that we don't know about fireflies," says Marc Branham, an entomologist at the University of Florida in Gainesville. He and Michael Greenfield, of the University of Kansas at Lawrence, deciphered the code of fireflies called Photinus consimilis, which range from the Ozarks to northern Florida. They emit several flashes in succession. Rapid flashers attract the most female attention, the researchers found. When a female likes a male's flash, she responds with more flashes, and her flashes are brighter.
Other males advertise their intentions succinctly. Male Photinus ignitus fireflies, for example, found from Maine to North Carolina, employ just a single flash. Could that lone burst of light somehow be sexy or not sexy?