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.

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?

Five years ago, Lewis and then-student Christopher Cratsley discovered that variations in the single flashes of Photinus ignitus do say something about a male's suitability. Males with longer flashes—which females prefer—were more generous with what biologists call their "nuptial gift," a coiled package of sperm and protein. During copulation, the male deposits the gift in the female's reproductive tract, where it provides nourishment for the female and her developing eggs. By controlling fireflies' access to one another in the lab, Lewis and another former student, Jennifer Rooney, demonstrated that a female who receives more nuptial gifts produces more offspring.

As researchers learn more firefly secrets, they've found similarities to animals such as frogs and insects that vocalize their sexual pleas. "Females in many groups seem to prefer higher-energy courtship signals," says Lewis. In fireflies, females are wowed by faster or longer flashes; in some frogs, crickets and katydids, females like longer, louder or faster calls. In other words, a female wants a mate who works hard to get her attention. Such devotion could be a sign that he has good genes or can provide well for her.

Not all flashing fireflies are hunting mates. Some are just hunting. James Lloyd, a firefly expert recently retired from the University of Florida in Gainesville, discovered that female Photuris fireflies mimic female Photinus fireflies. When an unsuspecting male Photinus is drawn to the flashing in the grass, she eats him. This behavior, called aggressive mimicry, isn't unusual in the animal kingdom, says Lewis. Bolas spiders emit an odor similar to a female moth's to attract male moths as prey. And certain coral reef fish imitate wrasses, which clean other fish's scales, to get near other fish and take a bite out of them. But Photuris females, nicknamed femmes fatales, are masters of disguise: they can mimic the female response signals of whatever firefly prey is nearby.

Fireflies live dangerous, showy lives, and are easy prey for bats and spiders, as well as other fireflies. For protection, many species produce bitter-tasting chemicals, such as varieties of a compound called lucibufagin. Predatory Photuris seldom devour Pyractomena fireflies, for instance, possibly deterred by their awful flavor. Photinus also produce chemical deterrents, but theirs don't work against Photuris females. In fact, Photuris females may even benefit, retooling the bitter chemicals they eat to bolster their own defenses.

One night near the end of firefly season, Lewis was disappointed that she no longer saw any predatory Photuris. She turned her attention toward the grass, where a pair of Photinus had been flirting intently, but the male had left without mating. People often think of males as being perpetually ready to mate, says Lewis, and think of females as picky. At the beginning of the firefly season, that's largely true. But toward the end, fewer Photinus males are left, and they've used up their limited resources mating with successive partners, while females gained nourishment in the exchanges. If a male thinks a female doesn't have many eggs left, he'll take his precious nuptial gift elsewhere.

Looking at the abandoned female, I take an amateur stab at a penlight flash. To my amazement, the Photinus lights up, hopeful. Lewis isn't surprised. "She's pretty desperate at this point in the season," she says.