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.