In the warm, lazy nights of summer, glimmers of light begin to sparkle—drifting down city streets, suburban backyards, and country meadows. The fireflies are out. Blinking "I'm here, I'm ready, I'm here, I'm ready," fireflies use their lights as mating calls. Winking with light, the male puts on his best flashes while the female hides in the grass, watching the show. When she's ready, she answers him with a similar flash of her own.

Like nearly all insects, fireflies are most active in the summer, when warm temperatures and plentiful food sustain them. While their cool glowings haven't been directly connected to circadian clocks, their flashy signals are predominantly active at night, and the flashes themselves are a clear example of a cycle, or oscillation.

A Brief, Shining Light
Fireflies, also called lightning bugs, are actually beetles. They're in the insect family Lampyridae, meaning "shining ones" in Greek. Like little lamps, they usually come out only at night. And that light is the firefly's last hurrah. They live for three years, and for the first two they're larvae, feasting on bugs and slugs. In their last summer, they emerge as full-fledged fireflies.

The firefly's light is actually a cool chemical reaction. A small organic molecule, luciferin, mixes with an enzyme called luciferase in the presence of oxygen in the firefly's abdomen. The energy for the reaction comes from another molecule, adenosine triphosphate or ATP. About 90 to 98 percent of the energy spent by the insects to generate light actually produces light, and only 3 percent, heat. That's very efficient, especially when you consider that a light bulb gives off 97 percent of its energy as heat.

A Glowing Clock in Time
Recently, scientists used the molecular secret of firefly glow to shed light on how our bodies keep time. They teamed luciferase to a gene that helps regulate circadian rhythms. This gene codes for a protein called PER, and the ebb and flow of per protein is one of the gears of the biological clock. When scientists linked the luciferase gene to part of the per gene in fruit flies, they hoped to find the per gene turned on in the heads and eyes of the little flies. But they were startled to see glow in the flies' bodies, and even out near the tips of their wings. The firefly's gleam showed that fruit flies and most likely all animals have molecular clocks in virtually every cell of their bodies. And, stranger still, these clocks kept on ticking even when disconnected from the fly's brain. Scientists believe that these tiny clocks help keep all animals on a set schedule, telling them when to mate, when to feed, and ultimately, when to die.

Falling for the Flashiest Males
There are more than 2,000 species of fireflies. They can be found everywhere from West Virginia to Malaysia. Some don't glow. Others have colors ranging from yellow to orange to space-alien green. They tell each other apart both by color and in their blink patterns. For example, a common North American lightning bug flies in a J-shaped pattern. So when J females see an attractive J-flying male, they respond in kind, telling him to come down to the tall grasses to mate.

No matter what the pattern, recent research also shows that all female fireflies are attracted to males that can flash the fastest. In a study at the University of Kansas, scientists simulated male pattern flashings and found that females were most attracted to lights that were so fast they would never be found in nature. Apparently, the only reason fireflies aren't little strobe lights is that they are limited in how quickly they can make luciferase. Scientists aren't sure why females like the fast flash, but they suppose that females might believe that faster flash equals stronger genes.

Some female fireflies have also learned that faking flashes is an easy way to a quick meal. In the species Photuris pennsylvanica, females mimic the "I'm available" flashes of females from other species, and then they capture and eat the anticipating males. This bait-and-switch attack is called aggressive mimicry. When the female Photuris is done snacking, she switches her blinking light to that of her own species. But the male Photuris has a few tricks up his sleeves, too. A male often flies around faking the flashes of other fireflies, searching for a response from a femme fatale of his own species while she is pretending to be another. Once he lands and, with his feelers, tentatively determines she's his type, then he has to persuade her to mate and not eat.

All Together Now
Perhaps the oddest phenomenon of the firefly is some species' ability to blink in unison. Trees filled with fireflies have been known to blink in rhythm—as though someone is flipping a light switch on and off. Fireflies in Thailand and Malaysia are famous for this behavior, and in the early 1990s scientists found fireflies in Tennessee that also blinked in unison. Scientists are not clear why it happens. Some believe that the synchrony guarantees a nice, dark moment for flying males to see the faint answers of the females. Others think that males are trying to beat the clock and be the first to flash to impress the ladies. Still others think that by coordinating their blinking, the collected males send a strong signal that will attract the females even from quite far away to their resident tree.

These cycles of birth and death and light and dark are what guide the firefly through its short life. As summer turns into fall and the seasons change, the fireflies come to the end of their three-year life cycle—leaving behind their eggs to start over again and, in later summers, to light up the trees once more.