The War of the Fireflies

Most people know that fireflies light up to find a mate. In the North American night sky, the light show is put on by the males. The females, meanwhile, hide in the grass below, and when they see a signal from a male of their species, they flash a response. This glowing back-and-forth continues until the male locates the female, flies to her, and they mate.

There are around 2,000 species of fireflies worldwide. The ones you see most in North America belong to two main genera: the smaller, yellow-glowing Photinus and the larger, green-glowing Photuris. Different species have their own unique flashing patterns—the color, duration, interval, number of blinks, and flight path all vary. Often, fireflies of the same genus look very similar, making their light pattern the key to telling them apart. This unique "flash code" allows males and females of the same species to recognize each other, preventing harmful cross-species mating—a mechanism evolutionary biologists call "reproductive isolation."

For example, a common Photinus firefly, known as the "Big Dipper," emits a yellowish-green glow. The male flies low, flashing a half-second light every six seconds as he rises, forming a "J" shape in the air. A female on the ground, if she sees this, will wait about two seconds before flashing a half-second response. Other species have different patterns: some flash for two to three seconds every few seconds, others blink rapidly three times every few seconds, and still others use a pattern resembling Morse code's "dot-dash" (one quick flash followed by a long one).

If you can read the codes, you can identify the species dancing before your eyes. In fact, if you use a small flashlight to mimic a male's pattern, you might even get a response from a female of that species.

In the 1960s, Cornell University entomologist James E. Lloyd used this trick to catch the female fireflies he wanted to study. He was primarily interested in Photinus fireflies, but he was puzzled to find that female Photuris fireflies also responded to the Photinus male signal he was mimicking. Why would they respond? Was their reproductive isolation not so strict after all?

The answer came on the night of April 6, 1965. In Florida, Lloyd was trying to find a female Photinus firefly, using a flashlight to mimic the male's signal (two quick flashes with a two-second interval, repeated every four to seven seconds). He got a strong response, but upon getting closer, he discovered it was a large female Photuris firefly, whose male's signal was quite different (a single flash every three to five seconds). Lloyd stayed and watched the female. Over the next half hour, she attracted twelve male Photinus fireflies. The last one exchanged several signals with the female before landing near her. A dozen seconds later, Lloyd turned on his light and saw the female Photuris firefly holding the male Photinus firefly, chewing on his back.

While most fireflies eat nectar and pollen as adults, or nothing at all, the female Photuris firefly was an aggressive predator, using a deceptive signal to catch other fireflies. The reason for this, in addition to nutrition, was a surprising discovery made by entomologist Thomas Eisner and his colleagues at Cornell.

This brings up another question: fireflies glow in the dark, so aren't they vulnerable to predators? If the light is for love, then why do their larvae and eggs also glow, even in species where the adults don't? It turns out that firefly eggs, larvae, and adults all contain toxins similar to those found in toads. Birds, spiders, and other predators learn to avoid this poison. The light, therefore, is also a warning: "I am toxic, don't eat me!"

The Photuris firefly, however, does not produce this toxin itself. When a female eats a male Photinus, the toxin from the male transfers to her body, and then to her eggs and larvae. So, the female Photuris firefly primarily preys on the males of other species to acquire a chemical weapon for her own defense.

Interestingly, male Photuris fireflies also carry the toxin, and scientists suspect they get it by directly preying on male Photinus fireflies in the air. Whether they do this in the wild remains a mystery.

Behind the romantic flicker of a firefly, a story of deception, poison, and a brutal battle for survival is hidden, revealing a world that is both cruel and astonishingly clever.