Fireflies Are “Cannibals”—And More Surprising Facts About the Summertime Insect

By Jason Bittel

For many people, nothing signals the start of summer quite like a field full of flickering lightning bugs.

There are about 2,000 firefly species—actually beetles—that are known for flashing their evening lights in temperate countries worldwide. (See related video: “The Science of Summer.”)

A time-lapse composite image of fireflies taken in Grand Ledge, Michigan, in 2013. Photograph Vincent Brady, National Geographic Your Shot

In the Great Smoky Mountains National Park in the eastern U.S., there’s a species of firefly (Photinus carolinus) that takes the showy spectacle to a new level.

In an attempt to woo a female, thousands of P. carolinus males flash their lanterns on and off at the same time, creating a synchronous bioluminescence display unlike any other on Earth. Every June, people come to the national park to witness the spectacle, which has become such a popular tourist attraction that you’d have an easier time getting into a Rolling Stones concert. Advance tickets sell out in a matter of minutes.

So for those of you who missed out on the natural light show this year, here are some amazing firefly facts to ponder on those warm summer nights. (See National Geographic’s photos of extreme summer adventures.)

Time Is of the Essence

For P. carolinus, the synchronous display is an event that’s simultaneously silent, rhythmic, tranquil, and frenetic, said Lynn Faust, a naturalist and the author of a forthcoming book called Fireflies, Glow-worms and Lightning Bugs: A Field Guide to the Fireflies of the Eastern U.S. and Canada.

“Peaceful to us,” she said, “life or death for them.”

Faust explains that synchronous fireflies live just two to four weeks after they reach their adult phase. That means each night is a desperate attempt to find a mate and pass on their genes. And if you’re a small black insect trying to find other small black insects in the dark, it helps to have a chemically activated beacon on your backend. (See other pictures of glowing animals.)

In P. carolinus, the synchrony goes like this: Around 9:30 p.m., a cloud of male fireflies starts to flash. Each male emits six quick blinks, then takes a break for about six seconds. Over time, the insects manage to coordinate their individual pulses to the point where the whole forest throbs neon green, then goes wordlessly dark.

The light show, as Faust calls it, is highly dependent on temperature, moisture level, and elevation—but given optimal conditions, the display can go on past midnight. (See pictures of summer scenes.)

“Femme Fatale” Cannibals

Of course, there’s a downside to lighting up the night. As every little kid with a Mason jar knows, it makes the insects awfully easy to detect.

Among those taking notice are the females fireflies of another genus, Photuris. Entomologists have nicknamed these insects “femme fatales” because they eat P. carolinus fireflies—and they have some flashy ways of doing it.

Photuris fireflies attack synchronous fireflies in the air, a maneuver known as “hawking.” But that’s not how the femme fatales got their nickname.

“These fireflies flash the wrong signal and pretend like they’re a female Photinus carolinus,” said Rebecca Nichols, an entomologist for the Great Smoky Mountains National Park. “The male will come down thinking he’s going to mate, and then the Photuris will grab him and eat him.” (Related: “Flesh-Eaters: 5 Cannibalistic Animals.”)

The femme fatales are such skillful mimics that they can even switch between the flash signals of several species, depending on what’s fluttering around that night. Cannibalism offers extra nutrients to females at a time when they, too, will be mating and laying eggs, but a balanced diet isn’t the only thing they get out of the meal.

“Poisonous” Bugs

Many fireflies produce defensive compounds similar to the venom found in toads. Predators find these compounds distasteful and have learned to avoid eating fireflies altogether.

Fireflies in the genus Photuris don’t have these compounds, but research has shown they’re able to absorb them by eating other fireflies. What’s more, the chemical defenses also pass into the Photuris fireflies’ eggs to safeguard the next generation.

Blood Defense

Synchronous fireflies aren’t totally defenseless though. When attacked by the femme fatales, the insects discharge a bit of their blood, which scientists call “reflex bleeding.”

For most predators, the blood offers a taste of the defensive compounds mentioned above and sends them packing, but for the femme fatale fireflies, it’s the stickiness that causes a problem. Faust wrote in 2012 in the Journal of Entomology that the blood “coagulates into a sticky mass” in the cannibal firefly’s mouth, sometimes giving the synchronous firefly enough time to escape.

But even reflex bleeding has a workaround. Faust has observed the femme fatales stalking and stealing fireflies from the webs of orb weaver spiders. These fireflies have already dispensed with their reflex bleeding and have been essentially gift-wrapped by the spider. The femme fatales will even do battle with the spider over a meal, though sometimes this means turning into a meal themselves.

“Recurring Miracle”

Though the Smokies firefly display is pretty much over, luckily the species can be found throughout the southern Appalachian Mountains in the eastern U.S. In fact, Pennsylvania will be hosting its second-annual Firefly Festival later this month. And, of course, the bug is found around the world, especially in warm parts of Asia.

To Faust, seeing the synchronous fireflies “never gets old—it is a recurring miracle each year, just like the first wildflower, the fall leaves, the first hummingbird,” she said.

“I like to be up there the very first night the first male emerges and flashes. It gives me a kick and reassurance that life goes on.”

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