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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  • Brian

    They’re eating another species, so not cannibals. Title is very misleading.

  • Ima Ryma

    A male firefly, just weeks to live,
    I focus on seeking Ms. Right,
    Cuz I got genes I gotta give,
    So I go blinking through the night.
    A chemical butt beacon sends
    Code that I wanna get it on.
    Whether or not it works, depends
    On how lucky a hand I’ve drawn.
    Cuz there’s a femme fatale firefly
    Who flashes signals of pretense,
    Hoping to fool a sex starved guy,
    Then make a meal out of us gents.

    As I was looking for my mate,
    Guess who came to dinner – too late!

  • Judy Pichon

    I enjoyed this article. Thanks!

  • Walter

    Turns out those sweet little fireflies are pretty hardcore! 😉

  • Don

    They aren’t even members of the same genus. Maybe the definition of cannibal should be added to the list of surprising facts.

  • Piper

    I agree, Brian. They’re not even the same genus. It’s like how some humans in the world eat monkey meat. We don’t call them cannibals, though they are cousins of their prey.

  • Afsaneh

    It makes me surprise

  • Nicholas Jenkins

    Different genus, same species. Similar to American and Asian, both still human but of a different genus. If I ate an Asian it’d still be cannibalism.

  • Peggy Butler

    Lynn is an amazing women. She came to Kellettville, PA 2 years ago and brought her team here to study the fireflies in the Allegheny National Forest. We learned so much from her, and sure enough found P. carolinus right here in my backyard. We just love this little critters now.

  • Christine Dell’Amore

    Hi everyone. Thanks for your comments about the term cannibal. Since most people think about fireflies collectively, I think it’s OK. However I did add quotes to make it more accurate. Thanks!

  • BIgBossOgg

    I wish someone could explain why we no longer see them in central Florida. I’m a child of the 60’s, and they were everywhere up until about 20 years ago.

  • Bubba

    Nicholas, all living humans are the same genus (homo) and species (sapien). Asians and Americans are all homosapiens. Homosapiens long ago killed off (outcompeted) all other species in our genus.

    Also, if two organisms are the same species they must, therefore, be the same genus.

  • slvia

    still patiently waiting for them to show up…..thanks for the great article

  • Joseph Higginbotham

    I remember firefly flashes in backyards, open fields and woods, we don’t see them anymore in southwest Louisiana. Is there an explanation?

  • Anita

    Discovering the fireflies in the wetland behind my house was one of the most amazing experiences. They are very rare in Colorado. Unfortunately they are in danger of being displaced by a developer with plans to build a road right through their home. Still hoping that empathy, sanity and the wetland protection laws will prevail and their rare habitat will be saved! Loss of habitat and light pollution is probably why they are a vanishing species.

  • Leila Pereira

    To all the people wondering what happened to their fireflies: human encroachment, loss of habitat, and climate change are just a few reasons why.

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