11 Illuminating Facts About Fireflies

Nature's light show.
Nature's light show. / diliananikolova/iStock via Getty Images

No summer evening is complete without watching—and sometimes catching—fireflies. There are about 2000 different species of lightning bugs, and there's still a lot that scientists don't know about them. Here are a few things we do know.

1. Fireflies are not actually flies—they’re beetles.

Up close, it's easier to see that fireflies are beetles. And like all other beetles, they have hardened forewings. Fireflies use their forewings—also called elytra—for balance while in flight.

2. Fireflies are bioluminescent.

Fireflies putting on a show in Japan.
Fireflies putting on a show in Japan. / tdub303/Getty Images

According to LiveScience, the light is produced when oxygen is mixed with a pigment called luciferin, an enzyme called luciferase, and a chemical called adenosine triphosphate that provides cells with energy. The final part of the formula is uric acid crystals, which are located in the cells that make the light and shine the light away from the firefly's body. (The light-emitting part of the firefly is called a photic organ, by the way.)

3. Male fireflies focus on finding a mate.

At the nucleus of their night flights, the brightness of their glow, and their flashing patterns is one thing: reproduction. These guys are intent on mating. Typically, the females sit immobile and only flash back when they see a male with a particularly impressive display.

4. Each firefly species has its own flash pattern.

As males fly through the air searching for a mate, each uses a “flash fingerprint” specific to its species. According to the American Museum of Natural History, some firefly species only flash once; others use “flash trains” of timed bursts of light; some fly in specific J-shaped patterns; and others shake their abdomens side-to-side, so it looks like they’re twinkling. Scientists can use these distinct patterns to determine how many species are in an area.

5. Some fireflies sync up.

Synchronous fireflies flash in unison, putting on an incredible performance. Males do this to communicate with and attract females (but people often flock to the sight as well). There are a few amazing places to see these types of fireflies, but you should plan in advance: Viewing the fireflies at Great Smoky Mountains National Park is such a huge tourist attraction that visitors have to enter a lottery to win tickets to the spectacular light show.

6. Flashing fireflies are rare west of the Rocky Mountains.

There are a few exceptions [PDF], but for the most part, flashing fireflies don't live west of the Rockies. Whereas flashing fireflies communicate with their flickers, non-flashing ones use pheromones to stay in touch with one another.

7. Fireflies can be poisonous.

Not a snack.
Not a snack. / Jeremy_Hogan/iStock via Getty Images

Not only do fireflies taste nasty, they can cause some actual harm. When predators attack, fireflies kick into a process called reflex bleeding. They shed drops of blood that contain bitter-tasting chemicals that are poisonous to vertebrates, including lizards and sometimes birds. Snacking on a firefly likely wouldn’t kill a person, but it still isn’t wise to eat one.

8. In much of Europe, female fireflies don’t actually fly.

European female fireflies remain flightless into adulthood and take on the form of a worm that glows rather than flashes.

9. Female fireflies can be cannibalistic.

Some adult fireflies don’t eat at all, and many who do tend to feed on mites or pollen. However, the femme fatales of the genus Photuris like the taste of their own. Using what is known as aggressive mimicry, the female of this particular subfamily waits for a male firefly to flash, then imitates that male's flash pattern, suggesting that she is a receptive mate. After luring him in, she chows down.

10. Firefly larvae aren't a picnic, either.

These glowing worms will follow the slime trails of snails and slugs, bite and inject them with a paralyzing neurotoxin, and chow down.

11. Firefly populations are declining.

People will often fondly recall summer nights spent in a backyard filled with glowing beetles before lamenting the lack of lightning bugs around today. And sadly, their anecdotal observations have been proven correct: Firefly populations are declining. A 2021 study found that 14 out of 128 Lampyridae species are now at risk of extinction. Factors that are affecting other insects, like light pollution and habitat loss, are largely to blame.

A version of this story originally ran in 2013; it has been updated for 2022.