Photograph by David Liittschwager, Nat Geo Image Collection
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Fireflies streak across a Tennessee summer night.

Photograph by David Liittschwager, Nat Geo Image Collection

How Fireflies Glow (and What Really Turns Them On)

A new book details the lights, loves, and science of fireflies.

Summertime, and we’re all looking forward to some July 4 fireworks, not to mention some smaller lights in the nighttime sky.

Fireflies are a summertime staple, so Weird Animal Question of the Week is taking the author’s prerogative to ask a twofer, first about the lights and then the loves of fireflies: what turns fireflies on?

First Light

The enzyme that turns on the rainbow of colored lights that occur in different firefly species is called luciferase. Sara Lewis, author of the beautiful new book Silent Sparks: The Wondrous World of Fireflies, says what first hit the switch was really a glitch.

The firefly gene that codes for luciferase is very similar to a common fatty acid-making gene, Lewis says. It's likely a duplicate of that gene acquired a mutation that caused it to produce a tiny bit of light in a distant firefly ancestor. (Related: How Do Fireflies Glow? Mystery Solved After 60 Years).

Over eons high concentrations of that light-producing chemistry evolved along with specialized tissue, to create a whole new “light organ,” Lewis says—the firefly lantern.

Watch: Fireflies Glowing in Sync to Attract Mates

These fireflies coordinate their flashes. Here's why.

Turn-Ons Include: Flashing

For fireflies, “light is the language of love,” says Lewis.

In other words, they light up to look for romance.

Males broadcast a signal that’s code for their species. For example, "I’m a big dipper firefly," a common eastern U.S. species, and "I’m a male," over and over. Females respond only to signals of their own species, flash specific patterns in return, and have their own ideas of what’s sexy.

Watch: How do fireflies glow?

Big dipper ladies prefer flashes of longer duration. In Photinus consimilis, faster flashers are the stud muffins.

Once she finds the right male, their flashing conversation can go on for hours, Lewis says, partly because the ladies play hard to get, responding only to every fifth flash or so. They can be on the same blade of grass and he’ll still spend a long time blinking and searching for her return flash.

Once they find each other they’ll turn the lights out and mate, tail-to-tail, for hours. That way, the male blocks any rivals from accessing the female that night.

But the next night she’s on to flip another fellow’s switch.

View Images

Fireflies, or lightning bugs, hold many secrets.

Bright Young Things

The glow isn’t always about sex, in fireflies or other insects.

Philip Koehler, an entomologist at the University of Florida, says the cave-dwelling glowworm, or fungus gnat (Arachnocampa luminosa), clings to rock walls and catches insects in its “sticky mucous threads.” The prey is drawn in by the glowworm's bioluminescent abilities, similar to a porch light that attracts insects.

And fireflies use their glow to also turn predators off. The insects contain toxic compounds, known as lucibufagins, that are strong enough to kill small animals, such as lizards and birds.

Juvenile fireflies live underground. Like the colors on a monarch butterfly warning predators that they’re toxic, the glow of these subterranean babies lets underground predators, who can’t see colors, know to back off. (Related: Fireflies are “Cannibals” - and more Surprising Facts about the Summertime Insect.)

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Glowworms in Australia light up to attract prey.

Dim all the Lights

In her book Lewis lists ways to make your yard more inviting to fireflies, including one standard recipe for romance.

“If we keep our lights low or out they can see each other and find each other,” she writes.

The love can keep going, for the fireflies and those of us who can’t resist even the tiniest fireworks.

Watch: Luminous fireflies create scenes similar to Vincent van Gogh’s "Starry Night" in this spectacular time-lapse video from filmmaker Vincent Brady.

Weird Animal Question of the Week answers your questions every Saturday. If you have a question about the weird and wild animal world, tweet me, leave me a note or photo in the comments below, or find me on Facebook.