What Makes Fireflies Glow?

When ancient humans saw mysterious blinking lights over field and stream, they sometimes attributed the light to dragons, gods or demons. Reports in early religious writings from China and India hold the earliest recorded discovery of the true source of the lights. The glow came not from deities or monsters, but normal, mortal animals: fireflies.

Greek and Roman scholars made the first thorough examinations of fireflies (which aren’t actually flies, but a family of beetles known as Lampyridae) and other “luminous organisms.” Aristotle described almost 200 marine species with the strange power to glow.

Centuries later, the phenomena of bioluminescence (from the Greek bios (“living”) and the Latin lumen (“light”))was fairly well known, but still poorly understood. While Shakespeare mentioned the “effectual fire of the glow-worm” in Hamlet, English explorers missed their chance to land on a poorly defended Spanish Cuba, mistaking fireflies for Spanish campfires and deciding they would be outnumbered.

In 1887, French pharmacologist Raphael Dubois made a giant leap in figuring out the secrets of bioluminescence.

During one of his experiments, he took tissues from a bioluminescent clam called the common piddock and ground them up. He found that if he put the ground tissues in cold water, they glowed for a few minutes. He’d extracted the animal’s light-producing chemicals. When he put ground tissues in hot water, there was no glow, but adding the hot water to the cold water made the light come back on. He called the hot water extract luciferin (from Lucifer, Latin for “morning star”) and the cold water extract luciferase.

American biologist Edmund Newton Harvey continued on the path that Dubios had forged and spent most of his career looking for luciferin and luciferase in almost every luminous organism he could find. He discovered that luciferins and luciferases from different animals were not interchangeable; he hypothesized this was because bioluminescence and its systems had evolved to fit the various needs of different species.

You Light up My Life: The How’s and Why’s of Bioluminescence in Fireflies
The tag team of the luciferase enzyme and the luciferin molecule is the key to turning on a firefly. To make light, luciferin combines with adenosine triphosphate (ATP), a high-energy molecule that powers cells, to form luciferyl adenylate and pyrophosphate. These compounds bind to the surface of luciferase. Luciferyl adenylate then combines with oxygen to make the molecules oxyluciferin and adenosine monophosphate (AMP). Rapid energy loss from the excited oxyluciferin results in it giving off visible light.

The wavelength of this light is between 510 and 670 nanometers, making it appear like a pale yellow or orange-green color to us. In the area of the body where the light-making reaction happens—called the photic organ or the lantern—there are uric acid crystals that help reflect the light away from the abdomen.

For firefly larvae, bioluminescence is a defense against predators. Most firefly larvae produce chemicals within their bodies that are toxic—or at least taste terrible. Their glow warns predators that they won’t be a pleasant meal, and that trying to eat them isn’t going to do anyone any good.

Biologists think adult fireflies used to also use their glow for defense, but it eventually evolved as a tool for mate selection and communication. At certain times of night when they’re active, male fireflies will begin flashing a light pattern specific to their species. Females of the same species will watch and if a flashing male catches a female's eye, she will respond with the same pattern, on a short time delay. A flash dialogue ensues as the male locates his lady fair and flies to her to begin mating. Female fireflies are known to be fond of certain flash characteristics, like longer flash duration and bigger lanterns, and will preferentially respond to and mate with males who have more attractive glows.

Males looking to mate walk a thin line between sex and death every time they flash their light. Females of the Photuris genus of North American have figured out how to turn amorous males into an easy meal. They’ve developed an ability to replicate the mating flash code used by the Photinus genus. The Photuris females will flash back their hacked code in response to males, and when the poor suckers come looking for some loving, they walk into a dinner date that won’t end well.

Not only do the femme fatales get a meal, but they also pick up an insurance policy against getting eaten themselves. Photinus fireflies have a natural defense against predators in the form of steroidal chemicals called lucibufagins, which Photuris fireflies lack. When a female Photuris cannibalizes a male Photinus, though, the toxins slip into her bloodstream. She’s now got a defense against hungry predators and can even pass the protective chemicals onto offspring.