It’s one of the typical signs of summer in some parts of the United States: fireflies that sparkle in the night. The ability of fireflies to produce their own light is called bioluminescence, which occurs in select animals, bacteria and fungi around the world. Most of these creatures live in caves or oceans. But a handful live where humans can see them, including the more than 2,000 species of beetles that make up the firefly family.
So we know what the effect is called. But how do fireflies (family Lampyridae), also called fireflies, create these dazzling displays?
The key to their light is a chemical reaction based on a compound called luciferin Timothy Fallona biochemical geneticist at the University of California, San Diego.
Luciferin makes light by losing electrons—a process called oxidation—in the presence of adenosine triphosphate (ATP), a molecule that provides energy for cells, and magnesium. This reaction is mediated by the enzyme luciferase. Fireflies have light organs in their abdomen where these reactions occur which contain a layer of crystallized uric acid which helps to reflect and amplify light.
This luciferin and luciferase utilization system has evolved independently several times in bioluminescent animals, including another group of luminous beetles called Sinopyrophoridae.
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Only in the last few hundred years have scientists begun to understand how some living things can create light. One of the first people to make progress in this regard was a 17th-century Fellow of the Royal Society in Oxford who discovered that air was essential for a bioluminescent sponge to glow.
Indeed, oxygen is one of the main ingredients for the bioluminescence of fireflies.
Glow starts early for fireflies. The pupae—and even the eggs—are able to glow, possibly as a signal to predators that they will not feed well, as some are poisonous due to chemicals called lucibufagins, which they synthesize from their diet. When fireflies complete metamorphosis and reach adulthood, they make new light organs. But the overall system is the same, with the light coming from inside special cells found in light organs on the underside of some fireflies, turning them yellow, orange, green or even blue.
These cells are full of luciferin and luciferase, as well as an unusually large number of mitochondria. These tiny organelles pump the ATP the fireflies need to trigger the chemical reaction. Fireflies turn their light on and off by the flow of oxygen to these cells. Lack of oxygen means darkness. Too much oxygen? You have a glow.
This on-off switch is important for US East Coast fireflies, which use flashing signals as adults to find the right breeding partner. Finding mates as a beetle can be difficult, and each flashing species has evolved its own light sequence to distinguish itself from the others. That makes bioluminescence “a Morse code love song to light,” he said Lynn Faust, an independent firefly researcher based in Knoxville, Tennessee. “With their extremely short adult lives, it’s a race against time” to find a mate.
Researchers have come a long way in understanding the science behind firefly bioluminescence. But countless species of fireflies remain undocumented in Asia and Africa, and researchers are still struggling to understand how the beetles first evolved their specialty of glowing in the dark 130 million to 140 million years ago, according to research published in BioRxiv preprint database.
“The biggest unanswered questions are, for example, what are all the genes that are involved in bioluminescence?” Fallon said.
A big breakthrough came in 1985 when researchers discovered the gene responsible for the production of luciferase. This enzyme is now used in biomedical research to artificially illuminate specific proteins in plants and animals. In 2024, researchers in Wuhan, China discovered two more genes in Aquatica leiia rare aquatic firefly, which they suspect may help position the lantern — the adult firefly’s glowing organ — in the abdomen and activate light-generating genes such as luciferase.
In addition to deciphering fireflies’ bioluminescence, scientists are still learning what makes fireflies tick in the wild. In recent years, fireflies have declined in numbers due to overpopulation, habitat loss and climate change. “We know so little” about fireflies in their natural habitat, Faust said. “How on earth can we preserve and protect [them] if we barely understand their most basic requirements?”