It’s one of the quintessential signs of summer in parts of the United States: fireflies twinkling in the night. The ability of fireflies to produce their own light is called bioluminescence and is found in certain animals, bacteria and fungi around the world. Most of these creatures live in caves or oceans. But a handful live where people 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 lightning bugs, 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 to cells, and magnesium. This reaction is mediated by the enzyme luciferase. Fireflies have light organs in their abdomen where these reactions occur, which are a layer of crystallized uric acid that helps reflect and amplify the light.
This system of using luciferin and luciferase has evolved independently several times in bioluminescent animals, including in another group of glowing beetles called Sinopyrophoridae.
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Only in the last few hundred years have scientists begun to understand how some living things can make light. One of the first people to make progress in this was a 17th century member of the Royal Society in Oxford, who discovered that air was essential for a bioluminescent fungus to glow.
Indeed, oxygen is one of the key ingredients for firefly bioluminescence.
The glowing starts early for fireflies. Pupae – and even eggs – can produce light, possibly signaling to predators that they will not eat well, as some are poisonous due to their chemicals called lucibufagins, which they synthesize from their diet. When fireflies finish metamorphosing and reach adulthood, they make new light organs. But the overall system is the same, with light coming from special cells found in the light organs on the underside of some fireflies, allowing them yellow, orange, green or even blue.
These cells are packed with luciferin and luciferase, as well as an unusually high number of mitochondria. These tiny organelles pump out the ATP fireflies needed to initiate the chemical reaction. Fireflies turn their lights on and off due to the flow of oxygen to these cells. No oxygen means darkness. Lots of oxygen? You have a glow.
This on and off switch is important for fireflies on the American East Coast, which as adults use flashing signals to find the right breeding partner. Finding mates as a beetle can be difficult and each flashy species has developed its own light sequence to distinguish itself from the others. That makes bioluminescence a ‘Morse code love song in light’, the researchers say 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 numerous firefly species remain undocumented in Asia and Africa, and researchers are still struggling to understand how beetles first developed their glow-in-the-dark specialty 130 to 140 million years ago, according to research published on the preprint database BioRxiv.
“The biggest unanswered questions are: What are all the genes involved in bioluminescence?” Fallon said.
A major breakthrough came in 1985 when researchers discovered the gene responsible for making luciferase. This enzyme is now used in biomedical research to artificially light up specific proteins in plants and animals. In 2024, researchers in Wuhan, China discovered discovered two more genes in a Aquatica slatea rare water firefly, which they suspect may help position the lantern – the glowing organ of the adult firefly – in the abdomen and enable 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, firefly numbers have declined due to light population levels, 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 nature? [them] when we barely understand their most basic requirements?”