How do fireflies make their own light?

Under a firefly’s abdomen sits a light organ. Inside it, a substance called luciferin meets an enzyme called luciferase, plus oxygen, and light appears. Remember those names once; after that, “light-making substance and enzyme” is enough.

National Geographic explains that fireflies combine oxygen with that substance in special cells to make light with almost no heat. Scientific American also notes that oxygen, calcium, ATP, and more take part. In a simple picture, when the ingredients mix, energy is released as light. Unlike a bulb, it does not get hot, so a small body can flash safely.

A bulb gives off lots of heat with its light. Firefly light turns nearly all of the energy into light—“cold light,” or bioluminescence. Smithsonian Ocean also summarizes that light appears when the light-making substance reacts with oxygen. Some sea creatures use similar bioluminescence, but fireflies finely control the reaction in their abdominal light organ.

So “a fire burning in the belly” is wrong. Nothing scorches or heats up like a flame; a body chemistry reaction becomes light. How efficient it is varies by source—some say around the 90 percent range.

The light is usually a mating signal. Males send a flash pattern unique to their species, and females answer in kind. Interval and length differ by species, which helps many kinds share one night without mix-ups—like each species using its own Morse code.

When larvae glow, many accounts treat it as a “I taste bad” warning to predators. National Geographic notes adult flashes may serve both courtship and distaste warnings. Details of how flashes turn on and off still differ across studies.