How do fireflies light up?

It's one of the quintessential signs of summer in parts of the United States: fireflies twinkling in the night. Fireflies' ability to produce their own light is called bioluminescence, which is found in select Animals, bacteria and fungi all over 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 beetle 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, said Timothy Fallon, a 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 abdomens where these reactions occur, which contain a layer of crystallized uric acid that helps reflect and boost light. 

This system of using luciferin and luciferase has evolved independently several times in bioluminescent animals, including in another group of light-up beetles called Sinopyrophoridae.

Related: Which group of animals has the most species?

Only in the last few hundred years have scientists started to understand how some living things are able to make light. One of the first people to make headway on this was a 17th-century Royal Society member in Oxford who discovered that air was essential for a bioluminescent fungus to glow.