How do fireflies glow?

With a chemical reaction inside their own bodies, and almost no heat at all. A firefly combines a substance called luciferin with oxygen, and nearly all the energy comes out as light rather than warmth, which is why it is called cold light.

Each species flashes its own pattern, like a name written in the air. And the glow is brief: a firefly can spend a year or more underground as a larva before its few weeks of light.