How_do_fireflies_glow_

Fireflies glow because they produce light by a biochemical reaction.
In the lower abdomen of fireflies, there is a chemical called luciferin and an enzyme called luciferase.

When oxygen enters into their lower abdomen through tracheal tubes, it basically reacts with luciferin in the presence of luciferase. This reaction produces light.

This production and emission of light by a living organism is called bio luminescence. Bio luminescence is a cold light. This means the light produced generates very less heat. Thus, preventing the firefly's abdomen from getting overheated. However, one more interesting fact is that not only adult fireflies but also their eggs and larvae are bio luminescent.