A firefly controls the beginning and end of the chemical reaction, and thus the start and stop of its light emission, by adding oxygen to the other chemicals needed to produce light. This happens in the insect's light organ. When oxygen is available, the light organ lights up, and when it is not available, the light goes out. read more
Luciferin and oxygen react, an nzyme called luciferase catalyses the reaction. The details are well covered in Wikipedia. read more
The compound is called luciferin. As air rushes into a firefly’s abdomen, it reacts with the luciferin, and a chemical reaction gives off the firefly’s familiar glow. This light is sometimes called “cold light” because it generates so little heat. read more