Which part of the firefly lights up in the dark?

The part of the firefly that lights up in the dark is located in its abdomen. This bioluminescence is produced by a chemical reaction involving the enzyme luciferase, the substrate luciferin, oxygen, and adenosine triphosphate (ATP). Fireflies use this light primarily for mating signals and communication. The glow can vary in color, typically ranging from yellow to green or blue.