The firefly also commonly known as “Lightning Bugs” produce a light within their bodies! How do they do this, you ask? Well it is through a process called bioluminescence . Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bioluminescent bacteria. Each blip of light is produced by a chemical reaction between a special protein found in the fireflies abdomen called luciferase, a pigment called luciferin, and oxygen. Once the chemical reaction begins, the result is a bright little light.
So why do fireflies light up their little abdomens? Male fireflies cruising for mates flash a species-specific pattern to announce their availability to receptive females. An interested female will reply, helping the male locate her where she’s perched, often on low vegetation.
Sources:
[1] http://wonderopolis.org/wonder/how-do-fireflies-glow/
[2] http://animals.howstuffworks.com/insects/question554.htm
[3] http://insects.about.com/od/beetles/a/10-Cool-Facts-About-Fireflies.htm