Fireflies glowed through bioluminescence-a chemical reaction in their bodies resulting in the emission of light.
It was something Justin had learned a long time ago, in the classroom of Jeff Emerson, his next-door neighbor and high school science teacher. For whatever reason, it was one of those things that had stuck in his head. Maybe it was because, at the time, he'd hated Chemistry class with a passion, and the idea of chemicals making a bug's butt turn green was, to him, the coolest thing the subject had to offer.