PHFireflies light up to flirt
As Marc Branham, an entomology professor at the University of Florida explained to Scientific American, a firefly's light itself is a result of so-called "You do not have permission to view the full content of this post. Log in or register now.." Oxygen combines with calcium, adenosine triphosphate (ATP), and the chemical luciferin, and light is produced with minimal energy being lost to heat. Fireflies mainly emit light to attract mates (via certain light patterns) or to defend territory (that's why when you put them in a jar, they light up, as a defense mechanism).
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