I just saw a firefly outside.
Apparently the taillight contains two rare chemicals, luciferin and luciferase.
update: 2011Mar28: Oh noes, a broken link!! Here’s a shittier article about fireflies from the same domain as the initial good link. It has no picture, no pretty url and nothing about luciferin or luciferase. If this one ever breaks then I’ll just link to the Wikipedia article.