How do fireflies light up?
(Q&A by Erica Dean)

First of all, there are special cells on a firefly's
abdomen. These cells contain a chemical called
luciferin and produce an enzyme called luciferase. To
produce light a couple things must happen.
First, the special cells create thousands of
luciferase enzymes. As this happens, luciferin bonds
with adenosine triphosphate (ATP) found inside the
cell. This produces luciferyl adenylate and
pyrophosphate (PPi):

luciferin + ATP --> luciferyl adenylate + PPi

Then, the luciferyl adenylate combines with oxygen
brought in through a tube in the abdomen called the
abdominal trachea. When this happens, oxyluciferin
and adenosine monophosphate (AMP) are produced.

luciferyl adenylate + oxygen --> oxyluciferin + AMP + light

The reaction that takes place to produce the
oxyluciferin and the adenosine monophosphate releases
quite a bit of energy, exciting the atoms involved.
This excitement creates the light that we see through
the thin layers of the firefly's abdomen.
The light that is produced is between 510 and 670
namometers giving it a yellow-green colour. This
light is used by the fireflies for mating and
confusing their predators.

Source: http://launch.yahoo.com/u2