The glow of a firefly's butt

In the butts of fireflies there is an enzyme (big macromolecule like a protein) called Luciferase. When luciferase is exposed to the common energy currency among life forms, ATP (Adenosine Triphosphate) it glows. The more ATP the brighter the glow. A PhD student in our lab uses this firefly butt enzyme to measure the concentration of ATP in a groundwater samples. Why would she do that? She does it to measure the activity level of bacteria in the water. The more ATP, the more active they are. It strickes me as amazing how useful a bugs butt can be. Who knows what other insect appendages will become useful next?