In the book Fireflies, by Judy Brinckloe, a boy is so proud the jar of fireflies that he caught, which to him was like owning a piece of the moon. But he soon realizes he must let them go or they will die.

Fireflies, or lightning bugs, are beetles, not flies or bugs. They are known for their glowing abdomens, which you may be able to see from your window on summer nights. Male fireflies use special flashing patterns to attract the females. The male flashes and the female answers with a different flash. The male then follows the light to her. The messages can help fireflies find mates, but they can also attract predators. To protect themselves many fireflies have toxic chemicals that make them inedible to most predators.

Information from:
Deyrup, Mark. Florida's Fabulous Insects. Tampa Bay Florida: World Publications, 2000.