fireflies a question about them?

From Wikipedia: "Fireflies overwinter (some species for several years) during the larval stage. Some do this by burrowing underground, while others find places on or under the bark of trees. They emerge in the spring."

Wiki does not say what happens to adult fireflies at the end of summer. It might be that they don't outlive the summer, leaving their larvae to emerge as the next generation.

The lives of fireflies go in cycles. Adult fireflies only live about 1 to 3 weeks. During this time they reproduce by laying eggs. The larvae stage of the firefly may last one or two years, depending upon the species.

So, basically it goes something like this: egg, larvae, adult, death. Then a new batch of of eggs repeat the cycle. When the weather is cold the larvae overwinter as the poster above explained.