If dragonflies are the insects of Japan's day, then the mysterious, magical fireflies are its bugs of the night.

About now, firefly grubs will be emerging from rivers around the country. They've been living in the mud in larval form for the last year, and are ready to start the final, crucial part of their lives. They will pupate, and in June the chrysalis will split and the adult firefly — actually a beetle, because like all beetles they have a hard wing case while flies don't — will emerge.

Fireflies have an unmistakable and quite beautiful feature: They generate their own light source through a process known as bioluminescence.