Lightning bugs or fireflies are not flies, but they are beetles that can produce lights. Lightning bugs can light up during all stages of their lives, including larvae and pupae. In some species of lightning bugs, the eggs glow as well!
Percabeth Fanart by Burdge bug This sketch. Burdge must be a wizard because this is pure magic. I have no idea how she manages to convey such deep emotion in seemingly simple sketches like these but I hope that she never stops