Thunder bugs were large, docile insects who inhabited the forests during the New Age, most notably within the Darklands. They had bioluminescent properties and were capable of giving off a self-sustaining light source, usually yellow or orange or white in color. A thunder bug's abdomen remained illuminated for minutes at a time, and was sufficient to illuminate a small room. They were known to bite off fingers if someone tried to pet them.
The butterfly people of the forest provided for the thunder bugs, encouraging them to remain in the forest with their favorite foods, in the hopes that they would remain and provide a light source after nightfall. They seemed particularly fond of starflower nectar and sourfruits.
Thunder bugs were a necessary invention for the world of the Butterfly Princess, whose people do not use traditional souces of light such as lanterns, torches, etc. Thunder bugs are essentially gigantic domesticated lightning bugs.