Upon arriving at the Northern Air Temple, the mechanist found a room filled with natural gas. As he got closer to the room, the torch he was using as a light source caused an explosion. Afterward, the mechanist began using fireflies, a non-flammable light source, in order to see in the dark corridor leading up to the room.
When Team Avatar arrived at the temple, the mechanist showed Sokka the location of the room, using fireflies contained within lanterns as a light source. Sokka noted that they do not provide a great amount of light.
The firefly is characterized by a pair of slender, jointed antennae, forewings, and a narrow prothorax. It has a mechanism of bioluminescence that likely works by emitting a light from the abdomen. The color of this light appears light green or light blue.
The ancient Chinese sometimes captured fireflies for the purpose of lighting. It was also a widespread practice in ancient India. However, these were short-term solutions and the use of other light sources became more prevalent.