Glow-worms

An insectoid life-form also found on Roundworld, this beetle generates a low luminescent light.

Caged glow-worms are used by Dwarfs to provide a suitable level of light in their deeper mines. When Samuel Vimes was under arrest in the halls of the Low King for lèse-majesté, a cage of them was put in his cell as a courtesy detail for a human not used to absolute darkness.