A story told, staged, or written within the work itself, which mirrors or comments on the main story.
In Demons
But mark, mark, madam, the cockroach does not complain!
The fable of the cockroach, recited in the middle of the scene, is a story within the story. The captain uses it in place of an answer, and the image of the cockroach thrown out says what he refuses to say in prose.