An ichneumon is a type of bug that lived long, long ago during medieval times. It was a very strange bug because it would lay its eggs inside other bugs, like caterpillars.
When the ichneumon lays its eggs inside the caterpillar, it doesn't hurt the caterpillar at all. But as the eggs hatch, the baby ichneumon bugs will eat the caterpillar from the inside out! Yuck!
Even though it sounds really mean, the ichneumon is actually helping the ecosystem. It helps keep the population of caterpillars from getting too big and helps make sure that other bugs can have enough food to eat.
Even though we don't see ichneumons very often anymore, they played an important role in medieval zoology and helped scientists understand more about how bugs interact with each other.