Pupae are the inactive stage in the metamorphosis of many insects, such as butterflies and moths. During this stage, the insect undergoes a complete transformation, changing from a larva (caterpillar) to an adult. They don't eat because they are focused on growing and changing their body structure.
Instead of eating, pupae rely on the energy stores they built up while they were a larva. This is why it's important for caterpillars to eat a lot!