Why does a caterpillar turn into a butterfly inside its chrysalis?

Answer: A growth program inside it rebuilds its body into butterfly parts.

A switch to a different species gives it wings and a new body. — Wrong. A caterpillar and the butterfly it becomes are the same species at different life stages, not two species swapping places. The adult looks very different, which makes the “new species” idea understandable, but its genes are still the ones it hatched with. The change is a programmed rebuild, like a tadpole becoming a frog.

A growth program inside it rebuilds its body into butterfly parts. ✓ — Correct! Inside the chrysalis, the caterpillar's old body parts break down and its cells reorganize into wings, legs, and other adult organs. A built-in growth program controls this change, and hormones set it going when the caterpillar has stored enough energy. The animal stays the same individual; it has just rebuilt itself for a different way of life.

A diet of leaves slowly builds wings on the outside of its body. — Wrong. Leaves give the caterpillar energy and building materials, so food does matter for growth, but it does not turn into wings from the outside. The wings form from the caterpillar's own cells inside the chrysalis. Metamorphosis is triggered by hormones and a built-in growth program, not by the shape of its food.