No matter what their eventual size, all butterflies start out as tiny eggs. Female butterflies deposit their eggs on or near specific plants called host plants. In 1-2 weeks the eggs hatch to release tiny caterpillars which immediately begin feeding on the host plant.
Caterpillars have very specific tastes.
In fact, the larvae of each butterfly species feed only on certain, very specific, plant species. Butterfly larvae are highly efficient eating machines. It is their sole job to eat and grow, and they do both at astonishing rates. Unlike humans, butterfly larvae do not have internal skeletons. Instead, they wear their skeletons on the outside of their bodies.