Here is a little information about the stages that a butterfly goes through to be a butterfly.
A female butterfly will mate, then lay her eggs, after the eggs hatch they become caterpillars. As the caterpillar grows it will start shedding its skins, more like what a snake does. When they have shed their last skin they inter into a cocoon which is a protective shell and that is then called the pupa stage. Inside the pupa case, the caterpillar will then start transforming into a butterfly; it make take a couple of weeks or longer before it is ready to break out of its shell. The butterflies below are called Monarch.