The answer to the question..
Why? Because that's a question my niece asked me yesterday, which I didn't have the answer to at all! I think I thought that they didn't eat and they just die... but I was wrong, they do eat! Mostly liquid and vegetarian, though there is one which basically eats aphids, and another which can digest pollen. There's lots of other interesting Q&A about butterflies too..
Extended set of Q&A