Well, if you look at the picture below carefully, you will notice that there is like a rolled up ribbon -like thing where the mouth should be. That thing, is called a 'proboscis', which is a turbular sucking organ. When the butterfly has found nectar, it extends its proboscis like a party blow-out.
A Monarch Butterfly
A butterfly's proboscis is like a party blow-out.
A butterfly's proboscis up close.
