A butterfly's tongue is called a proboscis. It is a very complex system that is used to ingest the nourishment that the butterfly needs to be able to mate and lay eggs.
The proboscis of a butterfly is made up of two parts. Each side has nerves, a trachea and muscles. The two parts fuse together much like a zipper. Once the proboscis is zipped up, it forms a food canal that the butterfly uses to suck up liquids.
Some butterflies shoot water from their rears to help dissolve some solids so that they may be sucked up through the proboscis. Others can regurgitate onto food to be dissolved.