Today I am mostly wondering

I shall start at the beginning with what began me wondering.

A friend of mine said that he had saved a fly's life the other day, how noble is that?! Most people wouldn't give a ****, well to be fair I'm not sure he did either. Anyway he saved this fly's life by accident really. It was lying on the windowsill, looking dead, he gave it a flick and it came back alive (briefly). So I suggested that he might have restarted it's little heart. He laughed.... there is a related story for part of the reason behind the laughter, but I'm not going into that...anyways I asked whether he was laughing because flies don't have hearts. He laughed again, but really he didn't know anymore than I did. So do they? My friend said that he thought all living creatures have hearts, but then another friend (note: I'm not clever enough to have realised this) pointed out that worms stay alive when they are chopped in half and so they must have at least 2 hearts or none at all. We presumed none at all. But that still left the question, do flies have hearts? Is there some kind of formula for knowing what creatures do have hearts? How little are fly hearts? Are they the same as human hearts only wee?