A housefly's mouth, called a proboscis, is like a ridged sponge. When it eats it regurgitates a bit of stomach fluid, and part of its' last meal, on its food (a nice thought, especially if the last meal was a cow patty or a dead opossum!!!). Enzymes in the stomach fluid help liquify the new meal which will also be lapped up by the sponge-like mouth. Note the hairs on the fly's body, these hairs make flies very good at spreading germs. The ants in this photo were drinking oozing tree sap.
I was very anxious for a warm day to go out and find insects to photograph with my new macro lens, I really was desperate if I had to settle for ants and a fly!