Flies are the fourth largest order of insects overall, with literally tens of thousands of different species inhabiting nearly every livable inch of real estate on the planet. The very size and diversity of the order renders most generalizations about their behavior meaningless, so we will deal here only with those fly species that qualify as pests.

As a rule, nuisance flies can be classified as either scavengers, parasites or both. The scavenger species feed on decayed or decomposing matter of all types, including animal corpses, excrement, garbage scraps, raw sewage and rotting fruits and vegetables, while the parasitic species consume the blood of just about any warm-blooded animal, including humans. Unselective dining habits have earned the insect a widespread reputation for spreading germs and disease. Let's take a closer look at a few of the major culprits, starting with the housefly (also known as the common fly).