Flies not only carry germs on their bodies. Each fly has a sort of pouch or reservoir which it fills when it eats. After taking a full meal at some filthy rubbish dump, the fly wings its way to some more comfortable spot where it can rest and digest its meal. There the fly transfers this filthy material from the pouch to its own stomach, often leaving parts of the meal behind. This is what causes “fly spots’’ on walls and ceilings, and perhaps even on your own dining table or kitchen sink, if you are unwise enough to allow flies inside your house.