Flies breath like all insects through openings in their body called spiracles. These openings lead the oxygen through a complex system of tubes which are called trachea. The trachea run through the whole insect body.
|Air can be pumped in by expanding the body. The amount of inspired oxygen is limited with such system. This may be the answer why insects don't grow much larger than 10 centimetre. In prehistoric times, when there was more oxygen in the atmosphere, insects could grow to much larger sizes. Fossils of dragonflies with a 70 centimetre wingspan were found.
To see what some insects do to breath under water click on the thumbnail.