There are more than 100,000 different species of flies. They are found everywhere in the world--even in Antarctica. They belong to the order of insects called Diptera which means "two wings". Most insects have 4 wings. On the bodies of flies, however, over time, this second set of wings developed into small knobs called halteres that are located behind their main wings. These knobs help keep flies steady and balanced when they fly and make them very agile. They can maneuver themselves into intricate flight patterns, they can hover and spin, and they can even fly backwards.
Source: Utah Education Network