True flies belong to the insect order Diptera. This means they all have two functioning wings. Sometimes the other pair of wings survive as vestigial 'balancers'.
While flies are often seen as dirty and disease spreaders, flies actually occupy a large number of niches. Some flies are carnivores and others are pollinators (a very important task in NZ with a low diversity of bees). And of course, some flies are nature's recyclers, and consume dead and decaying plant and animal matter.