Many insects that usually are seen flying have long been called 'flies' of various sorts -- dragonflies, mayflies, fireflies, caddisflies, butterflies and so on. But these all belong to different orders of insects, only distantly related to each other. In entomology, none of these are flies proper, insects of the Order Diptera, which are often referred to as 'true flies' or 'two-winged flies' because they never have more than one pair of wings, those on the middle segment of the thorax. Some insects in other orders have lost a pair of wings, sometimes the front pair, sometimes the back one -- but none of these, except some male scale insects, have the hind wings reduced to halter-like structures. We follow convention in writing the English names of Diptera as two words (crane fly, house fly) and those called flies in other orders as a single word (dragonfly, stonefly).
G.G.E. Scudder and R.A. Cannings.
Scudder, Geoffrey G.E., Cannings, Robert A.. 2006. The Diptera families of British Columbia. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2006MR134