Other Common Names
The common names of true flies (order Diptera) are written as two words: crane fly, robber fly, bee fly, moth fly, fruit fly, etc.
The common names of non-dipteran insects that have "fly" in their name are written as one word: butterfly, stonefly, dragonfly, scorpionfly, sawfly, caddisfly, whitefly, etc.
Explanation of Names
Diptera means "two wings".
Numbers
188 families worldwide
153,000+ described species worldwide
Identification
Adult flies, except for wingless species, all have two functional wings and two halteres. The halteres are club-like appendages that are essentially the modified hind wings. The only other adult insects that only have two wings in both sexes are the
Strepsiptera, which have the front wings reduced rather than the hind wings. Males of some species of Mayflies and scale insects have only front wings. A few tiny parasitic wasps, e.g. Mymarommatidae, have their hind wings reduced, but these can be distinguished from flies as the wasps have only one vein in their front wings and flies always have two or more veins in their wings as long as their wings are membranous.
Remarks
The current classification of this order in Bugguide uses several paraphyletic groups (groups which do not contain all of the descendants from a single ancestor, and thus are not natural clades). These groups are not used as formal groups in modern classfications, but are convenient for sorting out similar groups of flies. The classification followed by Bugguide is as follows:
Nematocera - A paraphyletic grouping of the most primitive flies. Have more antennal segments than the Brachycera.
Brachycera - A group of flies sharing a reduction in the number of antennal segments.
Orthorrhapha - A paraphyletic subdivision of the Brachycera, including those flies that don't have the circular pupal aperture of the Cyclorrhapha.
Cyclorrhapha - A monophyletic subdivision of the Brachycera. These flies have a shared trait of a circular aperture where the adult flies emerge from the pupal case.
Aschiza - A subdivision of the Cyclorrhapha that is probably paraphyletic. These flies lack the ptinal suture characterizing the Schizophora.
Schizophora - A monophyletic subdivision of the Cyclorrhapha, these flies have a suture on the front of the head where a balloon-like structure, the ptilinum, is inflated to open up the pupal case when the adult emerges.
Calyptratae - A subdivision of the Schizophora that is probably monophyletic; have several shared morphological characteristics including the prominent lower calypter on the wing.
Acalyptratae - A subdivision of the Schizophora, probably paraphyletic. Lack the morphological features that define the Calyptratae.
