The Brachyceran infraorder Muscomorpha is a large and diverse group of flies, containing the bulk of the Brachycera, and, in fact, most of the known Diptera. It includes a number of the most familiar flies, such as the housefly, the fruit fly and the blowfly. The antennae are short, usually 3-segmented, with a dorsal arista. Their bodies are often highly setose, and the pattern of setae is often taxonomically important.
The larvae of muscomorphs (in the sense the name is used here; see below) have a reduced head capsule, and the pupa is formed inside the exoskeleton of the last larval instar; exit from this puparium is via a circular line of weakness, and this pupal type is called "cyclorrhaphous" - it is this feature which gives this group of flies their traditional name, Cyclorrhapha. No other insects have a life cycle that involves a puparium.
The name Cyclorrhapha is used, in various modern classifications, to represent either a subgroup within the infraorder Muscomorpha, or simply a rankless group within the Brachycera. In either case, the Empidoidea are the sister taxon to the Cyclorrhapha. In the present classification, as the Muscomorpha is used to refer to the sister taxon of the Empidoidea, the names "Muscomorpha" and "Cyclorrhapha" are effectively synonymous (though not entirely interchangeable: for nomenclatural purposes, it is always considered better if the endings of names of similar rank are consistent, and since all the other infraordinal names end in "-morpha", the use of "Cyclorrhapha" as an infraordinal name would be inconsistent).
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License