There are 11,000 described species worldwide and about 2,100 described species in America north of Mexico; estimates are that there are more than 20,000 total species were they all to be described. This family of flies are predaceous on other insects and occupy a wide range of habitats. They are considered beneficial for controlling many of the pest insects.
This fly receives its name from its aerial swarms, which make the fly seem to be dancing on air.
North American Dipterists Society has lots of good info
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