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"Most of the Muscomorpha are further subdivided into the Acalyptratae and Calyptratae based on whether or not they have a calypter (a wing flap that extends over the halteres)."

in wikipedia..

it is funny to see then in http://www.ento.csiro.au/biology/fly/fly.html that

Rhagadolyra magnicornis is an Acalyptrate fly and HAS 1 calypter! (one, at least visible). I know in Biology, there are always exceptions. But they could choice a better model to avoid confusion... is Rhagadolyra magnicornis an exception? Or the division Acalyptrate / Calyprata is no sense now...

"The name refers to the lack of calypters in the members of this group of flies." in glossary of diptera... it must be important to note that there are exceptions... and why, if possible.
I cannot get to find any more clues about characters of acalyptrate flies.. please give me more info. Thank you.