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I found this old thread http://www.diptera.info/forum/viewthread.php?thread_id=33392 in which some evidence that F cuprea consistently has a few dark hairs at the posterior end of femur 2 , while ruficornis doesn't. The latter is described as having a duller and darker abdomen. On both counts I think that makes my flies F. cuprea but would still welcome confirmation and any other tips on separating the two.