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I've applied the key to my two species and here are the results:

festiva + hind tibia with 7 bristles. The two small ones close together at the top are hairs. The 2nd and 4th bristles from the top are very short but under the microscope are clearly bristles not hairs. There's a useful image of histrio in the gallery showing the extra bristles. E. festiva large, 10mm. Male?

hilaris + feathery arista. Fly 8mm. Female?

I hope these check out ok and add to the observation overall. Are males bigger than females or is festiva simply bigger than hilaris? E. hilaris seen investigating holes used last year by large Crabronid wasps including Ectemnius.

(now any chance of a key for Hydrophoria?) ;) (I expect that's actually quite lengthy)

many thanks,

Jeremy