Brachyopa

These probably cannot be identified without capture and detailed examination.
As I prefer a living mystery to a dead certainty, these photos are therefore, "for illustration purposes only".
In those species whose Antennal third segments should have pits of distinctive shape,
1 or 2 of the photos below appear to have enough resolution to show these, but they aren't exactly obvious.
All have an undusted scutellum except for Specimens #6, #7 and #8. In these 3, the amount of dusting looks
intermediate between the "fringe" of B. insensilis and the "half" of B. bicolor quoted in keys.

Brachyopa #1:

Male, by Beech tree:

Brachyopa #2:

Male, by Beech tree:

Brachyopa #3:

Male, on Beech tree:

Brachyopa #4:

Male, on Beech tree:

Brachyopa #5:

Male, by Ash tree:

Brachyopa #6:

Male, on Beech tree:

Brachyopa #7:

Male, on Horse Chestnut tree:

Brachyopa #8:

Male, on Horse Chestnut tree: