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Wait for Valery, he will surely help you, though males of this group are difficult. In the meantime use the "Forum search" button to get information about the characters of the species under concern - you will find a lot, e.g., that Chaetostomella has no presutural dorsocentral bristles. The most detailed discussions you find in the old threads, so sort in "ascending" order.
And why shouldn't the flies feed on Tanacetum nectar? The host plant of the larvae is not necessarily also the only food source for the adults. I saw them on various Asteraceae flowerheads.
Regards, Sundew