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Thank you, Jorge, that's very interesting! Euphranta connexa is said to predate seeds of Vincetoxicum (Apocynaceae-Asclepioideae), and the fly was runnning on the plants of our Asclepioideae beds! When I photographed it, it was on Asclepias syriaca, but the neighbouring Vincetoxicum rossicum already set fruit - perhaps there is a real interaction between fly and plants! I wonder whether Terellia has similar affinities, running on the same plants, and hope I'll get a species name for it (may be, by Valery).
Cheers, Sundew