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Hoi Benoit,
Yes, your guess is right, they are all Chloromyia formosa.
This species is usually quite commen in the summer.
Characteristics: no thornes on the scutellum, cell in the middele of the wing with 3 veins coming from it, the fourth is not coming out of this cell but before, eyes hairy.
This means it is a Chloromyia, and it is C. formosa because first tarsal segments of leg 2 and 3 are brownish, wing is light brownish and the hairs around the ocelli from this females are light couloured!