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thanks Michael for your answer
...and no, this fly was not seen in miles (150+km) from any sand dunes, but on wet grassland next to a creek in the east part of belgium, near german/dutch border..
Delia penicillosa is not listed in the Grootaert-species list for Belgium, as well it isn't on two other observation-sides......or is there a synonym name used in the Lowlands ? ..

but unfortunately I've got no other shots from the abdomen from below...but I will check for certainty tomorrow ..thanks again...Chris