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On the one hand, there are several photos of such "striped" Scenopinidae flies in the internet (e.g. https://diptera.i...pid=168512) without a species name. On the other hand, we have only three members of the family in Germany, the Scenopinus species S. fenestralis, S. glabrifrons, and S. niger. Only S. niger has black legs. So it is this species! The white "stripes" seem to be the membranes between the tergites that become visible if the fly expands its abdomen. In contracted state, the whole abdomen looks black as usual.
Regards, Sundew