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Hi Gerrit, looking at this photo and your others on iNaturalist I do not think this is a fly larva. In this photo as well as the one on iNaturalist here -- https://www.inatu.../161853074, you can see dark knobs on the thoracic segments that would appear to be leg stubs. Also on both photos I think I can partly make out a black head capsule although the head is never in full view. And, those almost look like ampullae (essentially, dorsal prolegs) on the back of the larva. Taking all those things together, I would guess beetle larva.