Northern Germany, Hamburg-Flottbek, Botanic garden, in a small artificially produced rivulet, 19th of August 2018, Outsidephotos: kuv.
... Please help to get more ID.
Kuv
I'm pretty sure that it is Odontomyia but with that small size I couldn't tell. Maybe juvenile O. ornata or any other Odontomyia. But my best guess is still ornata but full grown larvae of that species are about 30mm