Through the previous and pragmatic help of Atylotus, I could easily ID this diptera larvae
And better, I can observe this larva alive, grazing and shredding green and filamentuous algae
Thank you Cranefly
Here is the must
Video clip on the latest moments of a larva
just becoming a pupa and fixing itself in a wood piece crack
Another one, older pupa is fixed on a Lemna stem
All these larvae were found in a very green sample: many Stentors polymorphus rotifers (clusters) and the very nice flatworm Dalyellia viridis(https://www.youtube.com/watch?v=Urq1i3goepo)
These Setacera larvae and pupae were very nice to observe