These larvae 5-6 mm length have been found in the same 50 ml sample in filamentous algae of a freshwater pond
in Belgium (close to the Netherlands) . I found two larvae and I made the video clip with both organisms, alive.
One of both was full of blood and the other was quieter?
I also found a pupa which I supposed with your help it could be Clinocerinae Wiedemmania (previous post)
I know we cannot link larvae and pupae immediately, but perhaps here they are from the same species