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I found this larva on Petasites hybridus in northern Germany, lower saxony,
last week.

Which family/genus/ could it be?

Gerrit

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I got more pictures if needed

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Larvae that are this colour usually turn out to be Cecidomyiidae. There are many free-living species that don't form galls. They are either predators or feed on fungi. I suspect that your larva is an aphid feeder.

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In this case I would rather suspect that it feeds on fungi -
do you see the orange dots? I think it is a fungi, and it is spread all over Petasites hybridus, where I found the larva.

Thanks for your help!