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Very interesting. I saw several similar larvae being carried along with a moving mass of Sciaridae in Durham, North Carolina (USA), 17 June 2025. An expert correspondent identified them as something in the Schizophora. I speculate this is an actual symbiotic (or predatory?) associate, as there were several of these larvae in the moving mass, and they made no effort to move out of it. iNaturalist observation:
https://www.inatu.../291125656
I retained several specimens in 70% ethanol and will work on some more detailed images. Hints about what structures to look at would be helpful.
In the not-too-distant future I hope to isolate DNA for barcoding from these and the Sciarids.
Edit. Attaching image (600 pixel width). Please see iNaturalist link for full size images.