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Xylomyidae, and most likely Solva marginata. The larva looks a bit like a Stratiomyidae, but has a different cuticular pattern, a differently shaped analsegment, rows of tooth like tubercles at the anterior part of some abdominal segments, but most obviously a shiny bright yellow pro- and mesothorax. Another typical feature (not seen on the photo) is the toothed anus. I guess this must be S.marginata, for the other species of Xylomyidae (Solva and Xylomya) do not occur in Belgium (www.faunaeur.org), but you have to ask that a specialist on this matter. See also the breeding effort of a Czech specimen by Miroslav (http://www.dipter...d_id=34921).