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I have no experience with larvae of the "rat-tailed maggot" type, but they belong to several genera (Eristalis, Eristalinus, Anasimyia, Helophilus, Lejops, Parhelophilus)... In http://eco-eclect...ae-in.html it is stated that Helophilus larvae have an almost black tip to their posterior breathing tube. That would fit your picture! However, the tails seem a bit longer. The cited book by Rotheray (1993) should be informative!