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Adrian Pont's (world specialist on Muscidae) note on Pegomyia bicolor emphasises Paul's point specialist may look at completely different characters then a 'general dipterist'.I did not know that "hind tibia. Two anterodorsals and two posterodorsals do not occur in combination like that in muscids" now we all do which shows how useful :D:D, Diptera.inf is. Adrian's second point "and this looks for all the world like a Pegomya" illustrates another point - some flies have a "jizz" ie. they are recognizable by characters not easily defined in the same way that a familiar human face can be recognized from a partial photo.A reference collection goes a long way to help since as Paul says using keys other than on specimens is very difficult. I endlessly repeat that systematic entomology is a very visual science and photos are assuming a very important role. Even if your fly cannot be identified it may illustrate some feature that will prove of great illustrative value in the morphological part of the glossary Am I right Paul?