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Somatic sex in most cases of flies is determined by two sex chromosomes X and Y, or more precisely, unlike in mammals, for example, by the ratio between autosomal chromosomes and the X chromosomes. In females the ratio 1:1 (that is, 2:2), in males 1:1/2 (2:1) It happens sometimes that one of the Xs in females is lost during somatik cell divisions, and, if that happens early during embrionic development, the resulting fly is a mozaic of male and female parts of the body. This is the likely explanation for the Tachinid fly mentioned by Theo above.

In Drosophila, and most likely in other flies as well, there are a handful of genes responding to the autosomal - X ratio. and take part in the determination of somatic sex. When inappropriately expressed (e.g. mutant in both copies), they porudce true intersexes with characters intermediate between the two sexes. I think this latter possibility may apply to the fly in the photos

Henrik (Gyurkovics)
once a Drosophila geneticist :D