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The halters are in fact modfied hind wings and as far as we know play a role during flight, but as you can imagine not for lift. :) It mainly has to do with balance. There are mutants in Drosophila where the halters truely develop a wings.

It will not be possible to identify you gnat to genus from this picture alone. There are several truely small details that determine which genus you're dealing with, e.g., the number of palpal segments, the presence/absence of a comb of setae on the tip of the fore tibia and the placement of small setae in that area.

If you can provide details, well, we may have a chance. ;)