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If you take the alternative with excavation of T1+2 not extending to posterior margin you end up at a group of species with wider frons. As I interpret the keys it can't be Bithia immaculata because the abdomen is not immaculate. I can not distinguish demotica, modesta, and proletaria. Cerretti's key says distal half of scutellum yellow for modesta and scutellum almost entirely yellow for proletaria. Your fly might fit either alternative.