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Well your confusion is understandable since that's a female from a gnoristine genus erected after publication of the Vol. 1 of MND, Acomopterella Zaitzev, 1989 (New data on taxonomy of Diptera, Mycetophalidae [sic!] of North America. - Zoologicheskii zhurnal 68(12):134-137, in Russian.). The type species, A. arnaudi Zaitzev (synonym of A. fallax (Sherman, 1921), described from males collected in Alaska and Canada (NWT); Sherman's material was from BC (Savary I., Vancouver). Zaitzev compares Acomopterella with Dziedzickia (with much shorter and more oblique r-m), Acomoptera and exotic genera of Austrosynapha and Schnusea. Zaitzev expressly mentions uniformly dark brown abdomen for his males, Sherman says the dark brown color 'usually more dilute' in females. Abdominal segments of your female appear brown with yellow posteriorly; this fits better A. fasciata Taber, 2015 from Michigan (a single male), in my opinion rather doubtful species, probably identical with A. fallax.