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Hi Nikita,
Thanks for looking at the photos. When I try to ID something as a muscid, it could very well be a grasshopper :-) Here's what Snyder's paper says about M. flavicornis:


Mydatea flavicornis COQUILLETT, 1902, Proc. U. S. Natl. Mus., vol. 25, p. 122; STEIN,1920, Arch. Naturgesch., sect.A, vol.84, p.26; MALLOCH, 1921,Canadian Ent., vol.51, p.10; 1923, ibid.,vol.55, p.220.

MALE: Length 5mm. Very similar to impedita Stein, differing from it in having the humeri entirely darkened. The scutellum with a row of distinct setulae along the ventral margin on the basal half.

Midtibiae with only two median posterior bristles. Hind tibiae with two anterodorsal and two or three anteroventral bristles.

Abdomen with distinct dorsocentral vitta, and the fifth sternite with only two or three bristles on the disc of the processes.

FEMALE: Length 6 to 7mm. Similar to the male, differing from it in having the front one-fourth of head width at vertex and of almost uniform width throughout. The humeri distinctly yellowish as in impedita Stein.

SPECIMENS EXAMINED: Five males and four females from Missouri, Wisconsin, Indiana, Pennsylvania, New York, and the holotype male in the United States National Museum from Quebec."

He uses the ventral scutellar bristles as the distinguishing character between M. flavicornis and M. impedita in couplet 4 ("Lateral ventral margins of scutellum with some black bristles" vs. "Lateral ventral margins of scutellum bare"). Whether that's actually correct, I'll leave to you :-)

Regards
Steve