According to the Manual of Nearctic Diptera, there are 2 subgenera in the Nearctic: Homoneura and Tarsohomoneura, with respectively 41 and 8 species. Both are reported as widespread and there is at least one picture in the guide of the latter subgenus:



R.M. Miller published in 1977 reviews on the taxonomy of the genus (which then included genus Mallochomyza as a subgenus). I don't have access to these however, but surely species lists can be found there.

From pictures, species in either subgenera can easily be placed in the right one by looking at the hind tarsi however; in Homoneura they are pale, whereas in Tarsohomoneura they are at least partly blackened.

The postocellar setae are also different. In the former group, they are "equal to or longer than ocellar setae, gradually convergent and with their bases about as close together as posterior ocelli". In the latter subgenus, they are "usually shorter than ocellar setae, strongly convergent, their bases much farther apart than posterior ocelli" (quotes directly from the Manual of Nearctic Diptera).