These flies are very common on vegetation around my house right now, and this appears to be
Neogriphoneura sordida. Comments on idntification:
Neogriphoneura is a monotypic genus in North America (at least) and I believe I've been able to rule out similar flies in that family. Refer to the
first image of the species (same location) identified by an expert on BugGuide. Compare, also, the careful
series by BugGuide contributor Scott Justis.