Moved from Conopinae.

The hyaline discal and costal cells; dark scutellum; and small, barely-visible dusted-white humeral spots...inset from the margin of the lateral edge of the thorax; together with the uniformly black cheek visible in your 3rd image...all cinch the species ID here as P. furcillata, rather than P. marginata or P. tibialis.

In fact, the range given for P. tibialis ends slightly south of Maine (in Massachusetts), while P. marginata is recorded from Florida to New Hampshire...though I guess it might sneak into Maine. But P. furcillata is definitely found in Maine...it's the most northerly (and easterly) nearctic Physocephala species, reaching to at least Nova Scotia (maybe beyond...I'd guess not many entomologist oventure very far beyond there often enough to know for sure).

Also visible in your third image is a bulge under the 4th sternite (though it's actually attached to te 5th sternite, but folded forward to rest below the 4th sternite). That's the female's "theca", used to grasp the unfortunate bee host for the Physocephala female's off-spring in mid-air flight, so she can insert an egg between the bee's tergites.

Moved from Thick-headed Flies.

Moved from Flies.