Some kind of fly, I think

No idea what this is. Am puzzled by the light-colored sacs in its armpits (wingpits?). The flower itself was only about 3/4 inch across. (I should note that the EXIF data say it was taken with a 135mm lens. It wasn't. It was taken with a 105mm AI-s Micro-Nikkor with a TC-14a teleconverter, for 147mm or thereabouts.) Update: Roy Lowry, the great macro photographer who also exhibits pictures on Ipernity, let me know that the sacs are used to stabilize the fly as it flies. A little research revealed that they are called "halteres."