Is this a Conopid playin' Polistes? - Physocephala

Maybe not only playing...

but perhaps preying on Polistes, too. Conops (and related genera like Physocephala) are parasitoids of adult Hymenopterans, and local paper wasps could well be a part of the maggots' "menu". Should you often - if not always - find these flies where there are many paper wasps, then such a relationship is virtually sure.
The two black structures beneath the abdomen are used to thrust an egg between the unfortunate host's tergites (this takes a fraction of a second, often in flight). As a result, males lack them: this superb shot is one of a female.

… Richard Vernier, 22 February, 2006 - 8:04am

...

I was wondering about that. It seems that this fly is described as feeding on flowers, but I watched this one for a couple minutes, and it didn't seem to be feeding at all. Perhaps it was waiting for a host to approach.
-Sean McCann

triatoma.blogspot.com

… Sean McCann, 22 February, 2006 - 8:12am

Pollinators

Conopids' long proboscis indicate that they feed on flowers, they prefer Asteraceae, that seems to be the right side for that proboscis. Also they are known to be pollinators of some flowers.

… Beatriz Moisset, 22 February, 2006 - 8:31am

Physocephala

Yes, it is a Conopidae, and because of that lump on the back femur it must be in the genus Physocephala. The color is different from the others, may be somebody can ID it to species.

… Beatriz Moisset, 22 February, 2006 - 7:14am