(Extracted and adapted from a previous post at Voyages Around My Camera)
While it is a flower feeder, this thick-headed fly (likely Physocephala furcillata of the Family Conopidae) is a parasite of solitary bees and sometimes wasps. It can attack in mid-air, bringing its victim to the ground. It then forces eggs between the abdominal segments where the parasitic larva will later develop.
(Thanks to Jason Dombroskie at the U of Alberta for the I.D. Taken with a Nikon D80 with the Tamron 90mm macro lens on a Kenko Pro 1.4x tele-converter and a Nikon SB-600 flash with Lumiquest soft box)
28/12/09 . See more on this fly at The Home Bug Gardener