I have been chasing butterflies lately and have spent a lot of time by common milkweed, which brings in all kinds of butterflies. It brings in other pollinators as well–bees and wasps, primarily.
On three different occasions I have run across a small fly that looks quite a bit like a wasp.
This is a thick-headed fly, Physocephala tibialis. I think this is a female. The abdomen is shaped a little like a clamp–compare it to one of those old-fashioned can openers that have the pointed hook. And it works a little like that, also.
The female attaches on to a bee (the suspected host is a bumblebee, Bombus bimaculatus), and deposits an egg under the integument of the exoskeleton. The maggot then lives as an external parasite on the host, gradually eating away at it from the outside. Apparently it does not kill the host.
But as with lots of other insects, the complete details of the life cycle have yet to be figured out.
These flies are supposedly relatively common, but I do not recall seeing one before this year.