I think this is the same bee I saw on tarweed on 8/17/2016. At that time, Pierre Martineau wrote: "I think you'd like to know that the "iridescent fly" (images 3-7...) is actually a bee; I believe it is a species of Ceratina, a genus of (cute) small carpenter bees." I should have known: no halteres. To which Pierre adds: "Bravo Dan for the "no halteres" comment! makes me feel quite proud of my student :-) Also, carefully looking at your pictures at the highest resolution might reveal one on which you would be able to tell (I was not) that there are in fact 2 wings folded on top of each other; this would rule out a fly.

"Another striking (yet more difficult to formalize precisely) bee character on your pictures are the 'elbowed', ant-like antennae. AFAIK, no fly has such antennal structure" Though I can't definitively distinguish two sets of wings, the bent antennae are obvious here. Trail 15, 11:04:45 am.