It sounds a bit incoherent, but that was certainly my first reaction when this cloud of flying things dive-bombed my head as I was out hiking a couple of days ago.

At first I thought they were some sort of predatory flies, dropping in to make me dinner.
But then some of them got around my face, and inside my glasses, and when I could see them, I realized "Oh <bleep>, those aren't flies, they're wasps!"
It was then that I decided to run for safety, or at the very least, far enough down the trail that the swarm could declare victory and go back to doing whatever it is that little wasps do in this part of the country.
But that didn't work. Most of the <bleeping> swarm stayed right with me. On my arms, on my back, on my hat, even in my mouth -- I killed two by drowning! "Aiiiiieeeeee!!!!"
I catch on a little slow sometimes.
About 50 yards down the trail, it gradually dawned on me that I wasn't getting stung. I wasn't even getting bitten.
In fact, not much bad at all seemed to be happening, except for the sheer annoyance of having apparently hundreds of these things buzzing around my head. "What's going on here?"
For posterity's sake, I whipped out the camera and snapped a couple of shots over my own shoulder. (Sorry for the focus -- apparently I forgot to push the "macro" button.)
Then I hauled out an empty water bottle and collected a few of the critters for later examination.
Ants, they were. Flying ants.
All I can guess is that these critters stay together by swarming around distinctive colors or odors, and for whatever reason, I was the highlight of their day. You can see in the picture that they seem particularly attracted to the short piece of red Velcro on my pack strap.
I've posted some closer pictures over in macro.
Anybody who can offer some more information about what these things are and why they were mobbing me, I'm all ears!
Thanks,
--Rik