Millions and Millions of... bugs? In winter?

While walking through my woods this morning, I noticed that the snow that fell a couple of days ago was covered with a thin layer of black "dust."

However, when I put my finger down in the dust, the "dust" motes would jump out of the way, like flees.

I wonder what kind of bug this is, and what kind of life cycle it has if it is blanketing the snow with millions of themselves.

In other news: I can see from the tracks and scat that the elk came through the yard in the last 12 hours.