Snow flies or snow fleas?
OK, I was making a pun about the snow flying when I posted the question, but I am real glad you meantioned the snow fleas.
There have been days when it was not quite spring that I have seen the snow just black with them. They really seem to like the sunny sides of the ski tracks or boot tracks.
It is always hard to convince people who have never seen them that they exist until they see them, and it is always worth seeing their expressions when they realize what they are seeing.