strange bugs

I was walking home the other day when a tiny speck of red caught my eye against the light gray of the sidewalk. It was moving, and I would have thought it was an ant except that it was too red. It was some kind of strange little bug the like of which I have never seen before. After a moment I realized there were lots of them, and they were piling onto the seed of a maple spinner. They varied among themselves, but were all fairly wide and flat for their size. I've asked and looked around, and I think they might be nymphs of the milkweed bug.