Natalie Hernandez in BugGuide identified the aphid as Macrosiphini, a “tribe” of aphids. She adds, “Tough to ID it further from this angle, the shape and texture of the head between the antennae (dorsal view) separates some of the similar genera. Reticulations on the siphunculi are important too but tough to see in photos, and this one isn’t helping by breaking the siphunculus that is in view.” The siphunculus is a small tube for emitting pheromones; I have not the foggiest idea where the siphunculus is in this picture.
Mark Sturtevant, who provided many helpful tips on macrophotography, wrote me,
Aphids are weird. Late in the season like this they will likely start laying eggs soon to overwinter. But during the main part of the season they give live birth – regularly pooping out a young-un, which is simply dropped behind the parent, facing in the same direction as the mother [see the video here]. Furthermore, unless they are tended by ants, which tickle them for nectar and make them move, most aphids in a colony will all be facing the same direction. I think the reason for that is because what I just mentioned: They are born facing the same direction as their parent, and there they pretty much just sit. They grow, and soon start pooping out their own youngsters, until you have a dense colony, all facing the same direction which is the direction they were born (!).
While this is happening, if you look closely at a “big” one you can often see pairs of red dots inside the abdomen. Those are the compound eyes of their unborn young.
And other weird things: These live births are done by parthenogenesis, so the babies are clones of the mothers. Aphids are all female for most of the season.
And then there is another weird thing. Their bright colors (green, yellow, red, depending on the species) are apparently from genes lifted from fungi who-knows-how-long ago. This seems a case of gene transfer from fungi to animals.
And then there are all the parasites and predators they get. You can sit for hours watching a large aphid colony and see lots of drama. Live births, and various forms of horrible death.