Fellow bloggers are so helpful! In my previous post, I was stymied and asking for help; within a couple of hours, Christopher, then Seabrooke and Neil were there with information and ideas on where to find more. (Thanks!)
So I Googled and examined my tree, and looked at photos on BugGuide. Here's what I've found out, so far.
First, the mummies. (I love that concept; mummies in my maple tree!) A small wasp lays one egg in a live aphid. The aphid continues to develop, fattening up as the wasp larva matures (almost as if it were pregnant). It changes colour, becoming silvery-brown. When the wasp pupates, the aphid dies; later, the adult wasp cuts a hole through the mummy's back. These are the holes I saw.
I examined all the branches that I could reach on my maple tree. I found no more mummies, but plenty of aphids:
Brown aphid on maple flower.
This aphid has the same pattern of hairy studs that the mummies had; two lines of studs on the back, one on each lower edge. Here's that photo of a mummy, again:
BugGuide has a photo of a mummy colony, and one showing the hole with a sort of trapdoor lid. These are not the same species of aphid; they were found on bamboo, and have no studs.
Next, the parasite: Google (and Neil's hint) gave me Aphidiinae, small black wasps that lay eggs in unfortunate aphids, and a photo that almost matches the ones I found.
One of the mummies in my tin seemed to have something dark in the abdomen, even though the hole had been cut through. I wondered if maybe the wasp was still inside, so I replaced the lid and waited. Sure enough, in the morning, this wasp had emerged. He has a narrow wasp waist, a sharply-pointed rear end, and those kinky antennae Christopher had noticed. I found several others like him on the tree.
"Each aphid parasite species attacks only a few aphid species and they will not attack any other (nonaphid) group of pests." From Illinois Natural History Survey.There were a couple of green spitbugs on the maple. It looks like they don't have to worry about the wasps.
Also on the tree, confusing the issue, were many tiny black flying critters, about the same size as the wasps (1/8 inch, more or less, not counting that extra-long wing):
It turns out that they are the winged form of the aphids; I found a photo here. They are Periphyllus, or maple aphids, as Neil had suggested.
I've collected a fresh batch of maple leaves, and shut them up in the aquarium. They came with a batch of aphids, and at least one wasp. I hope there are more. I want to see if I can catch them laying eggs.
At least one website calls these wasps "beneficial". It all depends on your point of view. If you are a gardener, they're great; not so much if you're an aphid.