|Wasp in a sumac gall - REK|
That evening when I cut into the largest smooth sumac gall there was a small 5mm black insect laying among the living aphid larvae. The gall had been intact with no visible entry point. I photographed it with the microscope and sent it off to Chris Barnhart for an explanation.
"Appears to be a gall wasp, Torymidae (Superfamily Chalcidoidea, the Chalcid wasps). I think that it developed in there (grew from an egg laid in the gall by the parent) and you opened the gall just before it would have emerged. They have to chew their way out. Yours grew up as a larva eating aphids, I suppose." CB
Since then this ID was confirmed by Bugguide. Key features of this Torymidae family are their metallic bodies and the large hind legs. They are small, from 1.1mm to 7.5mm, (less than 1/3") and have a long ovipositor, sometimes longer than their body.
They are commonly parasitoids of gall forming insects although some species are plant eaters and just take over the gall for housing. Many use their ovipositor to inject their egg into a gall. There is a wide variety of behaviors depending on the species. Parasitoids are parasites that eventually sterilize, kill or even consume their victims. Other Torymidae are inquilines that develop inside the gall, living in the gall space and likely eventually killing the original inhabitant.
The study of parasitoid wasps is quite complex and I haven't scratched the surface. I was unable to find any information on the genera or species of wasp associated with the sumac leaf gall aphid, Melaphis rhois and my photographs are not sufficient to narrow down the choices. I am left with the wonder of this microcosm, a wasp that grows up in an aphid's gall while the sumac continues a healthy life.
More detailed information on the Torymidae family is at this link.
The parasitic way of life has evolved many times as described in National Geographic.