Cynips rugosa
Round, hard galls, sessile, on the branches of Quercus prinoides. Size from 1/2 to 1/4 of an inch in diameter. They are attached to the branch by a small point. The surface when green is smooth and often quite red on the side exposed to the sun. When fully ripe they have a shrunken and shrivelled surface and the color varies from an ashen hue to a dull brown. The free larval cell is surrounded by a yellowish brown cellular mass, too dense to be called spongy, which fills the entire gall. This species has been known to me for many years, but until lately I have considered it a variety of C. globulus Harris, but a careful study convinces me that it is a distinct species. C. globulus is only on the white oak and rarely more than one or two galls in a place, and has even when dry a smooth surface, while C. rugosa is often found in clusters of four or five and even more, so closely compressed that the galls are of an angular or cuneate form. The flies are all females and they mature and leave the galls late in the fall, at least I have never been able to find them in the galls in winter.
”- HF Bassett: (1881) New Cynipidae (1881)©
Reference: https://www.biodiversitylibrary.org/item/22092#page/110/mode/1up