This appeared in a jar of Quercus gambelii leaves collected a week earlier. The leaves had Brachys and Cameraria leaf mines; I searched each leaf carefully and couldn't find any galls.
On red maple. First two images unretouched. Third through sixth look a little unnatural because they have levels adjusted to bring some details out of the shadows.
On red maple. Host association with Acer rubrum: https://www.zin.ru/animalia/coleoptera/pdf/clark_ledoux_et_al_2004.pdf