Callirhytis eldoradensis (Hymenoptera: Cynipidae)
Mitchell Canyon, Mt Diablo State Park, Contra Costa County, California
November 27, 2015

The growth of this Quercus agrifolia acorn was stunted by a gall wasp. The wasp has already developed and left; its round exit hole is visible here. The larval chamber of the wasp is under the cap but outside the embryo of the acorn.