Many insects form detachable galls on oak. All but two of these are cynipid wasps. The two exceptions are the oak gall midges Polystepha pilulae and Polystepha globosa.

Oak leaf gall midge (Polystepha pilulae) is a long-legged, 1 ⁄16″ to ⅛″ long, mosquito-like fly (midge). Adults are impossible to identify by appearance in the field. However, the species can easily be identified by the gall it produces. Galls appear always on the upper surface of northern pin oak, northern red oak, and possibly black oak leaves. They are hard, 1 ⁄16″ to 3 ⁄16″ in diameter, and irregular in shape. They are green when they first appear in the spring, soon turning red or magenta. As they age they become brown and crusty. They can be easily detached from the leaf surface.

Oak leaf gall midge (Polystepha globosa) forms similar spherical galls on the undersurface of the leaves of black oak and possibly other oaks in the red oak group.

http://minnesotaseasons.com/Insects/oak_leaf_gall_midge_(pilulae).html