Neuroterus quercusbaccarum

The common spangle gall on the underside of leaves and the currant gall on the male catkins or occasionally the leaves, develop as chemically induced distortions on pedunculate oak (Quercus robur), or sessile oak (Quercus petraea) trees, caused by the cynipid wasp Neuroterus quercusbaccarum which has both agamic and bisexual generations. Previous names or synonyms for this species are Neuroterus baccarum, N. lenticularis, N. malpighii, Cynips lenticularis, C. quercus-baccarum, Spathegaster baccarum, S. varius, S. interruptor.

Source: Wikipedia — Neuroterus quercusbaccarum (CC BY-SA 4.0)