Silk button galls

These little round 'silk button' galls are the result of a little wasp Neuroterus munismalis, which has 2 generations a year. the sexual generation (needing both males and females) causes blister galls on oak leaves whereas the agamic (all females that don't need a male to reproduce) generation cause these Silk button galls on the undersides of oak leaves.
The gall is a single cell holding 1 wasp larva which although mature in August will remain in the gall on the ground over winter emerging as the sexual generation of tiny wasps from February to April.