Galls on the catkins of an oak. It seems that the wasp responsible for this particular type of gall (Andricus grossulariae) has an even more complex life cycle or perhaps generation cycle than many insects.
These spherical galls appear in the spring, growing, as here, on the catkins. They are for the sexual form of the insect. A second form grows later in the year, on young acorns. It is for an asexual (parthenogenic) form, which I will now be looking out for.