This year, many acorns in our area fell prematurely, plagued by some kind of warts or gall-nuts which I see for the first time:

Some of the acorns are covered by two or three warts at a time, so that the actual nut itself is practically non-existent.

Judging by the hole present in all the warts, I'm guessing those were houses for some insects or larvae. Unfortunately, warts on acorns available for inspection (those which already fell) are already abandoned by their hosts.

Does anyone know who those guys are and how they look like?