Willow Grove, Montgomery County, Pennsylvania, USA July 2, 2009 Size: 1 cm or more
Is this the wooly oak gall? On the underside of an oak leaf.
It is similar to this. There are some references on that page, such as Callirhytis sp.
The inside of the gall was empty, except for these tiny eggs. I doubt it that this is the gall-maker.