The leaves of this White Oak in Green-Wood have refused to fall. They held up to Sandy, the Nor’easter a week later, and all the rest of the winter so far. Actually, some oaks are tenacious leaf-holders, only shedding them just before, or just as, new leaf growth begins to bud.
So I got to take another look at the hedgehog galls I noticed back in August. They’ve lost their lurid coloring. 
The exit holes: the female wasps within (there are only females this generation) should have emerged during autumn to deposit over-wintering eggs on the tree’s buds (I looked but wasn’t sure what I was looking for); the generation from these eggs on the buds, which will hatch out in the spring, will be the reproductive one, producing both males and females.
Remember that these galls are created by the tree itself, as a kind of containment system around the irritation of the gall wasp’s eggs and then larvae. The miniscule wasp is forcing the tree to house and protect its tender larvae, chemically commanding the tree to work for it (most galls are harmless to the host plant). The image above shows where the gall was attached, extended, connected to the leaf. And inside?
Sliced roughly in half, the tunnel is clear but the chamber within is filled with frass- or sawdust-like material, presumably left over from the adult insect cut its way out.
Oaks being gall-magnets, the same tree had examples of another type of gall. These were on the twigs, not the leaves. Now, I don’t know which type of insect produces this gall, I’m guessing another wasp, but then I’m no Alfred Kinsey.
Some of these also had had exit holes:
Some didn’t. I sliced one of the latter in half:
A tiny egg-shaped structure was inside the cavity within. You can see where the plant ends and the animal begins. And inside of this eggy cocoon, the larva:
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