One of the most unusual galls found on oaks is the light green, sometimes speckled, ball-like Roly-Poly Galls produced under the direction of the gall wasp, Dryocosmus quercuspalustris (family Cynipidae). The specific epithet reveals one of the most common hosts of this gall-making wasp: Quercus palustris is the scientific name for Swamp Spanish Oak (a.k.a. Pin Oak) with palustris being Latin for “swampy” or “marshy.”
However, the gall-wasp has a much wider gall-making palette including many members of the red oak group. The galls may also be found rising from both leaves and catkins. This is a bit unusual for gall-making arthropods with most targeting specific plant structures.
The hollow galls are around 1/2" in diameter. The “roly-poly” name comes from the unattached, white, seed-like structure that rolls around inside the galls. The structure houses a single wasp larva. I like to imagine newly emerging wasps staggering around after spending time rolling around inside the galls. Probably not true, but it's an entertaining thought.
An alternate common name sometimes used for the galls is the much less descriptive Succulent Oak Gall. Although I’ve never found a reference explaining this name, I believe it refers to the fleshy walls surrounding the roly-poly structure like the flesh of a cantaloupe.
Plant galls provide both a home and food for the developing gall-maker. But they don’t always protect the developing wasp as illustrated by the images below. When I saw the hole in the succulent gall-wall, I thought it was an adult emergence hole. However, cutting the gall open revealed a concavity around the hole. A reasonable interpretation is the wasp larva became a bird meat snack; a fate not shared by its neighbor.