Apple oak gall (Amphibolips confluenta or Amphibolips quercusinanis), backlit to show leaf-like venation. Second photograph is a cross section showing where the wasp larva develops. I’d love to know what the spotting does, if anything.

Cynipid wasps love oaks for some reason — over 3/4 of the 1300 species use Quercus as host. People argue about why that’s the case (e.g., Ronquist et al. 2015). Even Alfred Kinsey the sexologist weighed in, back in the days when he was obsessed with gall wasps.