Andricus kollari larva (a type of wasp) inside an oak marble gall. This was a dead one. I found an old fallen gall and noticed it didn't have an exit hole, so took it home and cut it open.
Here are two photos of marble galls I took last September:
@jennywren It's a gall. You will see these on oak trees - the wasp species that cause these lays eggs in the leaf buds which then cause a distorted 'leaf' growth in the form of these galls. The larva grow inside the gall and eventually emerge as a wasp.
