A bit of a nerdy photograph I'm afraid. This is a marble gall I found today on a fallen oak branch. The round gall is abnormal cell growth around one of the oak buds, stimulated by a small wasp Andricus kollari. The gall provides a home for the developing larva of this wasp. It is a busy place in the late summer as it also often provides a home for other insects (inquilines) and for parasites that feed on the larva.
I sliced through this gall to see what I could find. There were a number of small exit holes, like the one on the top half, showing that this larva did have lodgers, some possibly not very nice. The chamber I've cut through, that you can just see on the upper left side of exposed section, was probably where the Andricus larva lived. It may have emerged successfully or possibly have been killed by parasites. The tiny white spot in the chamber is a minute live mite that may have been living on the larva. Such a lot to learn about this area of natural history.