What is inside the oak apples?

Although an oak apple looks like a fruit, it is actually a deformed leaf hanging by its stem or petiole on the underside of oak leaves. Gall wasps form oak apples. In summer, the female wasp injects its eggs into the leaves of oak trees. The larvae that hatch inside the leaf are small and round. As they grow, they cause a chemical reaction that forms a all around the larvae.
These larvae eat and grow within the gall before emerging as adult wasps.