The oak gall constitutes an amazing relationship between plant and insect. These balls are formed when the eggs of a tiny wasp are laid on the oak's outer bark near leaf clusters. The tree forms a protective cocoon around the wasp as it develops which protects it from predation. Once the wasp emerges, the gall is left as a hollow shell. This one was made by the gall wasp Loxaulus maculipennis.