Spangle galls on the underside of leaves of pedunculate oak, Quercus robur. The picture, taken in late Summer, shows common spangle galls caused by the cynipid gall wasp Neuroterus quercusbaccarum. A gall is produced when a female wasp lays an egg inside a leaf. The plant reacts by forming a cavity, before the larva hatches. The larva crawls into the cavity and begins to feed. The gall grows out through the leaf surface, reaching the stage shown here about six weeks after the larva hatched. By late Autumn the circular discs that enclose the central cavity, become woody. The galls fall to the ground, where they serve to protect the larva over the winter. The larva emerges in the following Spring