Галл дубовой орехотворки Andricus caputmedusae / gall from Andricus caputmedusae oak gall wasp.

Plant galls (or cecidia) are abnormal outgrowths of plant tissues, similar to benign tumors or warts in animals. They can be caused by various parasites, from fungi and bacteria, to insects and mites. Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified. This applies particularly to some insect and mite plant galls. The most common galls formed by Hymenoptera - gall wasps. About 1300 species of this generally very small creature (1-8 mm) are known worldwide. Most species of gall wasps live as gall-formers on oaks. One of the most well-known of these oak gall wasps is the common oak gall wasp (Cynips quercusfolii), which induces characteristic, 2-cm in diameter, spherical galls on the undersides of oak leaves.