An abnormal growth (often a swelling) of plant tissue, caused by external stimuli, generally by insects such as gall wasps (Cynipidae), and gall midges (Cecidomyidae) but also by mites, aphids, psyliids and some Lepidoptera. Sometimes galls are formed by bacteria, parasitic fungi or other diseases of the plant so galls induced by insects are called insect galls.
Links: http://en.wikiped.../wiki/Gall
Does someone has a scan of Nartshuk E.P. 2003. Key to families of Diptera (Insecta) of the fauna of Russian and adjacent countries. Proceedings of the Zoological Institute Vol. 294: 1-252 for me?