The distal lobe of the posterobasal portion of the axillary membrane that joins the hind margin of the wing to the thorax is called the upper calypter. It begins the membrane folds over then lower calypter. It is usually larger than the lower calypter, but in some groups, e.g. Tabanidae, Acroceridae, and many Calyptratae, the lower calypter is larger than the upper one.
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?