Ovovivipary is a phenomenon where eggs not only develop in the female body, but also hatch there, and that the larvae are 'born' only when the conditions are favourable.
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?