The side if the fly that we would indicate as 'the back' in human terms. In practice this means the side where the wings are placed (or would have been placed if the insect is apterous).
Adjective: dorsal.
Opposite: venter.
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?