One or more setae on the most lateral part of the mesonotum immediately above the notopleuron base of the wing, so situated between the lateral margin of the mesonotum and (if present) the intra-alar setae.
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?