A pair of bristles that usually arise from the ocellar triangle near the anterior ocellus but that are sometimes displaced, usually outside the ocellar triangle near the anterior ocellus.
The image is a temporary link to:
http://upload.wikimedia.org/wikipedia/commons/c/cf/Head-diptera-muscoidfly.gif
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?