A revision of the four Afrotropical and Palaearctic Sphyracephala Say (Diptera, Diopsidae) with an illustrated overview of the other five Sphyracephala.

In the Afrotropical Region, Sphyracephalabeccarii (Rondani) and S.munroi Curran are found, the former just extending into the Palaearctic Region. In the latter region, S.babadjanidesi Zaitzev occurs in the Balkan and Caucasus Regions and S.nigrimana Loew in Far Eastern Russia and North-Eastern China. The European stalk-eyed fly S.europaea Papp & Földvári is proposed as junior synonym of the Eurasian stalk-eyed fly S.babadjanidesi. In North America S.brevicornis (Say) and S.subbifasciata Fitch occur. The four true Holarctic Sphyracephala are shown to reach their northern limits between 45°30'N and 48°20'N. These species hibernate and show characteristic clustering behaviour in spring and autumn. The four Palaearctic and Afrotropical Sphyracephala are redescribed and extensively illustrated. A key is given to all nine Sphyracephala presently recognised. The subdivisions within the genus are discussed. A cladogram, based on morphological, molecular, wing morphometric and allometric considerations, is presented. It shows the two species groups recognised. Each species group is divided into two subtaxa. Geometric morphometric analysis supports the grouping in four subtaxa as well as the synonymy of S.babadjanidesi and S.europaea. In Sphyracephala, both sexual monomorphism and dimorphism with relation to eye span occur. The allometric lines for males and females of eight species are compared. A clear link is found between allometric slopes and the four subtaxa distinguished. In Sphyracephala, female-biased, balanced, and male-biased sex ratios are found. A remarkable case of female-biased sex ratio distortion is reported for Sphyracephalabeccarii. In Continental Africa and the Arabian Peninsula, a balanced sex ratio was found, while in Madagascar a female-biased 2:1 ratio was found. This represents the first case of a female-biased sex-ratio in a geographically isolated population of a monomorphic diopsid.