Cross-mating experiments are reported between adults of the typical, darkclubbed form of Syntomosphyrum glossinae Wtstn., obtained from pupae of Glossina pallidipes Aust. from Kenya and of G. palpalis (R.-D.) and G. morsitans submorsitans Newst. from Northern Nigeria, and the ‘white-clubbed’ form of Syntomosphyrum, originally reported by Waterston from Nyasaland, obtained from pupae of G. morsitans Westw. from Tanganyika.

Fertilised eggs, giving rise to offspring of both sexes, were produced only by crosses between the strains of typical S. glossinae. Crosses in either direction between the ‘white-clubbed’ form and typical S. glossinae resulted in all-male progeny, presumed to develop from unfertilised eggs. It is considered that this is evidence of a physiological barrier to reproduction between the two forms. The ‘white-clubbed’ form is therefore regarded as a distinct species.