Relationships of the Musca domestica subspecies calleva and curviforceps (Diptera: Muscidae) in Rhodesia

Field investigations on Musca domestica houseflies in Rhodesia demonstrate a greater degree of contact than previously reported between domestic M. d. curviforceps and feral M. d. calleva at their habitat interface. There was no evidence of any seasonal disappearance of hybrid flies; stable, intermediate, largely domestic populations are a feature of areas of sympatry. The existence of almost pure calleva feral populations in these areas was verified, though a slight degree of introgression with curviforceps was demonstrated for the first time. Gene‐flow between the subspecies was studied and a new explanation for the one‐way gene‐flow, based on habitat choice of hybrid flies, is advanced. Frons ratio, widely used in distinguishing different forms of housefly, showed serious limitations as a taxonomic character. The ratio, intended as a size‐adjusted measure of frons width, is not independent of size, and frons width and head width (shown to be a good measure of size) are at best weakly correlated. It was confirmed that the frons width of male calleva × curviforceps hybrids is stabilized at the fronJ width characteristic of curviforceps although laboratory breeding results indicate that intermediate frons widths are to be expected. The stabilization is due to selection for broader frons width in the domestic habitat. The nature of the selection is explained, and evidence is produced regarding its basis.