Glossina morsitans and G. swynnertoni were allowed to emerge for three days from pupae placed in each other's habitats and in that of G. pallidipes.

The morsitans in the habitat of swynnertoni were in poorer condition, and survived less well, than in the previous year. This may have been because the release was done a little later in the season, or because the freshly emerged flies were not up to the nutritional standards of a year or two before.

They were, however, nearly as heavy, in later life, as wild morsitans in their own home, and this is contrary to previous experience.

In relation to initial weight, swynnertoni, whether in their own home or as aliens in the habitats of other species, were heavier than morsitans, and in fat and water the alien swynnertoni were heavier than the native ones. However, the difference between the two species may possibly be caused by the fact that swynnertoni, after taking its first meal, cannot apparently fall much below its initial weight at emergence, as morsitans and particularly pallidipes are able to do, so that very hungry morsitans may be represented by swynnertoni which have been cut off by death, and are not available to the catchers.

The males of morsitans and swynnertoni are relatively inactive in early life, which has an important bearing on estimates of population and mean life made from marked flies. The females on the contrary are relatively inactive in later life.

Generally the relation between wing-fray and age found in the previous year is confirmed, except one aberrant result when swynnertoni was released in the habitat of morsitans. Scanty data suggest that the rate of wing-fray during the rainy season is not very different. As before, the females, especially in later life, fray their wings more slowly than the males.

Teneral (unfed) flies below the average size are comparatively non-viable, and about 10 per cent, are too small to survive to obtain their first meal. There is no suggestion that the same applies to non-tenerals.

The external morphology of early larvae is described; the first instar is provided with an egg-tooth in the mouth, which makes it incapable of feeding. Both the first and the second instars are spiny.

Although copulation between morsitans and swynnertoni is random, insemination from interspecific matings often fails, and the sterile egg is aborted. In such cases the left ovary ovulates before the normal time.

The flies and pupae live at about the temperature of the standard climate as recorded in a Stevenson screen ; possibly the flies are a little cooler and the pupae a little hotter.