65 



places ? Is there an actual predominance or disproportion, or are the 

 males simply easier to catch than the females ? 



It is impossible here to provide answers to all these conundrums, 

 some of which, moreover, as already stated, are still far from being 

 solved. We shall therefore confine ourselves in the present chapter 

 to dealing with the principal ones among them, and certain kindred 

 questions. 



SEASONAL MIGRATIONS. 



Statements by Messrs. LI. Lloyd and R. W. Jack, with reference to 

 the dry season concentrations of G. morsitans in Northern and Southern 

 Rhodesia, have been dealt with in Chapter V. (see p. 49). 

 Similar seasonal migrations occur in the case of other species, such as 

 G. brevipalpis, and, as regards West African forms, we give below 

 some interesting observations upon this subject by Dr. E. Roubaud 

 (I23a), taken from a recent publication by this authority dealing with 

 the Tsetse-flies of French West Africa. 



" The climatic and hydrological modifications that supervene in the 

 course of the seasons," writes Roubaud, " exert considerable influence 

 upon the geographical distribution of Tsetse-flies, and upon their 

 relative abundance in a given locality. Thus, should it have been 

 impossible to discover the presence of any of these insects at the 

 crossing of a watercourse in the dry season, even though the stream 

 had not entirely disappeared, it would not necessarily follow that 

 Tsetse are never to be found at that spot ; indeed, it is even probable 

 that in the winter season the same watercourse might be infested with 

 Glossina to a serious extent. 



" Generally speaking, at the commencement of the rains a sudden 

 apparition of the different species of Tsetse-flies is seen, and the insects 

 all at once become numerous in many localities whence they had more 

 or less completely disappeared in the course of the dry season. The 

 phenomenon occurs not only in the case of riparian species, but 

 also, though not so markedly, among the Tsetses of the savannahs. 

 In each instance it is dependent upon the seasonal hygrometric 

 modifications. 



" The fact is that the onset of the rains, with the profound modifica- 

 tions in the hygrometric state of the air occasioned thereby, and their 

 repercussion upon the hydrographic condition of the watercourses 

 which is, as we know, very great in French West Africa produce 

 migrations of Tsetse-flies. These migrations, when determined by 

 the pluvial factor, are generally directed from the north towards 

 the south in the case of the Sudanese species, and from the south 

 towards the north in that of the species of the coastal belt. The 

 flies established beside the watercourses of the Sudanese zone in 

 the dry season are drawn towards the south before the coming rains, 

 at the epoch of the rising of the waters, which is influenced by the 

 earlier rains of the southern regions. Conversely, when the period 

 known as ' the little dry season ' supervenes in the coastal regions, 

 the Tsetse-flies infesting the latter tend to move towards the north in 

 pursuit of hygrometric conditions characteristic of the height of the 

 winter season. Thus it is, for instance, that G. tachinoides, which is 

 absent almost throughout the year in Central Dahomey, nevertheless 

 puts in an appearance there at the commencement of the winter season, 

 during the months of April and May, when it may be met with down 

 to the environs of Abomey (7 N. Lat.), and perhaps even lower still. 



(5979) E 



