66 



When the rains have definitely set in, the fly leaves this zone of transi- 

 tory dispersion and travels northwards, into its normal zone ; it is 

 found as a permanently resident species in Upper Dahomey, to the 

 north of the llth parallel. 



" In the case of G. longipalpis, we have found that the opposite 

 phenomenon occurs : established in January-February in the coastal 

 region of Dahomey, it is not until May that this species appears in 

 Central Dahomey, towards the 8th parallel ; there it becomes very 

 abundant at the height of the winter season, in July-August, dis- 

 appearing for good in October and regaining its southern haunts, where 

 it then finds the humidity necessary to it. 



" The species that definitely prefer dry places, like G. morsitans, 

 appear to react in quite a different manner, since their period of 

 maximum dispersion seems to coincide with the end of the rains and 

 the commencement of the dry season. 



" In conjunction with the adjustments occasioned by pluvial condi- 

 tions, there is another meteorological factor which has a predominant 

 influence upon the relative dispersion of Tsetse-flies, namely, winds. 

 The east wind, known as the harmattan, which blows in the dry 

 season over a vast portion of West Africa, notoriously accentuates 

 the desiccating effects of the cessation of the rains, and, in consequence, 

 has a considerable influence upon the bionomics of Glossina. Under 

 the dehydrating action of this extremely dry wind, the mean hygro- 

 metric degree falls to an exceedingly low level, and Tsetse-flies more or 

 less completely forsake the haunts in which they are liable to be exposed 

 to its influence. Moisture-loving species, such as G. palpalis, naturally 

 react much more than the drought-loving species, but the latter are 

 likewise not indifferent. 



" G. palpalis is extremely sensitive to the harmattan, and under its 

 influence it forsakes all the small watercourses where the vegetation 

 is of little density, even when they are not entirely dried up, and : 

 concentrates in haunts affording the best cover, in the vicinity of! 

 large sheets of water. Thus it is that during a large portion of the 

 year this species is scarcely to be found throughout the northern part 

 of its zone, and it exists there only in areas of extremely limited extent. 

 The typical drought-loving species, G. morsitans, however, is also very 

 markedly subject to the influence of the desiccating wind. In the 

 zones in which it is met with, its dispersion tends to become much more 

 restricted from the commencement of March, when the harmattan is 

 in full blast, than towards the close of the winter season, and the fly 

 also shows a greater tendency to take refuge in the vicinity of the 

 streams. 



" In the case of each species of Glossina, there are thus zones of- 

 permanency (permanent haunts) in which the species persists throughout 

 the year, and zones of dispersion (temporary haunts), in which its. 

 presence is more dependent upon seasonal, meteorological circum- 

 stances. Migrations take place from the zones of permanency to the. 

 zones of dispersion, when the season in the latter becomes favourable 

 to the life of the flies, and conversely .... 



" Far from being merely a matter of curious interest, the seasonal 

 migrations of Tsetse-flies are of very great practical importance. All ; 

 the causes that inhibit, even temporarily, the dispersion of these 

 insects exert upon the regions concerned an economic influence of 

 primary value, by liberating them more or less completely from the: 



