﻿190 JAS. J. SIMPSON — ENTOMOLOGICAL 



their insect faunas must constantly intermingle, and it would be surprising if they 

 were distinct. The Lake Chad system is therefore the only one which can be 

 considered as topographically distinct from the others. 



It is extremely difficult to express in general terms the distribution of the 

 various types of vegetation in this area, but in the coast region the mangrove 

 holds exclusive sway, while in Southern Nigeria and the southern parts of 

 Northern Nigeria the fresh-water swamp forest is predominant, especially in the 

 valleys of the rivers. Further removed from the larger streams, there is a certain 

 amount of mixed deciduous forest, while in the north of Northern Nigeria the 

 savannah forest, and ultimately almost pure savannah, is found. 



In the south of Southern Nigeria the climate is equatorial, i.e., there is no true 

 separation of dry and wet seasons ; in the northern districts, and in the south of 

 Northern Nigeria, there is a transition stage betAveen this form and the true 

 tropical form, in which the dry and wet seasons are perfectly distinct and of 

 almost equal duration. Details of these forms have already been given in these 

 reports. 



How far, then, is the distribution of Glossina correlated with these phenomena ? 

 The species to which I would draw attention here are : — G. palpalis and G. 

 tachinoides in the palpalis-gvouip ; G. submorsitans and G. longipalpis in the 

 morsitans-gi'oup ; and G. fusca in the fusca-gvouip. 



Of these by far the most widely distributed is G. palpalis. This species is 

 ubiquitous along the coast region and follows the course of all the rivers. It is 

 especially abundant in the delta area and is found at all parts of the Niger within 

 British territory. In the smaller rivers in the Western Province of Southern Nigeria 

 it is found at considerable distances from the coast, while it is the prevalent 

 species throughout the Cross River, at any rate up to the point where this river 

 enters German territory. 



On the Benue River it has not been found further up than Loko, a station not 

 far from Lokoja. Consequently, this species exists wherever the rainfall is great, 

 where the dry season is not of long duration, where the vegetation is dense, and 

 always along the basins of rivers. Where any or all of these factors are less 

 accentuated, the number of individuals tends to decrease, for example, in the 

 higher reaches of the River Niger and on the Benue. 



The conditions which are most favourable for G. palpalis are, generally 

 speaking, most unfavourable for G. tachinoides. Where the country is open, the 

 vegetation sparse, the dry season well defined and the rainfall slight, there G. 

 tachinoides is most abundant. Consequently, this species is the predominant one 

 in the region bordering the Sahara in Northern Nigeria ; it is the only member 

 of the palpalis-groiip found in the Lake Chad area. 



We may therefore consider the dispersion of G. palpalis as coming from the 

 south and of G. tachinoides from the north, so that in the intermediate region 

 both species occur, and, according as to which of the two sets of conditions 

 already mentioned is most marked, there will be a preponderance in the number 

 of individuals of one or other of the species. A comparison of the map which 

 accompanies this report with that given for Northern Nigeria will show that this 

 is precisely what occurs. 



