12 R. W. CROSSKEY 



fauna in the Oriental and Australasian Regions is much smaller than that in each of 

 the other regions, and four subgenera have been recognized for the Australasian 

 (Crosskey, 1967a) : there is no recent re visionary work on the Oriental fauna but 

 from a preliminary study on this region it appears that almost all described forms 

 are assignable to only three subgenera (Eusimulium, Gomphostilbia and Simulium 

 s. str.). No complete subgeneric classification has yet been worked out for the very 

 large Simulium fauna in South America (approximately 200 species : Vulcano, 

 1967). 



A conspicuous feature of the world Simulium fauna is that different regions some- 

 times show segregates of forms that conform with each other in a large number of 

 characters but differ clearly in some others, the morphological similarity often being 

 accompanied by ecological preferences for a similar aquatic environment for the 

 immature stages. There is insufficient evidence to show whether the resemblances 

 are due to recent monophyletic relationship or whether convergence exists, perhaps 

 partly at least from adaptation to particular ecological niches, and it is best to regard 

 such pairs of closely similar yet slightly differing segregates as ' counterparts ' in 

 the different zoogeographical regions — since no clear conclusion can be drawn as to 

 how the similarity arose. In the Ethiopian Region several of the endemic subgenera 

 recognized have ' counterpart ' subgenera of the nature just described in other 

 regions, and to help towards a world picture of such resemblances the following list 

 summarizes the counterparts : 



Endemic Ethiopian subgenus Counterpart subgenus 



Pomeroyellum Rubzov Morops Enderlein (Australasian Region) 



Psilopelmia Enderlein (Neotropical Region) 

 Anasolen Enderlein Grenierella Vargas & Diaz Najera (Neotropical Region) 



Xenosimulium sgen. n. (Malagasy Region) 

 Freemanellum sgen. n. Grenierella Vargas & Diaz Najera (Neotropical Region) 



Metomphalus Enderlein Wilhelmia Enderlein (Palaearctic Region) 



Edwardsellum Enderlein Wilhelmia Enderlein (Palaearctic Region) 



The resemblances and differences between these counterparts are detailed later in 

 the text under the treatment for each Ethiopian subgenus mentioned. There are 

 no counterpart subgenera in other regions to the Ethiopian subgenera that live in 

 phoretic association with crabs (Lewisellum sgen. n.) or nymphal mayflies (Phoreto- 

 myia sgen. n.) or to the subgenus Meilloniellum Rubzov, to which both the phoretic 

 subgenera are apparently closely related : the phenomenon of phoresy with river- 

 crabs occurs only in tropical Africa, and phoretic associations with mayflies (although 

 recorded from the Oriental Region and Soviet Asia) are almost unique to the 

 Ethiopian Region, making the Ethiopian Simulium fauna especially remarkable for 

 the evolution of such exceptional behaviour. 



If it is assumed, as seems probable from their suites of characters, that Pomeroy- 

 ellum, Meilloniellum, Lewisellum and Phoretomyia are in general closely related to 

 Eusimulium, and that Metomphalus and Edwardsellum have close affinity with 

 Wilhelmia, it then follows that the overall affinities of the Ethiopian fauna of 



