46 R. W. CROSSKEY 



groups). The adults are inseparable, and agree for instance in having the base of 

 the radius bare, the fore tarsi slender, a very large basal tooth to the female claw, 

 bare pleural membrane, unarmed cibarium, similarly marked scutum, and exactly 

 similar form of male hypopygium (Text-fig. 95) ; especially characteristic is the 

 presence in the females of both Psilocnetha and Byssodon of an elongate thumb-like 

 or finger-like anterior process on each arm of the genital fork (a much more slender 

 and drawn-out process than occurs in other segregates), a striking feature that 

 supports the present conclusion that Psilocnetha must be treated in synonymy with 

 Byssodon. There are differences in the immature stages (indicated below in the 

 diagnoses of the species-groups), but the overall resemblance in larval facies also 

 supports the proposed synonymy : in both the Holarctic and the Ethiopian forms 

 involved the larval head is usually pale and weakly sclerotized, the postgenal cleft 

 is enormous and occupies most of the venter of the cranium, the mandibular serra- 

 tions are usually reduced to one, the rectal gills are much subdivided, and the 

 thoracic and abdominal cuticle is covered with small erect setae on the dorsal side ; 

 they even conform in detail of the small number of rows of hooks in the posterior 

 circlet. 



The new synonymy of Psilocnetha and Byssodon is supported by the unusual 

 ecological requirements of both the former in Africa and the latter in North America 

 and Eurasia ; the immature stages are found almost exclusively in the very largest 

 rivers and their more major affluents, where they often form the only component of 

 the Simulium s.l. fauna present. In North America Byssodon breeds mainly in the 

 rivers of the south-western half of the United States, including especially the 

 Mississippi River (type-locality of the type-species, meridionale Riley) and in 

 Eurasia in the large European, Russian and Siberian rivers (such as the Po, Danube, 

 Dnieper, Volga, Yenisei, Lena and Kolyma) ; likewise, the griseicolle-gvoup of 

 Byssodon occurs in the great rivers of Africa, including the Nile, Congo, Benue, 

 Niger and Volta. A biological adaptation clearly exists in all these forms for a pre- 

 imaginal existence in moderately swift but most often unbroken waters that allows 

 species of the subgenus Byssodon to thrive in long stretches of the very largest 

 rivers, where other segregates of Simulium s.l. are usually absent, although occa- 

 sional coexistence with other segregates sometimes occurs : for instance, S. 

 (Byssodon) macidatum in Europe may occur in admixture with S. (Wilhelmia) 

 equinum L., andS. (Simulium) colombaschense (Fabricius), and in Africa S. (Byssodon) 

 griseicolle sometimes coexists with S. (Meilloniellum) adersi Pomeroy or S. (Edwards- 

 ellum) damnosum Theobald. 



The ecological adaptation of Byssodon to very large rivers may provide a clue to 

 the otherwise rather anomalous position of this segregate in the fauna of Simulium 

 south of the Sahara. All the Simulium s.l. of the Ethiopian Region other than 

 Byssodon (and excluding the almost cosmopolitan subgenus Eusimulium) belong to 

 endemic subgeneric segregates, all with haired base to the radius, that appear to 

 have arisen after long isolation south of the desert and to have diverged from their 

 Arctogaean counterparts to such an extent that there is no longer a high correspon- 

 dence of characters. In Byssodon, however, the only subgenus in Africa south of 



