ii2 R. W. CROSSKEY 



the Simuliidae (e.g. by Rubzov, 1959-1964 ; Stone, 1965 ; Davies, 1966) ; the 

 same course is followed in the present work. The main reasons for this are the 

 presence in Simulium s. str. of some adult characters that are never found in Pro- 

 simuliine forms, viz. bare basal section to the radius, enlarged and flattened fore 

 tarsus, exceptionally enlarged and elongate styles of the male hypopygium, complex 

 toothed and beaked ventral plate, and the boldly marked scutal pattern (though 

 some or most of these characters are found in some other subgenera that are probably 

 quite nearly related to Simulium s. str.). 



The subgenus most nearly related to Simulium s. str. is clearly Odagmia, which 

 differs by having the pleural membrane haired (always bare in Simulium s. str.), by 

 the presence of a small claw-tooth in the female (claws nearly always simple in 

 Simulium), by the presence of blunt ventral papillae and undivided rectal gill lobes 

 (although exceptions to the latter do occur in Odagmia), and by the small rounded 

 or quadrate larval postgenal cleft (the cleft much larger and differently shaped in 

 the larvae of Simulium s. str.). These differences are not great, and it would not be 

 inappropriate to rank Odagmia as merely a species-group within Simulium s. str. 

 (the course recently adopted by Davies, 1966, in dealing with the British fauna, 

 where the ornatum-group is placed in Simulium s. str.), but I prefer — because they 

 are readily and consistently distinguishable in both adult and larval stages — to 

 recognize both Odagmia and Simulium as valid subgenera : this course has the 

 advantage of making Simulium s. str. more clearly definable than it would become 

 if Odagmia was merged with it. 



The same cannot be said of Gnus, which Rubzov (1959-1964) and Stone (1963, 

 1965) treat as a valid genus-group segregate distinct from Simulium s. str., mainly 

 because of the presence in the claws of the female of a small basal tooth, and the 

 loose open weave of the fore part of the cocoon, together with the long postgenal 

 cleft of the larva that meets or almost meets the hypostomium. Neither these nor 

 any other characters of the species that have been placed in Gnus, either from the 

 North American or from the Palaearctic fauna, offer sufficiently consistent differences 

 from the characters found in Simulium s. str. to justify the treatment of Gnus as a 

 separate subgenus and I here unhesitatingly place Gnus as a synonym of Simulium 

 s. str. When all forms of Simulium and Gnus are considered from the Holarctic and 

 Oriental Regions it is at once clear that many intermediate conditions exist for any 

 character (e.g. between the cocoon with very large openings, smaller fenestrations 

 and no fenestrations ; between a completely simple female claw and one with a 

 definite small tooth ; between a larval postgenal cleft that stops well short of the 

 hypostomium and one that reaches it widely and completely divides the postgenal 

 bridge) and that no combination of characters from different stages of development 

 is maintained throughout any given series of species with enough constancy to 

 enable any separate subgenus to be defined — and it is difficult to define species- 

 groups within Simulium s. str. reliably. 



If Gnus and Simulium s. str. are maintained as separate subgenera it becomes 

 impossible satisfactorily to assign many species either one way or the other. As 

 examples of this difficulty may be cited the following : Simulium nobile de Meijere 



