98 R. W. CROSSKEY 



streams, in rapids or in rock cascades, where the subgenus usually forms the dominant 

 component in the simuliid fauna throughout Ethiopian Africa ; ecologically the sub- 

 genus Metomphalus may be regarded in this area as the fast-water counterpart to the 

 slow-water subgenus Pomeroyellum (although the latter subgenus does contain a few 

 torrenticolous species that may cohabit in the same watercourses with species of 

 Metomphalus). A notable feature in this subgenus is the extreme abundance in 

 which the juveniles of many species occur, larvae and pupae sometimes densely 

 coating the smooth submerged rock surfaces of cascades or hanging in thick masses 

 on trailing roots. 



The species of Metomphalus fall into three main groups distinguishable by the 

 characters given in the key and definitions below. 



Key to the Species-Groups of the Subgenus METOMPHALUS 



i Larva with dense covering of large conspicuous scales on abdomen and dorsum of 

 thorax ; postgenal cleft subelliptical (Text-fig. 259). Cocoon without definite neck. 

 Pupal gill filamentous (Text-fig. 214). Body of 5* ventral plate subtriangular and 

 bordered with strong spines (Text-fig. 117) . albivirgulatum- group (p. 98) 



- Larva with thoracic cuticle bare and abdominal cuticle with minute inconspicuous 



hairs or scales only on posterodorsal region ; postgenal cleft not of this shape. 

 Cocoon with well developed neck. Pupal gill with stout primary branches and fine 

 secondary filaments or with fat tubular branches. 5* ventral plate not of this form . 2 



2 Pupal abdomen with the normal pair of hooks each side ventrally on segment 5. 

 Pupal gill with basal arms (Text-figs. 21 1-2 13). Larval abdomen posterodorsally 

 with sparse spinous hairs (Text-fig. 227) ; postgenal cleft subcordate or bluntly 

 sagittate (Text-figs. 256-258). $ scutum without bold pattern, usually with in- 

 definite pair of silvery triangles anteriorly . . . medtisae/orme- group (p. 100) 



- Pupal abdomen without hooks ventrally on segment 5 or with a single minute hooklet 



on each side. Pupal gill without basal arms (Text-figs. 215, 216). Larval abdo- 

 men posterodorsally with blunt flattened scales (Text-fig. -229) ; postgenal cleft 

 subcircular (Text-fig. 260). q* scutum often with very bold black and silver-grey 

 pattern .......... bovis- group (p. 99) 



albivirgulatum-group. Small species, wing length 17-2-3 mm. <J scutum with anterior 

 pair of subtriangular brilliant silver-white scaled patches. <J ventral plate complex, forming 

 right-angle in profile (Text-fig. 135), body of plate with strong marginal thorns, subtriangular. 

 $ cibarium unarmed. Pupal gill of 25-40 simple filaments coalesced only at extreme base. 

 Pupal abdomen with usual two hooks each side on venter of segment 5. Cocoon without neck, 

 although usually with a very narrow woven band connecting or almost connecting the two 

 anteroventral corners of the cocoon. Larval head elongate, postgenal cleft elongate sub- 

 elliptical ; hypostomium with two of the lateral serrations formed into rather definite teeth out- 

 side the main anterior row of nine teeth (Text-fig. 281). Larval thoracic cuticle dorsally with 

 large scales. Larval abdomen of unusual shape (Text-fig. 287), not noticeably swollen posteri- 

 orly, first few segments sometimes expanded mediodorsally (appearance of such swelling em- 

 phasized by clustering of the scales) ; almost whole abdomen densely covered with large scales. 



At present this group contains only one remarkable species that differs from all 

 other African Simulium s.l. except those in the subgenus Edwardsellum by having a 

 dense vestiture of large scales (immediately obvious with low-power magnification) 

 over the thorax and abdomen ; on this and other characters of the larva it could 

 well be placed in a separate subgenus, but the adult and pupal characters conform 

 closely with those of Metomphalus and it appears best to assign it to this subgenus. 



