48 R. W. CROSSKEY 



(despite the large claw tooth of the female and unarmed cibarium) than to true 

 Byssodon ; I exclude it from Byssodon as here defined. S. slossonae is known to 

 me only from description, from which I could not associate it with meridionale, type- 

 species of Byssodon, sufficiently closely for assignment to this subgenus. 



The affinities of Byssodon are very uncertain. Rubzov (1959-1964) places the 

 segregate (under the name Titanopteryx) between Cnephia and Eusimulium, a 

 position in his classification that implies that Byssodon is a rather primitive group, 

 despite the ' advanced ' character of bare base to the radius (a feature presumably 

 evolved more than once) ; the simple male hypopygium and female gonapophyses, 

 the rather fully scaled female abdomen, and slender fore tarsus recall Eusimulium 

 but the cuticular covering of setae on the larva show analogy with Edwardsellum or 

 Parabyssodon, and the divided fan-shaped setae occurring in some forms are remi- 

 niscent also of Pomeroyellum. Taking all the characters together it is impossible to 

 ascribe particular affinity to any other subgenus or group of subgenera with 

 confidence. 



The subgenus includes some species that periodically form serious outbreak 

 swarms, when biting of man and livestock becomes commonplace even though the 

 species are as a rule ornithophilic ; in this regard Simulium (Byssodon) griseicolle 

 along the Nile valley in the Sudan shows closely similar behaviour to that of S.(B.) 

 meridionale on the Mississippi River. 



The Holarctic and Ethiopian forms of Byssodon fall into two distinct species- 

 groups : — 



meridionale -group. Pupal gill arborescent, with 22-26 filaments. Pupal abdomen with 

 sparse backwardly-directed spine-comb dorsally on segment 8, sometimes trace of similar comb on 

 segment 7. Larval head with postgenal cleft broadly reaching base of hypostomium, postgenal 

 bridge therefore obliterated, sides of cleft slightly convex. Larval abdomen with dorsolateral 

 rows of swellings on first five or six segments, and with small subconical ventral papillae. 

 Thoracic and abdominal cuticle with flattened slightly scale-like simple setae. 



Included taxa. Palaearctic Region : Simulium (Byssodon) heptapotamicum 

 Rubzov ; S.(B.) maculatum (Meigen) including the supposed subspecies recognized 

 by Rubzov (1959-1964 : 259-262). Nearctic Region : S.(B.) meridionale Riley 

 [synonyms : occidentale Townsend, forbesi Malloch]. 



griseicolle- group. Pupal gill not arborescent, bifid or trifid with flattened lanceolate or 

 subcylindrical or subspherical branches. Pupal abdominal segments without dorsal spine- 

 combs. Larval head with very large subcircular postgenal cleft separated from base of hypo- 

 stomium by narrow but complete postgenal bridge (Text-fig. 262). Larval abdomen without 

 dorsolateral prominences and without definite ventral papillae. Larval thoracic and abdominal 

 cuticle with deeply divided compound fan-shaped setae. 



Included taxa. Simulium (Byssodon) bifda Freeman & de Meillon ; S.(B.) 

 griseicolle Becker ; S.(B.) tridens Freeman & de Meillon ; S.(B.) trisphaerae Wanson 

 & Henrard, 1944. 



Ungrouped species : Simulium (Byssodon) buxtoni Austen [synonyms : bipunc- 

 tatum Austen preocc, irakae Smart] from Middle East, known only from female 

 (see Crosskey, 1967&). 



