52 R. W. CROSSKEY 



tized, stiff, black and strong (Text-fig. 325) ; when the fan is open the sparse large 

 blackened rays are wide apart from each other and extraordinarily conspicuous. 

 The strong rays give an impression under ordinary entomological binocular micro- 

 scope examination of lacking the filter-apparatus, but phase-contrast examination 

 shows that this is present and well-formed. The fans when open form shallow but 

 convex baskets or brushes, and are not flat as in the copleyi-group (the only other 

 Simulium segregate in the African area in which the larval mouth-brushes are 

 strikingly modified from the normal pattern). 



Attention should be drawn to one other aberrant feature of the larvae of S. 

 (Dexomyia) atlanticum sp. n., the nature of the dorsal and dorsolateral parts of the 

 thoracic and abdominal cuticle. This is strongly rugose, deeply micro-fissured with 

 rounded or irregularly elongate raised areas between the Assuring (Text-fig. 319), 

 the roughness being easily visible as a coarse shagreenation under low-power 

 examination ; in addition the cuticle has a rather tough darkened appearance, is 

 conspicuously constricted between the segments, and the segmental spiracular scars 

 are black and obvious ; all these features, together with the vestiture of setae of 

 diverse sizes, give the larval body a most unusual appearance. The setae cannot be 

 looked upon as exceptional since somewhat similar ornamentation occurs in scattered 

 species in several subgenera of Simulium (e.g. Edwardsellum, Byssodon, Metomphalus) 

 but the shagreened cuticle appears to be unique in the Simuliidae : I know of 

 nothing else like it in Simuliid larvae, for it is of very much coarser texture than that 

 formed by the striations and plaque-like cuticular thickenings in the subgenus 

 Lewisellum. 



Included taxon. Simulium (Dexomyia) atlanticum sp. n. 



Simulium (Dexomyia) atlanticum sp. n.* 



Large species, approximate measurements : wing length 3-5-4 mm. ; pupal body length 

 4-5-6 mm. ; mature larval body length 9-1 1 mm. 



o*. Head : Normal, holoptic, areas of enlarged upper eye facets occupying most of head and 

 areas of lower eye facets and clypeus correspondingly reduced. Eye with about 25 rows of 

 upper facets. Clypeus dark brown, thinly pruinose, with much long soft pale hair. Antennae 

 1 1 segmented, scape and pedicel slightly enlarged and flagellar segments not strongly compacted. 

 Last segment of maxillary palp very elongate. Posterior surface of head normal. Thorax : 

 Scutum (Text-fig. 310) with bold pattern formed by large rich velvety black median area with a 

 broad black median vitta extending forwards to anterior margin, anterolateral corners, sides and 

 prescutellar depression much paler grey pruinose ; in some lights trace of two fine longitudinal 

 paler lines running through dark area and delimiting the broad black median vitta from the black 

 areas either side ; black areas covered with dark coppery bronze scales, pale areas with silvery 

 scales, prescutellar depression with some soft pale hair. Scutellum with long dark hair. Pleural 

 membrane with hair, mainly on upper part, sometimes sparse. Katepisternum bare. Post- 

 notum (postscutellum) bare. Legs : [Material dissected from pupae, leg colour therefore un- 

 certain but femora except for apices and perhaps most of tibiae apparently pale.] Fore tarsus 

 slender, fore basitarsus about six and a half or seven times as long as its greatest breadth. Hind 

 leg with pedisulcus and with well developed calcipala ; hind basitarsus greatly enlarged, length 

 only about three times as long as the greatest width (Text-fig. 317) or slightly less [hind tarsal 

 curvature of pharate specimen precludes accurate statement of ratio] . Wings : Costa and vein 



* See Appendix for additions to description 



