SIMULIIDAE OF AFRICA 49 



Subgenus DEXOMYIA sgen. n. 



Type-species : Simulium (Dexomyia) atlanticum sp. n. [Description below.] 



Diagnosis. <J$ : Basal section of radius haired. Pleural membrane haired*. Katepisternum 

 bare. Fore tarsus slender, fore basitarsus 6-5-7 times as long as its greatest breadth. $ : head 

 unusual, eyes relatively small in profile (Text-fig. 69), wide apart in dorsal view. Cibarium un- 

 armed. Tarsal claws with large basal tooth. Scutum without definite pattern, trace of broad 

 darker median vitta anteriorly. Abdomen evenly covered with pale hair. Seventh sternite 

 undeveloped. Gonapophyses with slightly produced and pointed apices. Paraprocts normal. 

 Spermatheca without internal hairs. 0* : head normal. Scutum with bold black and greyish 

 pattern. Genitalia with small tapering truncate styles shorter than coxites ; style with one 

 apical spinule ; coxite not produced beyond base of style ; ventral plate lamellate and trans- 

 verse with haired median keel and subparallel basal arms ; median sclerite long strap-like ; para- 

 meres long and slender, each with only one large very strong outwardly-directed parameral hook. 

 Pupa : Gill with four long filaments thick at base and strongly tapering ; gill much shorter than 

 pupal body. Abdominal onchotaxy strong and aberrant, usual four hooks each side dorsally on 

 segments 3 and 4 but segments 5-7 ventrally with irregular transverse rows of numerous hooks, 

 other segments dorsally and ventrally with spinous hairs or minute hooklets (total arrangement 

 as Text-figs. 323 and 324). Cocoon shoe-shaped, with neck. Larva : Head and mouthparts 

 aberrant from normal Simulium in several characters. Head capsule convex, cephalic apotome 

 broadest near middle and strongly contracted on posterior half ; cephalic fans forming curved 

 brushes, with a few exceptionally strong blackened rays completely distinct from the few finer 

 pale rays. Hypostomium atypical, large outer prominent groups of five blunt teeth each side 

 with deeply sunken median group of three teeth ; four or five main hypostomial setae in each 

 row with three or four more that are much smaller, slightly irregular, rows lying subparallel to 

 lateral margins of hypostomium. Head-spots positive, very bold. Postgenal cleft minute, an 

 inconspicuous pointed notch, very much shorter than postgenal bridge. Mandible with blunt 

 apical teeth, comb-teeth very strong and decreasing in size from first to third, first comb-tooth 

 blackened and resembling apical teeth ; two mandibular serrations, sometimes apparently 

 reduced from wear. Antenna very short, with four segments. Thoracic and abdominal cuticle 

 toughened, strongly rugose on dorsum, abdominal segmentation exceptionally well marked by 

 intersegmental constriction. Abdominal, and usually also thoracic, cuticle conspicuously 

 covered dorsally and dorsolaterally with large blunt black setae admixed with smaller more 

 spinous setae, thorax (sometimes including base of proleg) occasionally with sparse colourless 

 hairs. Ventral papillae absent, but last abdominal segment swollen laterally. Accessory 

 sclerites absent. Rectal scales absent. Rectal gills simple trifid, no secondary lobules. Post- 

 erior circlet with about 95-125 rows of 8-13 hooks. 



Bionomy. [Oviposition habit unknown.] Larval and pupal stages non- 

 phoretic ; attached to large loose stones in swift stream. [Female presumed able 

 to bite, habit unknown but presumed ornithophilic] 



Distribution. St. Helena Island, South Atlantic Ocean. 



Discussion. The new subgenus Dexomyia is erected here for a single new extra- 

 ordinary aberrant species of Simuliid that occurs only on St. Helena Island in the 

 Atlantic Ocean, and even here — despite search in many possible sites — is known 

 from only a single stream on the southern side of the island ; the species, Simulium 

 {Dexomyia) atlanticum sp. n., is fully described below, after this discussion of the 

 unusual characteristics and possible affinities of the new subgenus. 



In an earlier paper (Crosskey, 1965a) describing another species of Simulium from 

 St. Helena (viz. S. loveridgei Crosskey) brief mention was then made of the remark- 



* or bare in °- (see Appendix) 



