SIMULIIDAE OF AFRICA 77 



than normal (Text-fig. 292), main apical tooth greatly enlarged, other apical teeth and comb- 

 teeth reduced (Text-fig. 297) ; two mandibular serrations, proximal one sometimes reduced. 

 Antenna long and slender, with four segments. Thoracic cuticle bare. Abdominal cuticle bare 

 (except for some small colourless simple setae posterodorsally) and with striate, crazed, or 

 platelet-like microsculpture. Abdomen with posterior enlarged part unusually attenuate 

 (Text-fig. 284). Ventral papillae absent. Accessory sclerites absent. Rectal scales present 

 (? exceptions). Rectal gills with secondary lobules. Posterior circlet with about 120-160 rows 

 of 16-25 hooks. 



Bionomy. [Oviposition habit unknown.] Larval stages (except first instar) and 

 pupal stage phoretic, attached to riverine crabs of genus Potamonautes Macleay 

 (Crustacea : Decapoda : Potamidae) living in small river and stream habitats in 

 lowland and upland areas. Female mammalophilic, including anthropophilic. 



Distribution. Endemic subgenus in Ethiopian Region occurring in Cameroon, 

 Congo Basin, and eastern Africa from Ethiopia southwards to Malawi (Map 5). 

 Unknown from West Africa. 



Discussion. The most remarkable Simnlium ecologically are the forms found in 

 tropical Africa in which the larval and pupal stages live in a state of obligate 

 phoresis with river-crabs, a unique biological association occurring in no other 

 Simuliidae from any region. These forms comprise such a distinctive, morpho- 

 logically homogeneous, and ecologically clearly defined segregate in Simulium s.l. 

 that they are best treated as a subgenus on their own ranking taxonomically equiv- 

 alent with other subgeneric segregates ; the subgenus Lewisellum sgen. n. is there- 

 fore here erected for them, with S.(L.) neavei Roubaud as type-species. Hitherto 

 this species, and the others associated with crabs, have been placed in the hirsutum- 

 group (Freeman & de Meillon, 1953) of Simulium, or together with the phoretic 

 forms on mayfly nymphs in the weam'-group of Simulium (Crosskey, i960), or have 

 been placed in the genus Meilloniellum by Rubzov (1962). 



It is probable that the phyletic affinities of Lewisellum do, in fact, lie with Simulium 

 hirsutum Pomeroy and its allies in the subgenus Meilloniellum Rubzov, since there is 

 a close resemblance in the form of the ventral plate and styles of the male hypo- 

 pygium between the two groups (such that it is improbable that the resemblances 

 arose convergently). There is also a close concordance of other adult characters, 

 male and female, and of pupal form between Lewisellum and Meilloniellum although 

 the pupal abdomen in Lewisellum has a number of supernumerary hairs on the 

 dorsum that are not represented in the latter subgenus. The adults of Lewisellum 

 superficially differ much from those of Meilloniellum species by their large size (wing- 

 length 2 -6-3 -6 mm., usually about 3 mm., in Lewisellum, and about 2-2-5 mm - m 

 Meilloniellum) and by striking appearance through golden scaling on the scutum and 

 thick golden or golden and bronze-brown scaling on the abdomen. 



The larvae of Lewisellum species show a number of aberrant, or at least slightly 

 atypical, characters that set them apart from those of all other subgenera : these 

 include the reduction of the eye-spots, shape of the head and cephalic apotome, form 

 of the mandible, form of the mature larval hypostomium, abdominal shape and 

 cuticular surface pattern. Some of these are presumably adaptations for life on the 



