80 R. W. CROSSKEY 



Mandible atypical, main apical tooth usually very large relative to other apical teeth, comb- 

 teeth usually much reduced (Text-fig. 299) ; at most two small mandibular serrations, these 

 usually reduced or obliterated. Antenna short or very short, with four segments or with only 

 three segments because of loss of suture between the normal basal two. Thoracic cuticle bare or 

 with small pale hairs. Abdominal cuticle bare or with covering of minute colourless hairs, some- 

 times microstriate at least posteriorly. Abdominal shape atypical, posterior circlet in almost 

 ventral position (Text-fig. 285), anal sclerite almost terminal and not dorsal, arms of anal sclerite 

 unusually slender, last abdominal segment sometimes produced as a posterodorsal bulbous lobe 

 almost overhanging the anal opening. Ventral papillae absent. Accessory sclerites absent. 

 Rectal scales apparently absent. Rectal gills with numerous long finger-like secondary lobules. 

 Posterior circlet with about 90-100 rows of 14-23 hooks, usually about 94 rows of about 18 

 hooks. 



Bionomy. [Oviposition habit unknown.] Larval stages (with probable excep- 

 tion of first instar) and pupal stage phoretic, attached to nymphal stages of fluvial 

 mayflies (Ephemeroptera : Heptageniidae, Baetidae, Oligoneuriidae) living mainly 

 in stony torrents*. [Female biting preferences unknown.] 



Distribution. Endemic subgenus confined to the Ethiopian Region, known from 

 equatorial Africa from Ghana eastwards through Cameroon and northern Angola to 

 Uganda, Kenya and Tanzania (Map 6) ; at present there is a large break in confirmed 

 distribution in the Congo basin. 



Discussion. The new subgenus Phoretomyia is here erected, with Simulium 

 copleyi Gibbins as type-species, to include all the species of Simulium from tropical 

 Africa that in their immature stages live in an obligatory phoretic relationship with 

 nymphal mayflies. These forms are ecologically distinct from all other black-flies in 

 the Ethiopian Region, and from almost all other world Simulium ; simuliid-mayfly 

 relationships are unknown in the New World and in the Australasian Region, but 

 have been recorded in the Oriental Region and one Palaearctic species {Simulium 

 ephemerophilum Rubzov from Soviet Central Asia) of simuliid attaches to mayflies. 

 The Ethiopian species with this habit differ not only biologically but also morpho- 

 logically from other segregates of the Simulium fauna in Africa, and it is impossible 

 to place them satisfactorily in any of the already named subgenera ; they are there- 

 fore here placed in the new subgenus on their own, and ranked as an equivalent taxon 

 to the phoretic forms on crabs in the subgenus Lewisellum and to each of the free- 

 living segregates of subgeneric status. 



The species of Phoretomyia are fairly uniform as adults, but rather heterogeneous 

 in the larval and pupal stages, which are strikingly aberrant compared with those of 

 typical free-living Simulium ; the pupal abdomen often bears numerous additional 

 hooks that are supernumerary to the normal basic onchotaxy found in Simulium 

 pupae, the larval head is sometimes broadly bulbous with the cephalic apotome 

 widest near the middle (recalling the condition of the head in the genus Prosimulium 

 or in Gymnopais) , the cephalic fans may be curiously modified and the hypostomium, 

 mandibles and abdominal shape of the larva all differ from those of typical Simulium ; 

 in forms with additional pupal abdominal hooks there is a correlated reduction of 

 size of the cocoon, that may only cover the pupal abdomen. Some of these depar- 

 tures from the norm for the genus Simulium are clearly adaptations to life on the 



* Also attached to river prawns, see Appendix 



