78 R. W. CROSSKEY 



phoretic partner, although it is difficult to discern the significance of most of the 

 departures from the morphological norm in Simuliidae. The hypostomium (Text- 

 fig. 268), with a rather regular row of thirteen instead of the normal nine apical teeth, 

 has a form unique among black-flies, but it is important to note that this form is not 

 acquired until late on in larval development, for the early instar larvae have a 

 normal type of hypostomium with prominent median and corner teeth and an apical 

 row containing nine teeth altogether as in free-living forms ; the mature larval 13- 

 tooth row appears to be derived by an enlargement and forward migration during 

 development of the paired blunt serrations that lie immediately behind each corner 

 tooth in the normal hypostomium. 



The larval abdominal cuticle in Lewisellum species shows a more definite micro- 

 sculpture than in most Simuliidae and the nature of the cuticular thickening, 

 whether irregularly striate, with close-set ovoid plaque-like thickenings, or with a 

 crazed crack-like pattern, provides a taxonomic character for distinguishing species: 

 the character has been illustrated by Grenier & Mouchet (1959) and Lewis (1961). 

 It is not yet clear how valuable the existence of the microsculpture might prove as a 

 diagnostic subgeneric character. 



Mature larvae and the pupae of Lewisellum never occur, or at least have never 

 been found, in a ' free-living ' state attached to the normal types of substrate selected 

 by Simulium larvae, such as on rock, stones, trailing roots and grasses, fish-traps and 

 other objects in the watercourses, and it is clear that the phoretic association with 

 crabs is an obligate one for maintenance of the life cycle. The egg-laying habits of 

 the species of Lewisellum are still not known, but neither eggs nor first instar larvae 

 have as yet been found on the crab partners ; attachment to these probably occurs 

 at or as soon as possible after the second instar. Nothing is known of the mechanism 

 of crab location. 



Most of the African river-crabs belong to Potamonautes Macleay and the immature 

 stages of the species of Lewisellum are only known to be associated with this genus of 

 crabs. The species of Potamonautes occur widely throughout Africa in both the 

 warmer lowland and cooler upland rivers and streams, but crab and simuliid 

 associations are known mainly from cool highland habitats in East Africa (possibly 

 because these habitats have been more intensively studied than the lowland ones). 

 The subgenus Lewisellum is best known from such mountain areas as western Kenya, 

 Mount Elgon, Ruwenzori and Kigezi, and the Usambara Mountains, but it occurs in 

 many lowland localities in the Congo Basin, and is almost certainly much more 

 universally distributed in the equatorial belt of Africa than is apparent from the map 

 of present known distribution (Map 5) : nevertheless, most species of Potamonautes 

 appear to have a wider distribution than the black-fly species that are associated 

 with them. In East Africa nine out of the thirty-one species of river-crabs are 

 known to act as carriers in phoretic associations with black-flies (Williams, 1968), 

 but there is little or no evidence suggesting that a particular species of black-fly is 

 associated with a particular crab species. 



The subgenus Lewisellum appears to be absent from West Africa, since no crab- 

 simuliid relationships have been discovered in Nigeria or anywhere further to the 



