32 R. W. CROSSKEY 



if it is placed in an existing named genus this would involve aggregating forms in 

 which the whole fundamental morphology of the adult head-capsule is quite different. 



Up to now the form of the head has been little used in the taxonomy of Simuliidae 

 since it offers few characters, and the morphology of the posterior surface of the head 

 has not been studied from the taxonomic viewpoint. Before considering the 

 exceptional features shown by the head of Afrosimulium it is therefore necessary 

 briefly to consider the adult head morphology in other black-flies, which are typified 

 by the genus Simulium sensu lato (the head of other genera, Prosimulium, Twinnia, 

 Gigantodax , Austrosimulhim is essentially similar to that of Simulium with approxi- 

 mated postgenal lobes and at least weak sclerotization between the cardines but the 

 occipital foramen in these genera tends to be more ovate in the vertical axis than in 

 Simulium) . 



The head-capsule of Simulium in facial view is broader than its height in both 

 sexes (Text -figs. 61, 63), and the broad shape with convex outer margins is very 

 evident in posterior view also (Text-figs. 57, 59) ; in profile the head of the male has 

 the eyes rather distinctly subtriangular in general form (Text-fig. 65) with the area 

 of enlarged upper eye-facets at least as long as its height ; the female head in side 

 view, even if the eyes are relatively reduced as in some forms, usually shows no 

 definite angulation of the eye-margin posterodorsally (Text-figs. 66, 69), and the 

 head-height is usually only about a third greater than its length. The morphology 

 of the posterior surface of the Simulium head is very constant, only varying through- 

 out the genus by minor differences in degree of sclerotization and by very minor 

 differences in the proportions of certain areas : the occipital foramen is approxi- 

 mately as wide as its height or only very little longer in the vertical than the hori- 

 zontal axis (Text-figs. 57, 59) ; well sclerotized postgenal lobes have well formed 

 upper inner corners and mainly close the lower end of the foramen, and are only 

 narrowly separated by the postgenal membrane in the mid-line ; the cardines are 

 well visible in posterior view and the area between them is at least weakly sclero- 

 tized, so that they are never separated by a wide membranous area (the postgenal 

 membrane therefore does not reach as far as the posteroventral edge of the head 

 (Text-figs. 57, 59)). 



In Afrosimulium the head-capsule of both sexes in facial view (Text-figs. 62, 64) 

 is not wider than its height, and the relatively narrow shape and less strongly convex 

 eyes are well seen in posterior view (Text-figs. 58, 60) ; in profile the head of the 

 male has the eyes of an inverted pear-shape (Text-fig. 67) with the area of large 

 upper eye-facets higher than long ; the female head in side view is about two-thirds 

 higher than its length and has the hind margin of the eye distinctly angulate near 

 the top (as indicated by the arrow in Text-fig. 68). The posterior surface of the 

 head in Afrosimulium gariepense is completely different from that described above 

 for Simulium : the occipital foramen is sub-ovate in the vertical axis with the height 

 conspicuously more than the width, and the foramen is closed ventrally only by the 

 exceptionally wide postgenal membrane ; the postgenal lobes are large, very widely 

 separated by the broad unsclerotized postgenal membrane, and have no well formed 

 upper angle against the occipital foramen (Text-figs. 58, 60) ; the cardines lie in a 



