Mr. G. A. K. Marshall on the Genus Byblia. 337 



specimens exhibit a marked reduction, and, indeed, since my 

 arrival in Natal in August I have noticed that the dry-season 

 forms of many other species are, so to speak, " drier " than 

 usual — that is, they are smaller and more under-coloured. 

 As this year in Natal the summer was unusually dry and 

 hot, and was followed by a very mild winter, it strengthens 

 my belief that in South Africa the development of black 

 markings depends not so much on temperature as on humidity. 

 The occurrence of the wet-season Ibrm in South Africa may 

 be roughly given as from September to Aj)ril and that of the 

 dry form from April to September, though, of course, allow- 

 ance must be made for exceptional seasons and a certain 

 amount of overlapping. 



It is interesting to note that wherever B. ilithyia occurs in 

 the Ethiopian Region it is accompanied by a well-marked 

 variety, whereas in the Oriental Region it gives rise to no 

 variety at all. 



All the varieties may at once be distinguished from the 

 typical form by the absence of the transverse row of black 

 spots before middle on the upperside of iiind wing, in addition 

 to other characters, and so far as 1 have seen they never 

 exhibit any graduation into the typical form, though they 

 merge very much into one another. There are three well- 

 marked continental varieties, which are more or less confined 

 to certain distinct areas : — 



(a). Var. acheloia, Wallgr. 



This is the southern variety, and is the only one 

 found in the South-African subregion. On the eastern 

 side it ranges northwards to Nyasaland, and perhaps 

 even as far as the Equator, but on the West Coast the 

 Cunene River appears to be its northern boundary. 

 Apart from the distinctive character mentioned above, 

 ocheloui diflers from ilithyia in having all the black 

 bands on both surfaces more strongly developed at all 

 seasons and the hind marginal spots on upperside of 

 hind wing are smaller; on the underside of the hind 

 wing it differs in the wet season in the absence of the 

 subbasal and median whitish bands ; these are, how- 

 ever, present in the dry form, which only differs from 

 that of the type in the absence of the hind marginal 

 lunulate white line in both wings. Hewitson's figure 

 of B. ilithyia (Doubl. Gen. Diurn. Lep. pi. Ixviii. 

 tig. 1) represents the wet form of this variety. 



In South Africa the distribution of achelo'ia is some- 

 what curious. In the warmer low-lying districts, and 



