Butterflies of Southern Kordofan. 287 



Obviouslv the only really notable difference between 

 them from this point of view is the very small number of 

 new Pierines that Capt. Wilson collected. This may be 

 in part explained— but only in part— by the fact that 

 when on the White Nile I paid special attention to that 

 group and sent home long series. 



The extreme scarcity of Satyrines in the White Nile 

 basin is very remarkable. To my single specimen of 

 Yphthima asterope Capt. Wilson adds two Mycalesis milyas. 

 In the nature of things positive observations have much 

 more value than negative, and it is quite probable that a 

 collector working throughout the year might find many 

 more. Mv own observations in Cape Colony suggest that 

 when Satyrines are scarcest Pierines are most abundant 

 and vice versa. 



In the lists, I have marked with a D all the butterflies 

 that I consider to be "desert" insects; this, of course, by 

 no means implies that the species so marked are con- 

 fined to "desert" areas; far from it. The D indicates 

 that butterflies so marked— however extensive their dis- 

 tribution may be— are such as can live where " desert 

 conditions prevail. 



Deserts differ inter se; some are much more barren 

 than others, in some the prevailing surface is sand, in 

 others rock, while some are shingly. Their common 

 characteristic is aridity, an absence of rain for sufficiently 

 long periods to stunt vegetation. Much depends upon 

 season; Col. Yerbury says graphically : " Few . .have 

 any idea of the effect on ' the barren rocks of Aden of a 

 few heavy showers; how almost immediately, as if by 

 magic, vegetation springs up in every ravine and water- 

 course, accompanied by a tolerably abundant insect 

 fauna." * Under such circumstances the desert literally 

 rejoices and blossoms as the rose. 



The sand-dunes on our British coasts are true deserts, 

 small though they be; on them C. pamphUus, P. cardui, 

 A. aglaia, G. phlaeas, L. warns and astrarche may gener- 

 ally be found among the bents, while now and then a 

 Colias may be seen coursing over the sand ; such are desert 

 butterflies, though they may all be found inland living 

 under very different conditions. 



Looked at in this way, we see that whereas among 



* Journal Bombay Nat. Hist, Soc, 1892, p. 208. 



