202 THE COLOURATION OF INSECTS 



Poulton, from careful study of this genus, concluded 

 that " dry " forms are more procryptic, and that this 

 is due to the operation of Natural Selection during the 

 greater stress of the dry season, when insects are so scarce 

 that insectivorous animals need to work harder to obtain 

 food, and the risk to any particular insect is proportionately 

 greater.! Now the climate of the islands of Lake Victoria 

 has been shown to be more uniform than that of the main- 

 land, ^ and it is very interesting and significant that in 

 the case of the commonest Precis there (P. archesia), the 

 only form met with was the " wet " one ; and of P. sesamus, 

 the salmon-pink " wet " form was so much the commonest 

 that the appearance of a " dry " one was quite a memorable 

 event. During the campaign in German East Africa, I 

 first became familiar with the phenomena of a typical 

 dry season, and was soon impressed by the much greater 

 procrypsis of the " dry " Precis, and also of such Pierines 

 as Teracolus, Belenois, Pinacopteryx. When the grass is 

 quite dead and dried up it becomes brown, tinted with 

 pink or grey, and the dry season Pierines, when at rest, 

 harmonize most admirably with their surroundings on 

 account of the suffusion of the under surface with brown, 

 grey or pinkish scales, so that they are often exceedingly 

 difficult to detect after they have alighted. It is note- 

 worthy that, just as in the case of the moth above men- 

 tioned, this especially procryptic colouring is only found 

 wher*^ it is needed on the under surface, viz. on the whole 

 -of the hind wing, but only on the tip of the fore wing, 

 which is not covered by the hind wing in the position 

 of complete rest. The remainder of the under surface 

 of the fore wing is as in the " wet " form. 



The above examples of conceaUng colouration are all 

 procryptic ; that is to say, they are species, found highly 

 desirable by vertebrate enemies, which have acquired 



1 Trans. Ent. Soc, 1902, part iii, pp. 424-443. 



2 See chart. Chapter iii. 



