TRANSACTIONS OF SECTION D. 



907 



We determined to concentrate our attention on the pupae of certain butterflies, 

 this stage being especially suitable because the chrysalis is motionless, and, there- 

 fore, remains in any position in which it has been fixed until it is seized by an 

 enemy or emerges as an imago. Our object was to decide : — (1) whether there is 

 a straggle for existence during the pupal stage ; (2) whether the struggle, if it 

 takes place, is decided by the conspicuousness of the pupa. 



The inquiry was almost confined to the pupa of Vanessa urticte, which ranges 

 from a brilliant golden appearance through increasingly dark varieties up to black. 

 Seven degrees of colour variation were distinguished, as in the researches into the 

 sensitiveness of this pupa to its environment.* Captured larvae were placed in 

 boxes lined with gilt, black, and yellow paper, &c., so as to produce pupae with 

 diflierent degrees of colour, the aim being to obtain the most contrasted results. 

 The pupae were then fixed to the surfaces upon which they are known to occur 

 in Nature, and others upon which they may be supposed to occur — viz. the food 

 plant (nettle), tree trunks, fences, stone walls, and rocks — while a few were placed 

 on the ground. They were attached by small nails driven through the silken web, in 

 which the caudal hooks were entangled, or (in the case of the food plant) by sewing 

 the web on to the leaves or stem with green silk ; in other cases the hooks were 

 entangled in the outer part of a little plug of cotton wool, which was forced into a 

 crack in bark, wood, or stone. Careful notes were taken of the degree of colour, 

 method, and height of attachment, character of surface, and the date of all visits 

 until the pupa either disappeared, emerged, or died. With very few exceptions, 

 visits were made every twenty-four hours, and in many cases at much shorter 

 intervals. Over 600 pupae of this species were thus fixed, and of these about 

 550 disappeared or emerged. The experiments were conducted in three difi'erent 

 localities — Oxford, Switzerland, and the Isle of Wight — with very divergent 

 results, as will be seen from the following table : — 



It should be noted that the Swiss pupae which were not taken are marked 

 ' left ' because many of them did not emerge, but were removed at the close of the 

 visit and used over again. The results thus tabulated leave no doubt about the 

 existence of an immense amount of extermination at Oxford, and, although much 

 less, a large amount in the Isle of Wight, while there was comparative immunity 

 in the two Swiss localities. This strong contrast is probably to be explained by 

 the scarcity of small birds in the latter, and their abundance in England, and 

 especially in Oxford, together with the fact that the Oxford experiments were con- 

 ducted earlier in the year Other considerations point to the same conclusion — 

 viz. that birds are the enemies in question ; the inaccessible position of the pupae, 

 which were nearly always fixed at a height of about four feet from the ground ; 

 the fact that the hard caudal extremity was frequently left attached after the pupa 



' Poulton, Phil. Trans. Roy. Soc, vol. clxxviii. (1887), B. p. 320 ; and Trans. Eitt. 

 Soc. Lond. 1892, p. 362. 



