908 REPORT— 1898. 



itself had been taken ; the excessive variation in mortality in neighbouring locali- 

 ties, corresponding to well-known facts in the distribution of birds. A watch was 

 kept in localities where the pupae were known to disappear rapidly, and, in a single 

 instance, a great tit was seen to creep over the bark of a tree, from which the 

 pupa was then found to have gone. 



So far as the observations extended, we inferred that the comparative freedom 

 of the Swiss pupae from the attacks of Vertebrate enemies is compensated by the 

 far gi-eater destruction of the larvae by insect parasites. It is probable that the 

 birds which attack the English pupae benefit the larvae by keeping down the 

 number of parasites. 



In the course of the inquiry the possibility was suggested that perhaps the 

 birds actually see the pupae being suspended, and afterwards search the spot. 

 A large number of pupae were, therefore, fixed at night by the light of a lantern, 

 but, so far as we can judge from general impressions (the analysis of the results 

 being unfinished), no difference was caused. It was also thought that, when 

 several pupae were suspended in close proximity, the birds, after finding one or 

 more, might search the neighbourhood with especial keenness, so that the chances 

 of successful concealment would be smaller than those of isolated pupse. In order 

 to test this suggestion, a number of pupae were scattered over a large area, succes- 

 sive individuals being separated by a distance of about 100 yards or more. Here, 

 too, our present impression is that little, if any, difference was produced. Both 

 these modifications of the usual conditions of experiment were made in the Isle of 

 Wight. 



The question whether there is a struggle for existence during the pupal period 

 of Vanessa urticce is answered with certainty in the afiirmative as regards those 

 localities where small birds are abundant. In such places it is now proved that 

 there is a tremendous struggle with an immense mortality, in spite of the brevity of 

 the pupal stage (from ten days to three weeks in length). 



The attempt was also made to answer the second question whether the struggle 

 is decided by the conspicuousness of the pupa. First, as to conspicuousuess in 

 form, pupae were fixed to surfaces which unequally concealed them ; thus thorough 

 surfaces of stone and bark (rough-barked trees being almost invariably selected), 

 and the shelter afi'orded by overhanging leaves of nettle, concealed their rough 

 angular forms far more than the comparatively smooth surface of fences. Looking 

 at the table on page 907, it is seen that at Oxford butterflies only emerged from 

 pupae fixed to nettles, while in the Isle of Wight the mortality on fences 

 (90 taken to 8 emerged), was enormously greater than on bark (135 to 84), walls 

 (14 to 12), and nettle (20 to 15). When therefore the pupa is suspended from a 

 surface against which it stands out conspicuously, it is in far greater danger than 

 when it is fixed to one upon which it is concealed. This result is inexplicable, 

 except on the theory that the sense of sight is important to enemies in the dis- 

 covery of the pupae. 



Secondly, as to conspicuousness in colour. In Nature the golden forms of the 

 pupa of this species are produced upon nettle, the darker forms on walls, rocks, 

 fences, and probably bark ; furthermore the darkest varieties are produced on the 

 darkest surfaces. In fixing the pupae, part were distributed as they are in Nature, 

 while part were given a reverse arrangement — dark forms being fixed to nettle, and 

 golden forms to black fences and dark surfaces of bark, &c. Some of the experi- 

 ments gave extremely positive results, in that the mortality among the latter pupse 

 was far higher than among the former ; other experiments were negative. Until 

 the whole of the experiments have been analysed in far more detail than has as yet 

 been possible, we caimot make any statement as to the general bearing of the 

 inquiry upon the danger or otherwise of conspicuousness in colour, although the 

 danger of conspicuousness of form has been shown to be conclusively proved. 



It may be supposed that the experiments were vitiated by the accidental loss 

 ot the pupae. Many considerations, however, indicate that no serious error has 

 been introduced in this way. In the absence of enemies, in Switzerland, the pupae 

 remained suspended until they emerged or until we removed them, and this was 

 also the case in the places of small mortality in the Isle of Wight ; in many place? 



