An Experimental Enquiry into the Strngyle for Existence in Certain 

 Common Insects. By Edward B. Poultox, M.A., F.R.S., Hope 

 Professor of Zoology, Oxford, and Cora* B. Sanders. 



[An abstract of a paper read before Section D of the British Association at Bristol, 

 on Monday, September 13, 1898. Reprinted from the ' Report ' (pp. 90G-909) of the 

 meeting.] 



Many Lepidoptera have been proved to possess the power of adjusting the 

 larval or pupal colours to those of the immediate surroundings. This power can 

 only be exercised once in the case of the pupa (viz. at the end of larval life), and 

 rarely, if ever, more than once or twice in the case of the larva. Many naturalists 

 consider that the power is protective, and has been produced by the operation of 

 natural selection. Others have doubted this conclusion, and W. Bateson ' has 

 attempted to cut away the foundation of such an interpretation as regards the 

 pupa of Vanessa urticce by arguing that there is no struggle for existence during 

 this brief stage. This argument was opposed, and the lines of an e.xperimental 

 enquiry were suggested in the same year by one of us.- In the discussion which 

 followed a paper on mimicry, read before the Linnean Society on March 17, 1898,'' 

 it was strongly urged, especially by Professor Weldon, that such an experimental 

 enquiry should be conducted. Our present work is the outcome of that discus.sion, 

 and we desire to express our thanks to the Government Grant Committee of the 

 Royal Society for assistance in carrying on the investigation. 



We determined to concentrate our attention on the pupoe 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 struggle for existence during the pupal stage ; (2) whether the struggle, if it 

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



The enquiry was almost confined to the pupa of Vanessa urticce, 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 

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

 The pupae were then fixed to the sui-faces 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 degi-ee 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 different 



' Trans, mt. Soc. Lond. 1892, pp. 212, 21.3. 



■' Poulton, Trans. Ent. Soc. Lond. 1892, pp. 471-477. 



^ Poulton, Natural Selection the Cause of Mimetic Resemblance and Commoti 

 Warning CoUmrs. Not yet published. 



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

 Svc. Lond. 1892, p. 362. 



d4: 



