MELANISM 199 



generations, numbering 3,265 moths, were reared. No melanic 

 examples appeared during the course of these experiments, either 

 in the case of individuals fed on the treated food or in the controls. 

 It would seem, therefore, that Harrison's explanation of the 

 appearance of melanics in his cultures as the result of the treat- 

 ment to which the larvae were subjected has not been conclusively 

 demonstrated. The reader is also referred to the paper by 

 Thomsen and Lemche (1933). 



Hasebroek (1926) has studied induced melanism in industrial 

 areas, and arrived at the conclusion that it results through the 

 absorption of volatile substances via the tracheal system. He 

 discountenances the influence of compounds absorbed through 

 the alimentary canal. By exposing pupae of various species of 

 Lepidoptera to the influence of the vapour of pyridene, 

 sulphuretted hydrogen, both separately and in mixture, he was 

 able to produce a considerable degree of melanism. Also sewer 

 gas alone, ammonia, pyridin and chloroform in combination 

 caused melanic tendencies : methane and ammonia also produced 

 pronounced darkening. While his experiments are suggestive, 

 and of great interest, they are open to considerable criticism. 

 In the first place, he does not appear to have used stocks known 

 to be free from melanic tendencies, and, consequently, the 

 possibility that the melanism may have arisen independently of 

 the treatment cannot be wholly excluded. Secondly, most of the 

 species employed do not appear, in England, at any rate, to 

 exhibit any marked melanic tendency in industrial areas, so it 

 may be argued that his experiments do not necessarily bear upon 

 this local problem. Thirdly, no mention is made of any breeding 

 trials being carried out with a view to ascertaining whether the 

 induced melanism is inheritable. 



{e) Most entomologists are familiar with the results of Poulton's 

 experiments (1887, 1892) with reference to the reactions of certain 

 colour-sensitive Lepidopterous larvae and pupae. It will be recalled 

 that with regard to Vanessa urticce and V. io, and also Pieris 

 hrassicce and P. rapw, he showed that when their larvae take up 

 their final resting position upon surfaces, preparatory to spinning 

 their silken attachment, they are in a condition highly sensitive 



