I20 YORKSHIRE NATURALISTS' UNION, 



perfect insect ; a very feeble current resulting in the production 

 of black spots. Atmospheric electricity may perhaps be a factor 

 in the natural process, especially in arctic or high mountain 

 regions where it is most abundant ; but if so, how and why does 

 it act in this manner ? 



Mr. de Vismes Kane, in his address to the Barnsley 

 Naturalists' Society, * on the Variation of European Lepidop- 

 tera, properly pointed out that ' all naturalists are agreed that 

 the strongest developed variations are to be found generally in 

 places where creatures are cut off from intercommunication 

 with the rest of their kind by mountains, vast forests, the sea, 

 or other natural barrier. But, although, as he says, ' isolation 

 begets peculiarity,' I am unable to agree that this is the princi- 

 pal cause of the aberrant colouration of the Shetland insects. 



Mr. Lewis, in the paper to which I have referred, f argues 

 in favour of colour in insects being due to a ' photoplastic ' 

 process, ' the rays or wave-movements from the sun impressing 

 living organisms with the structure necessary for colour.' He 

 thinks that nocturnal insects are black because not affected by 

 direct rays of the sun. His theory, as I understand it, is that this 

 process is mechanical, effected by the varying rapidity of vibra- 

 tory motion in the different waves of heat-rays in the solar 

 spectrum, rather than chemical, through those rays acting upon 

 the constituents which go to form pigment. 



His theory is for the most part applied to brightly-coloured 

 Lepidoptera and Coleoptera exposed to much sun, and he pro- 

 duces a valuable array of facts in support of it. 



This theory appears to me* to be open to much criticism, 

 but for the moment we are concerned only to apply it to the 

 special instances of more or less melanic varieties. 



* The Naturalist, Nov., 1884, p. 82. 



t Transactions of Entomological Society, 1882, p. 504. 



Trans.Y.N.U., 1883 (pub. 1885). Series D 



