CHEMISTRY OF INSECT COLOURS. 167 



I may add that heat has nothing whatever to do with this 

 cyanide reaction, since the above results were obtained in the cold. 

 With regard to the chemistry of the reaction, my own notion is 

 that probably the red is produced by a combination of the 

 cyanogen radicle with the yellow pigment ; but all speculation of 

 this sort is better left for the future. It will, of course, be 

 understood that the views suggested by this reaction are scarcely 

 more than hinted at here ; a fuller discussion of the whole subject 

 is reserved for a later date.* 



VL— The Biological Aspect. 



This concluding section will be, as I have already stated, very 

 brief, since it is intended chiefly to draw attention, by quoting a 

 few examples, to the connection between ray artificially prepared 

 varieties and those found naturally occurring ; and my acquaint- 

 ance with the literature of "varieties," as well as with large 

 collections, is far too slight to allow me to go much beyond 

 general suggestions. 



First of all, however, it is necessary to refer to one question — 

 of some considerable interest — that is naturally suggested by any 

 account of chemical experiments on insect colours ; the question, 

 I mean, as to what influence the chemical nature of the soil or food, 

 and even temperature, may have on the colours of insects in the 

 natural state. There is, of course, a general impression among 

 entomologists that such influences may account for a large per- 

 centage of variation, especially when in any one district such 

 variation is in one general direction ; for instance, I quoted, in 

 the last section,t Dr. Seitz's statement that in one restricted 

 forest area of Brazil all the species of insects showed a great 

 tendency to blue coloration : Wallace, too, remarks I that *' in 

 Tropical Africa we find two unrelated groups (in the Nympha- 

 lidse and Papilionidse respectively) characterised by a prevailing 

 blue-green colour not found in any other continent," these groups 

 being Romalesoma and Euryphene, and Papilio zalmaxis and 

 several of the P. nireus group. And again, Wallace§ states that 

 "species of totally distinct groups are coloured alike in one 

 district, while in another district the allied species all undergo 

 the same change of colour. The most probable cause for these 

 simultaneous varieties would seem to be the presence of peculiar 



* May, 1891. Since writing the above I have made various further experiments, 

 details of which I, however, reserve for the present. Still, I may say that the 

 reddening effect has been obtained with several other yellow species, all of which 

 belong to the Rhopalocera. I have succeeded in reddening no yellow moth yet, 

 neither have I found a similar reaction to occur with any other nitrogenous com- 

 pound besides potassium cyanide. 



t Entom. p. 1.38, supra. 



X ' Tropical Nature,' p. 256. 



§ I do not remember whether Wallace expressly ascribes this phenomena to 

 any supposed local chemical influences or not. 



