168 THE ENTOMOLOGIST. 



elements or chemical compounds in the soil, the water, or the 

 atmosphere, or of special organic substances in the vegetation ; 

 and a wide field is therefore offered for chemical investigation in 

 connection with this subject," 



Now, a 'priori, this is undoubtedly a very taking explanation, 

 and it is one that I adopted with great enthusiasm at the com- 

 mencement of my experiments, expecting them, indeed, to throw 

 considerable light thereupon;* but I must admit that I have by 

 no means so much faith in the principle now ; indeed, I should 

 incline — so far as I can see any data for forming an opinion at 

 all — to ascribe to it exceedingly limited efiicacy. Of course, I 

 am very conscious of the fact that there is a vast difference 

 between experimenting by external reagents on the colours of 

 dead insects, and experimenting by means of food, that is, by 

 reagents internally administered to living insects, and leading to 

 we know not what modifications of their metabolic processes ; 

 but nevertheless, I think that the results of my experiments do 

 give us sufficient insight into the relations of the various colours 

 to make a cautious argument to the living processes legitimate. 

 Here I may recall the speculations that were referred to in the 

 second section of these articles ;t for — as was then pointed out — 

 if it had been found that certain reagents produced one coloric 

 change, and certain others quite another, and so on, tlun we 

 should have had considerable reason to suppose that some change 

 in the constituents of the food, the presence of some unusually 

 large proportion of one organic acid, e. ^., or of some unusual body, 

 as, for instance, hydrocyanic acid in the peaches and almonds, 

 might lead to some colour variation ; but as it is, the experi- 

 mental results give no support at all to such views ; and the fact 

 that chemically the most opposite compounds produce a similar 

 effect on the colours has made me very sceptical as to the 

 potency of variation in food to cause any marked coloric change. 

 I would not be understood to deny such influence in toto ; for 

 instance, it is stated — and, I suppose, on sufficiently good 

 authority — that dark varieties of A. caia may be obtained by 

 feeding the larvse on walnut leaves, and again, that good varieties 

 may be got by feeding on plants standing in salt water ; but 

 I doubt if any great coloric change will be obtained by any such 

 direct means. Let us look a little deeper into the question ; we 

 have three classes of colours, viz., the physical, the soluble 

 pigmental colours, and the "reversible" pigmental. Now, as to 

 the physical colours, any direct chemical action of the food| on 

 these is out of the question, and my scepticism concerning the 

 efiicacy of the first to cause coloric variation is strengthened in 



* Cf. Entom. 1890, p. 156. t Cf. Entom. 1890, pp. 156, 157. 



X I am confining myself to the question of the chemical influences of the food, 

 since it seems clear that any chemical idiosyncracies of the soil or the water would 

 only act via the food— viz., by affecting that. I will refer to the atmosphere and 

 temperature effects directly. 



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