﻿CHEMISTRY OF INSECT COLOURS. 373 



of giving each a seperate handling : for indeed, whilst red, both 

 logically and historically, follows yellow, yet it is impossible to 

 fully discuss the phenomena of yellow without constant reference 

 to red. I may add tbat in no group of colours is there more 

 temptation — and even necessity — for speculation and hypothesis 

 than in this : and in no part of my work have I had so many 

 warnings of the dangers attendant on too speedy generalisation 

 and the necessity for circumspection. It is fortunate that the 

 colours which most especially tempt one to such dangers should 

 at the same time preach the most emphatically against the 

 rashness which they provoke. 



Referring now to the tables on pp. 248, 249, what do we find 

 as the general results ? As to red (omitting the cases of one or 

 two species, as Vanessa atalanta and V. cardui, which can be 

 better explained after dealing with chestnut), we have a singular 

 unanimity, as interesting as important, and can lay down the very 

 general law* that chemical reagents change red to yellow ; or, 

 translating this from the language of empirical facts into that of 

 theory ,f that red is retrogressively modified into yellow — from 

 which it was originally developed. In the species experimented 

 upon we have every subdivision of the Macro-Lepidoptera repre- 

 sented, and it appears to me an exceedingly interesting fact that 

 red is chemically always one and the same colour, however 

 different in superficial appearance, J and however far apart phylo- 

 genetically be the species thus adorned : red is one and the 

 same in the Rhopalocera, Sphinges, Bombyces, Noctuse, and 

 Geometra3.§ 



We have therefore this general result : that by the action of 

 chemical reagents|| red is instantly changed to yellow, but there the 

 change stops ; and by no means can this yellow so produced be 

 altered or affected in any way. Here we find ourselves at once 

 face to face with the problem of the character of the yellow 

 pigment, and to a consideration of that we must turn before we 

 can proceed further with red. 



Again referring to the table on p. 248, what do we find is the 



* I omit altogether any reference to Papilio machaon, since the colour here — as 

 evidenced even by its superficial appearance — is not a true red. Any significance 

 that might be attached to this one species, as typical of the Papilionidte, is entirely 

 destroyed by the fact that in species of tropical Papilios sent me (e. g., P. hector, 

 marked with brilliant vermilion) red is instantly changed to yellow, as in all other 

 groups. 



t See pp. 370, 371. 



{ As exemplified, e. g., by the pink of Deiopeia and Sphinges, the scarlet of 

 Parnassius apollo, the brilliant reds of Bombyces, and the unhealthy reddish of 

 Xanthia. 



§ Red being so rare a colour in Geometraa, I was exceedingly anxious to ascer- 

 tain whether red Geometree would behave as all other red species ; and it was with 

 considerable interest that I examined the two species, Hyria auroraria and Acidalia 

 rubricata, kindly sent me by several correspondents. These species will again bo 

 alluded to in another section. 



II Note that red is far more sensitive to acids than to alkalies. 



