SCIENTIFIC NOTES AND OBSERVATIONS. 109 



selective power of a pigment contained in the scales or membrane of 

 the wing and those produced l)y any peculiarities of structure or form of 

 the scale. 



It is abundantly evident that wo have as yet got but little farther 

 in the general subject than I reached in the Bn'iish Noctnae, kc, vol. ii., 

 viz., that insect colours are the result of (1) refraction, interference and 

 diffraction ; (2) the selective power of pigmentary matter. Tliis is 

 granted. The argument so far seems to be ( I) whether certain of the 

 examples I instanced wei'e I'eally due to pigment or to refraction, 

 interference or diffraction ; and under this head we have had the green 

 coloration of Thccla rubi very fully considered ; (2) whether certain 

 white insects, wliicli I did not consider to contain what we have since 

 defined as " pigment-factor," but which I spoke of as " an unstable 

 l)igment," really do contain sucli pigment-factor. It is only by criticism 

 that any true result can be arrived at ; this being so, I have read with 

 interest and pleasure the remarks made thereui^on. Dr. Riding has so 

 far succeeded in showing that two examples that I practically stated 

 to be without such pigment-factor, viz., EucJilo'i cardamines and Hepialus 

 humuli (J , do contain it, although, as shown by my extended notice of 

 the former (I.e., p. vii.) and my remarks on the sexual dimorphism of the 

 latter (ih.), it will be seen that I quite recognised that even such whites 

 as these were linked by almost insensible gradations to yellow, and 

 that possibly the amount of differentiation varied in different specimens. 

 One of the most authoritative papers on the typical white scales of Pieris, 

 is that by Dimmock (Psyche, 1883, p. 66), who states tliat the white 

 scales of Pieris rapae contain air and no appreciable colouring matter. 

 I am afraid I rather carelessly stated my position when I said that 

 the white colour of normal Hepialns humuli $ s, Euchloe, Pieris, 

 &c., was wholly and essentially due to the surface reflection of the 

 incident rays of light. This, of course, is true as a matter of fact, 

 but, so far as I suggested that these were absolutely without jjigment- 

 factor, I was wrong, for I was then aware that there was some small 

 amount of colouring matter present in them. But the doubt expressed 

 in the earlier part of this discussion as to whether certain of these 

 " whites " Avere in reality due to some pigment-factor hardly exists 

 any longer. The presence of a pigment- factor in at least some Pieridae 

 has been set on firm ground by actual chemical experiment and research. 

 The statement made by Dimmock as to the cause of the wliiteness of 

 certain Pierid scales which he found to be hollow and to contain 

 scarcely anything but air is undoubtedly true for the individuals 

 on which experiment was made. It is evident, however, that sucli 

 is not always the case, and from Dr. F. Gowland Hopkins' paper 

 (Proc. Roy. Soc. Lond., 1894), referred to in Ent. Record, vi., p. 13, it is 

 clear that a pigment-factor is present in the wings of certain Pieridae, and 

 that the substance isolated, which has been found to consist of uric acid, 

 functions as pigment, so that the wish of Dr. Freer to have such jjigment- 

 factor isolated has not only l)een consummated but the actual composition 

 stated. I have no douljt that the })igmcnt-factor of other families of 

 butterflies will liave a somewhat similar chemical composition. It 

 appears clear from observations that I have recently made tliat even 

 individual S2)ecimens of the same si)ecies of Pieris may vary in the 

 amount of pigment-factor ]n-esent. Female specimens of Pieris rapae 

 for example, excel the males in tliis respect, and vary in the different 



