SCIENTIFIC NOTES AND OBSERVATIONS. 37 



Perry Coste {Entomologist, xxiii., p. 371) gives as "a good instance of 

 retrogressive naetamorphosis," or change to original type, the fact of the 

 green under-side of Thecla rnhi being changed by every reagent to the 

 brown of the upper side ; if he had gone further he would have found 

 that when wetted with water the same effect was produced. The green, 

 in fact, is what Dr. Riding calls a " potential " coloui-, produced by the 

 refractive projierties of the upper layer of scales, but that is no proof 

 that the original colour of the underside was brown, nor that because 

 ammonia turns some whites to yellow and some greens to the same 

 colour, that those greens and whites are in any way connected. In 

 fact, I do not see that an^'thing can be deduced from the behaviour of 

 chemicals towards the colour of insects, until the colour has been 

 dissolved out and isolated. All other true animal and vegetable pig- 

 ments are capable of isolation (such as the various blood pigments, and 

 their derivatives), and until this is done, I think we must consider all 

 colours in an insect's wing " potential," " physical," or what you will, 

 but not pigmentary. Even my pigment granules have slender grounds 

 to be called pigment, but how much less have those colours whose sole 

 claim to be considered pigments, rests on their naked eye aj^pearances. 

 The very scales of an insect's wings may be of different refractive in- 

 dices, and by their super-imposition may produce the various colours. 

 The upper layer of scales being wetted and otherwise damaged may 

 cause them to have refractive powers, and the under layer would then 

 shine through. Again, mineral acids act as dyes on many substances, 

 and may so act on the chitin. — R. Freer, M.B. Oct. 30th, 1894. 



I cannot say that I agi'ee with Dr. Freer that " the microscope is a 

 much better witness to the presence of pigment than the reaction of 

 chemicals." We do not go through the world with our eyes looking 

 through a microscope, and we must and do trust our own unaided 

 sense of sight when we speak of colours. After all, when we use the 

 word " pigment," are we not forgetting that there is, in reality, no such 

 thing as colour at all — no material, tangible pigment — and that the 

 word is merely a convenient one for describing a process that has 

 three essential factors — (1) a substance observed, (2) certain rays of 

 light with various rates of vibration per minute, (3) the eye of an 

 observer — all of which are wanted to produce the sense of colour in us? 

 Indeed, Avhat we call pigment should rather be called " jjigment-factor," 

 and with that meaning I use it. I do not think I have anywhere 

 stated that the action of chemical re-agents on the pigments of insects 

 has a phjdogenetic value. What I wrote was, that 1 had observed the 

 action of ammonia to vary in different specimens of <? Hepialns hnmuli, 

 in intensity and permanency (and jDresence according to some), and 

 suggested it as possible, that we may find such change taking place in 

 certain localities onl3\ I have, as yet, formed no opinion on the 

 subject, but at the same time I cannot admit there is an3'thing un- 

 reasonable in the supposition. Are not the colours of insects influenced 

 by locality (natural selection transmitting the varieties) ? Why should 

 not the chemical constituents of soil, absorbed by })lants and eaten by 

 larvaB with their food, influence the imago ? or the moisture and 

 emanations from vegetable and other surfaces, etc., of their environ- 

 ment? What do we know of tlie essence of variation ? Why, no two 

 people are alike, and probably no two insects — the Dromios wanting 

 carefurdifferentiation only ? How the chemical re-agents act on the 

 pigment to produce change of colour is another question. 



