36 THE entomologist's record. 



colour is due to tlie fact that, of the light which falls upon the wing, the 

 yellow only is reflected to the eye, the rest being absorbed. Now, in 

 the very nature of things, white bodies cannot be pigmented, that is, 

 they cannot contain a substance which absorbs part of the light and 

 returns the other part to the eye, for the light is all reflected. But, in 

 some of our white insects, there is undoubtedly a substance which is 

 not pigmentary in itself, but which, under certain chemical re-agents, 

 becomes such. Now, when it does this, it is a pigmentary substance, 

 but until it does it should not be called so. We want, however, a 

 name for this substance rather better than " pigmentary " white, and 

 Dr. Eiding has already suggested that we should term it " potential 

 white," a very useful and suggestive term. Now that whites are either 

 " potential " or " absolute " is certain ; which are " potential " is a 

 matter for enquiry. In The British Noctuae, &c. (Introduction), vol. ii., 

 I instanced cases which I considered fell under these heads. In some, 

 I probably erred, but the essay contains a full account of what was 

 available at that time. The matter requires, however, to be gone over 

 again very carefully. Dr. Freer says, " The reaction of chemicals may 

 mean anything." How? I would enquire. I feel satisfied that when the 

 white is a recent derivative from yellow, we shall get results in the 

 direction of " potential " whites. It seems that there must be in such 

 instances some trace, in a loose chemical form, of the pigmentary 

 matter so recently lost. I have, however, so much work on hand 

 now. that I cannot possibly give the necessary attention to the matter, 

 and Dr. Freer and Dr. Eiding will, I have no doubt, come to a common 

 understanding as a result of the discussion. — J. W. Tutt. Oct. 20th, 

 1894. 



I am afraid that in my last note I was neither exjjlicit nor clear, with 

 regard to the white-pigment question. The matter really seems to de- 

 pend on a definition of pigment. My idea is, that we call anything 

 pigment which absorbs any colour rays. Transparent substances 

 evidently contain no pigment ; nor do those colours and metallic 

 appearances which are due to the refraction of light through innumerable 

 transparent films of different refractive indices come under this head. 

 Such are the metallic colours in pupa?, in which the chitinous film is 

 alternated with fluid, and such I deem the cause of the metallic color- 

 ation in most insects. The component })arts of the metallic scales may 

 be of different refractive indices, yet no striation be visible in the scale 

 under the most powerful microscope. This metallic coloration dis- 

 appears with transmitted light, which seems to bear out this theory. 

 My reason for mentioning the pigment granules was, that in tliem there 

 is a visible expression of pigment analogous to the pigment found in tlie 

 higher animals. The granules are too few in light-colo\ired insects, to 

 influence their coloration in any way, but are exceedingly numerous in 

 the black parts of some insects, and in all melanic specimens ; in fact, 

 there seem to be two kinds of black — one, the black of Pierinae, Colitm 

 edma, and all butterflies which I have examined in which pigment granules 

 are comparatively scarce ; the other, in most moths in which they are 

 plentiful, especially in the case of Amphidasys hetularia. In a pajjer I 

 read before the Birmingham Entom. Society, I suggested that this pig- 

 ment might be an expression of energy. With regard to the coloration 

 of insects, I cannot conceive that their behaviour towards chemical re- 

 agents can be of any phylogenetic importance. For example, Mr. 



