110 THE entomologist's RECORD. 



broocls. Dr. Gowland Hopkins practically supports the entire assumption 

 on which that part of my paper (i?j-/<. Noct., vol. ii.) which pre-supposed 

 an ill-developed pigmental matter in certain white insects, was based ; 

 since he shows that the " wing scales of the white Pieridae contain 

 uric acid," this substance bearing the " same relation to the scale as do 

 the pigments in the coloured Pieridae, and therefore functioning 

 practically as a Avhite pigment;" such yellow pigment being 

 " artificially " produced by heating uric acid with water in sealed tubes 

 at high temperatures. This was formerly described as " mycomelic 

 acid," but Dr. Hopkins believes that " the substance described was 

 urate of ammonium coloured by a yellow body, and probably identical 

 with the natural pigment." The most startling remark, however, is, 

 that " the described uric acid derivatives though universal in the 

 Pieridae, are apparently confined to this group among the Rhopalocera," 

 followed by the statement that, " when a Pierid mimics an insect be- 

 longing to another family the pigments in the two cases are chemically 

 quite distinct." As I have long insisted on the fact that the wing 

 membrane itself functions largely in colour, I am much interested in 

 observing that the existence of pigments other than scale-pigments 

 is for the first time described ; substances, namely, which are found be- 

 tween the wing membranes, and " which in certain genera are the basis 

 of ornament." Dr. Hopkins also states that " the yellow Pierids on 

 emergence from the chrysalis are apt to void from the rectum a 

 quantity of uric acid coloured by a yellow substance which exactly 

 resembles the pigment of the wing." Although this may be generally, 

 I doubt whether it is absolutely, correct, the excretory matter being, I 

 believe, frequently milky and at other times red in colour. 



How far certain colours in insects are dependent on the selection of 

 certain light- waves by a pigment for reflection, how far on refraction, 

 interference or diffraction, is, however, a matter of the most intense 

 interest, and it is from this point of view that our discussion is likely 

 to prove of value, it is quite possible that many colours are due to a 

 combination of conditions, i.e., that they are partl}^ and perhaps 

 chiefly, due in some cases to pigment, but that at the same time refraction, 

 interference and diffraction are also partly responsible for the total result. 

 I feel satisfied that this is the case in many butterflies, and that, even in 

 some which only a few years ago I considered as being purely due to 

 refraction, diffraction or iridescence, there is considerable evidence of 

 pigment. I may say at once that I quite agree with Dr. Riding, that 

 there can be no such thing in reality as " black pigment." "What I wish 

 to insist upon is, that one form of black found amongst insect colours, 

 appears to have been reached through changes which have taken place 

 in the pigment-factor of the wing, forming in fact part of a well-defined 

 and well-marked genetic sequence, whilst other blacks are apparently 

 entirely independent of pigmental change, and due to some peculiar 

 structure of the scales or membrane. It is quite true that very few so- 

 called black colours are really black, those of the first group I have just 

 mentioned being, indeed, very rarely so ; I believe absorption blacks to 

 be usually the result of modified or changed pigment-factor, and hence 

 but rarely pure black ; blacks due to interference are probably the 

 most pure, if I may use such a term, of the so-called black colours. 

 The only logical explanation that I can suggest concerning Dr. Riding's 

 remarks on Thecla rxihi is, that the colour is a compound one, produced 

 in part by diffraction, the result of the striations noted, partly by the 



