﻿284 THE ENTOMOLOGIST. 



confession I shall consider myself very badly treated if any 

 readers accuse me of dogmatism or illicit assumption ; and as to 

 the appropriateness or otherwise of such working hypotheses, I 

 may remind them that these articles were headed " Preliminary 

 Notices," — being indeed merely an account (published at this 

 somewhat early stage for reasons explained in the first section) 

 of preliminary experiments on British Lepidoptera, by way of 

 preparation for a considerably more extensive enquiry. 



Now before considering the actual experiments, we shall, 

 I think, do well to clear the ground of one or two matters, rather 

 than plunge at once in medias res. These articles are entitled 

 " On the Chemistry of Insect Colours," and there has, so far, 

 been allowed perhaps a tacit assumption that there is a chemistry 

 of — all — insect colours. Whereas there is not. It is superfluous 

 for me to remind readers of ' The Entomologist ' that it has long 

 since been opined by various naturalists that in many cases the 

 colours of insects are due, — not to any pigment at all, but to the 

 minute physical structure of the surface ; and this class of colours 

 may be conveniently, though somewhat loosely, referred to as 

 physical colours, in distinction from chemical or pigment colours. 

 Readers of Wallace's delightful book, ' Tropical Nature,' will re- 

 member that he alludes on several occasions to these "physical 

 colours," — especially with reference to Lepidoptera and other 

 insects. For instance, he conjectures that " many of the glossy 

 or metallic tints of insects, as well as those of the feathers of 

 some birds," are due to interference colours ; and states also that 

 " The iridescent colours of the wings of dragon-flies are caused 

 by the superposition of two or more transparent lamellae, while 

 the shining blue of the purple emperor and other butterflies, and 

 the intensely metallic colours of humming-birds, are probably 

 due to fine strise. "* Then, again, Mr. Cockerellf -quotes Dr. 

 Dimmock as opining that the white of Pieris and the silver of 

 {e. g.) Argynnis are both due to physical structure, and not to any 

 pigment. So that clearly naturalists have long been fully awake 

 to the fact that in discussing the colours of insects (and other 

 groups) we must discriminate between the chemical and the physical 

 colours ; although I am not aware that there has so far been much 

 formal evidence forthcoming concerning these physical colours. I 



* On the other hand Wallace, I think, goes too far in stating that pigmental 

 colours "comprise all the opaque colours of flowers and insects." I should call 

 Pieris -white, for instance, a very opaque colour ; and among Heterocera opaque 

 colours are abundant which are— so far, that is, as I can at present determine — not 

 pigment colours, 



t Entom. xxii. 126-7. 



I At least, not until very lately. I see, however, by a recent article in ' Nature,' 

 that Dr. Hodgkinson has published a paper on the " Physical Cause of Colour," 

 recording experiments tending to show whether the colour is caused by a structure 

 of thin plates or of fine lines. I have, however, not yet had an opportunity of 

 consulting Dr. Hodgkinson's paper, 



