CHEMISTRY OF INSECT COLOURS. 169 



no small manner by the fact that so many of the instances that 

 are quoted by writers to support that view are really instances of 

 physical colour ; for instance, the blue-green referred to by 

 Wallace above is probably one of the physical colours, — -in the 

 case of P. niveus certainly so ; and, with regard to the general 

 variation towards blue observed by Dr. Seitz — well, my readers 

 may judge for themselves how heavy are the odds against any 

 blue being a pigment colour. Again, Wallace remarks that the 

 Lepidoptera of the Philippine Islands are characterised by the 

 prevalency of metallic markings : here is a clear local idiosyncracy, 

 but these metallic markings are physical, not pigmental. In the 

 next place, taking the soluble pigment colours, chestnut and yellow, 

 it does not seem eminently probable that any difference of food 

 could lead to a disappearance of these colours, that is, to a ivhite 

 variety. It therefore seems to me that the region for this hypo- 

 thetical action of the food is greatly narrowed, being, indeed, 

 reduced to the following peculiarities: that yellow or chestnut 

 may develop the progressive variation red, that red may (retro- 

 gressively) stop short at yellow or chestnut, that green may stop 

 short at yellow, or finally — a very bold speculation — -that a pig- 

 ment, which ought to develop as green, may — so to speak — take 

 another road after reaching yellow, and thence develop as red (or 

 vice versa). If, therefore, anybody chooses to believe that within 

 this restricted province coloric variation is caused by abnor- 

 malities in the food, there is nothing in my results or the de- 

 ductions therefrom to directly contradict such view ; but at the 

 same time, I am not aware of any evidence at all in its favour. 

 Nevertheless, in thus saying, I would not wish to express at all so 

 complete a scepticism as to the influence of the food in producing 

 slight differences, e. g., in depth or shade of colour, and so on ; but 

 I certainly demur for the present to ascribing much to this factor. 

 Now to state my own view — I should rather incline to look 

 upon the whole affair as a matter of indirect influence ; instead 

 of ascribing any given coloric variety to the presence or absence 

 of a definite chemical constituent in the food, I should suggest 

 whether the varying totality of conditions of the environment 

 generally, may not act by more or less damaging the normal 

 equilibrium of the insect's constitution : the various metabolic 

 processes we know are in the highest degree plastic, and it would 

 be easily intelligible that any unusual factor in the surrounding 

 conditions should react on the constitution, and so indirectly lead 

 to modifications in the pigments, &c., produced. An advantage 

 of this explanation would be that we could include in it all 

 varieties in pigmental colouring,* whether produced by food or 



* It is as though — if I may use the simile — we should, after looking though a 

 kaleidoscope, give it a good shaking ; it is quite certain that this treatment would 

 " affect the constitution " of the kaleidoscope, and lead to a new pattern being 

 produced ; but we are quite unable to prophecy any definite or particular path. 



ENTOM. — JULY, 1891. ? 



