591.1 173 



III. 



The Pigments of Animals. 



Part II. 



ANOTHER group of pigments which of late has been much studied 

 includes those which are physiologically the ordinary nitrogenous 

 waste-products of the organism in which they occur, or are produced 

 by a modification of these. This employment of waste products in 

 coloration is best exemplified in insects, especially Lepidoptera, but 

 is also a familiar phenomenon in fishes. As regards the latter class, 

 we have a beautifully illustrated research by Mr. Cunningham and 

 Dr. MacMunn (3), which contains many interesting facts as to the 

 part played by guanin in the coloration of the skin, especially of the 

 Pleuronectidae. In the case of butterflies there are numerous papers 

 which require more detailed consideration. 



The colours of butterflies are partly structural and partly due to 

 pigment. The following colours are usually due to pigments : — white 

 (in part), yellow, red (at least in part), brown, black (in part), and, 

 very rarely, green. On the other hand, so far as is at present known, 

 blue seems to be always a structural colour, green and white are 

 usually structural, as is also black in part; metallic or changing 

 colours are of course always optical effects. The distinction of the 

 two kinds of colour, however, presents considerable difficulty in 

 practice, and it is therefore not surprising that at this early stage of 

 investigation there should exist some confusion as to their relations. 

 This confusion has been to some extent increased by the fact that 

 many of the pigments have been investigated only from the chemical 

 side, and not by a combination of microscopic and chemical observa- 

 tions. A recent paper by Spuler (13), however, contains a careful 

 summary of other researches, as well as an account of his own 

 observations on butterflies' scales, which is of great assistance in the 

 interpretation of the result of others. In spite of this, the uncertainty 

 is such that the lists given above must be regarded rather as the 

 balance of probabilities at the present time, than as a statement of 

 assured fact. 



As structural and pigmental colours in butterflies can thus not 

 be very exactly separated, it is perhaps advisable to give some 

 account of Spuler's results. 



Spuler is primarily concerned with optical colours, which he finds 



