36 COLOUR IN NATURE 



3. Waste Products 



The next group, the pigments which are definitely 

 waste products, or are produced by the modification 

 of waste products, has only been seriously studied 

 very recently. The researches of Mr. F. Gowland 

 Hopkins in this country, and of Dr. Urech abroad, 

 as well as of others, have demonstrated within the 

 last few years the extremely interesting and im- 

 portant fact that the colours of the butterflies 

 belonging to the family of the Pieridae are due to 

 pigments which are modifications of the ordinary 

 waste products of the organism. Hopkins's discovery 

 that the yellow pigment which he calls lepidotic acid, 

 found in the wings of the Pieridae, occurs also as one 

 of the normal waste products of the organism, is one 

 of extreme interest to the comparative physiologist. 

 It is well known that in higher Vertebrates death 

 follows with extreme rapidity upon the removal of 

 the kidneys, and it is usually stated by physiologists 

 that it is the nitrogenous waste products themselves, 

 or their precursors, which poison the animal ; in the 

 case of butterflies, however, we have a modification 

 of uric acid stored up in the wings, functioning as a 

 colour-producing substance, while the same substance 

 is eliminated from the body as a waste product at 

 the time of the emergence from the pupa state. We 

 are thus forced to suppose that the wings of butter- 

 flies, being relatively non- vital parts, can have 

 poisonous substances stored up in them without 

 injury to the organism, and that therefore the 

 utilisation of waste products as colouring agents can 



