3 6 
COLOUR IN NATURE 
CHAP. 
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 
