190 THE FUNDAMENTAL ASPECT OF COLORATION 



The red and yellowish hypodermal pigment present in the 

 wing-scales of certain butterflies, particularly Vanessa io and 

 V. urticce, plays a direct part in the coloration of those insects. 

 The researches of the Grafin von Linden (1905) indicate that it 

 is derived from the chlorophyll absorbed during larval life. 

 The transformation commences in the epithelial cells lining the 

 digestive canal, which contain at first green granules derived from 

 the chlorophyll of the food. So long as the digestive juices of 

 the larva remain alkaline little change supervenes, but when 

 transformation to the pupa is taking place the gut contents 

 become acid and the colouring matter changes to red. During the 

 process of change the red colour results from the combination of 

 the actual pigmentary material with an albuminoid substance. The 

 final colour assumed by the pigment in the hypodermis depends 

 essentially upon its degree of oxidation ; in its reduced condition 

 it is red-carmine, while oxidation transforms it into a dull greenish- 

 yellow. The pigment is soluble in hot or cold water, in neutral 

 salts and mineral acids, but is insoluble in organic solvents. By 

 the nature of its crystals, its spectrum and other properties it 

 shows itself closely allied to bilirubin. On the other hand, it 

 resembles haemoglobin in that it represents the combination of an 

 iron-containing pigment with an albuminoid substance, and in the 

 facility with which it forms unstable combinations with oxygen. 

 Hollande (1923) maintains that the origin of the red pigment 

 from chlorophyll is insufficiently established, and claims that its 

 formation is practically the reverse of that described, in that it is 

 primarily hypodermal in origin. When pupation is about to occur 

 disintegration of the larval hypodermis ensues, and the red 

 granules which it contains become liberated into the blood and 

 are then absorbed by the digestive canal. Any excess of this 

 material passes into the gut cavity and is ultimately excreted to 

 the exterior. One is inclined to agree with Verne (1926) that the 

 processes described by the Grafin are better substantiated than 

 the more hypothetical origin explained by Hollande. 



3. Pigments of Protein Origin. The pigments coming under this 

 category are those of the melanin group. Knowledge of their 

 chemistry is still very restricted, but they have been shown to 



