FUNCTIONS OF PIGMENTS 197 



performs any direct physiological function or not in the life of the 

 individual. It has been suggested by Gerould that it may play 

 some part in the elimination of CO 2 from the blood, but there is no 

 data either for or against this idea. It may well be, as appears to 

 be the case with carotin, that its presence is incidental and non- 

 injurious, and consequently much of it, instead of being eliminated 

 from the system, accumulates within the tissues. It occurs in 

 larvae which feed exposed during daylight upon plants, as well as 

 in night-feeders which lie so well concealed during the day that 

 they are hard to discover. 



There are many leaf-mining dipterous larvae which feed upon 

 chlorophyll-bearing tissue and yet do not assume a green 

 coloration. In these cases the explanation may be found in the 

 biochemical nature of the secretion of the mid-intestine proving 

 to be destructive to that pigment. There is some evidence that 

 certain anthocyanins, derived from plants, may pass through the 

 walls of the digestive canal in a condition sufficiently unmodified 

 to impart their characteristic colour to specific insects. This has 

 been demonstrated by Hollande in the case of larvae of the beetle 

 Clonus olens, which feed upon the purple staminal hairs of 

 Verhascum. The presence or absence of derived pigments in 

 different insects is a biochemical indicator of certain features in 

 the constitution of these animals. The important role which such 

 pigments play in the ecology of the organisms would, therefore, 

 ap2:)ear to be of a secondary nature. 



(c) It is well known that waste products, such as uric acid 

 and its derivatives, become deposited in the wings of Pierine 

 butterflies. Why this should be the case has not been solved, 

 and in the allied family Nymphalidae no such transfer takes 

 place. Evidence of a similar phenomenon outside the Pieridae is 

 very uncertain. We know that comparatively slight biochemical 

 differences in the constitution of these substances produce very 

 marked colour differences among different species. 



(d) Much has been written with respect to melanism in insects, 

 mainly Lepidoptera. It has long been customary to refer to 

 all and sundry specially dark or black forms of species of this 

 order as being melanic. The term has been applied without 



