142 COLOUR IN NATURE chap. 



absent in the larvae when they do not occur in the 

 food. 



Mr. Poulton's Experiment 



For the purpose of this experiment Mr. Poulton 

 obtained a large number of the eggs of TrypJuzna 

 pronuba, the Yellow Underwing. Immediately after 

 hatching, the larvae were divided into three sets. The 

 first set was furnished with the yellow etiolated 

 leaves from the heart of a cabbage, the second with 

 the white midribs from the same leaves after the 

 removal of the whole of the blade, and the third with 

 the ordinary green outer leaves of the cabbage. All 

 the specimens were kept in the dark, in order to 

 avoid the risk of the conversion of the etiolin into 

 chlorophyll, and were only brought to the light to be 

 examined and fed. The larvae in the first and third 

 sets developed well and showed almost identical 

 colouring. In the early stages they were mostly pale 

 green, but as development proceeded the colour 

 deepened to dark green, and before maturity was 

 reached most had turned dark brown. In addition 

 to the conspicuous green or brown pigment in the 

 connective tissue, or in the epidermis, the cuticle 

 showed spots and patches of dark pigment, which 

 were most conspicuous in the regions where the cuticle 

 was especially thick, such as the head, the true legs, 

 etc. This pigment is called by Mr. Poulton "true" 

 pigment, and as is shown by the condition of the 

 second set of larvae is not dependent for its formation 

 upon the presence of pigment in the food. 



The larvae in this second set developed badly and 

 out of a large number only one reached complete 



