428 Mr. E. B. Poulton. Experimental Proof that the Colours 



It is of great interest that tlie etiolin should be as effective as 

 chlorophyll in the production of larval colours. It is, however, prob- 

 able that the difference between etiolin and chlorophyll is, chemically, 

 extremely small, while both appear to undergo similar changes in the 

 larval digestive tract, yielding a substance which becojnes dark 

 coloured on exposure to air, probably by oxidation. Thus of th9 two 

 heaps of faeces represented in Plate 3, fig. 1, that to the left had been 

 exposed to the air for some hours, and was dark brown, while that to 

 the right was fresh and of a pale-yellow tint. A cut midrib darkens 

 on exposure quite independently of the plant pigments as may be 

 seen in the same figure ; but the tint is different, and the depth of 

 shade far less than in the faeces containing abundant etiolin. The 

 fasces of the larvae fed on pigmentless food similarly darkened far 

 less rapidly and to a much less extent than those of the others. 



Although the results of these experiments are, I believe, completely 

 successful in establishing the conclusion they were intended to test, 

 it must be admitted that they point to the beginning of an investiga- 

 tion rather than its end. We now know that certain larval colours 

 are dependent on the existence of modified plant pigments, and this 

 naturally leads to an enquiry into the nature and causes of the 

 processes by which chlorophyll and etiolin are converted in the 

 animal body into a comparatively stable green or brown substance 

 far removed from its original position in the digestive tract, and 

 situated so as to form an important element in the effective colouring 

 of the individual. 



.'. DESCRIPTION OP PLATES 3 AND 4. 

 Plate 3. 



Fig, 1. A group of five larvae of Tryphcena pronuba in the 4th 

 stage, natural size. These larvae had been fed entirely upon the 

 etiolated leaves of cabbage. They had hatched September 7 and 

 8 (1893) and were painted October 10. Nearly all the shades of 

 colour observed in the larvae at this stage are represented in the 

 figure, four being various shades of green, and one brown. It is clear 

 from the figure that the larvae can form a deep green colouring matter 

 from etiolated leaves. There was, in fact, no difference in this 

 respect between them and larvae fed on green leaves. 



The marked contrast in colour between the green larvae and the 

 leaf is some indication of the change which the etiolin has undergone 

 in the larval body. The dark marks along the sides are due to 

 superficially placed true pigment, which is formed independently of 

 any coloured substance in the food plant. 



Two heaps of faeces are represented in the figure : that to the right 



