428 Mr. E. B. Poulton. Hxperimental Proof that the Colours 
It is of great interest that the 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 becomes dark 
coloured on exposure to air, probably by oxidation. Thus of the two 
heaps of feeces 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 feces containing abundant etiolin. The 
feeces of the larve 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 OF PLATES 3 AND 4. 
Plate 3. 
Hig. 1.—A group of five larve of Tryphena pronuba in the 4th 
stage, natural size. These larve 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 larve 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 larve can form a deep green colouring matter 
from etiolated leaves. There was, in fact, no difference in this 
respect between them and larvee fed on green leaves. 
The marked contrast in colour between the green larve 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 feeces are represented in the figure: that to the right 
