ISM.] 45 



trace of the ventral cord be discoverable on the under-side of the body. 

 And so frequently was this the case with the material first examined, 

 that I almost came to the conclusion that the presence of the former 

 presupposed the absence (of course only apparent) of the latter. Now, 

 strange to say, the want of agreement in the colour of the supra- and 

 infra-sesophageal parts of the nervous system was found to be con- 

 nected with the position of the larva in the mine. Like many leaf- 

 miners, a large number, probably one-third, of these larvae mine upside 

 down, I mean with the venter uppermost, and where this was the case, 

 there it was that the ventral cord was over-coloured and visible, whilst 

 when the back was uppermost, then the cephalic ganglia were the 

 parts to have their colour intensified. What is the meaning of this ? 

 Light being unquestionably the most general and potent factor in the 

 production of pigmentation, it is reasonable to imagine, that pouring 

 in through the transparent tissues of these small creatures on to the 

 nerve ganglia, it will, in the course of generations, exaggerate or in- 

 tensify their colour, and the more readily, because some amount of 

 pigmentation is always present in these parts, and appears to be 

 essential to their constitution. Hence, when the head is uppermost, 

 the light falls upon the cephalic ganglia and in long process of time 

 blackens them, but cannot so well reach the ventral cord, which is 

 protected by the contents of the intestinal canal and so retains its 

 primitive colouring ; on the other hand, when the venter is uppermost, 

 the condition of things is reversed, and the cephalic ganglia remain 

 unchanged or only slightly affected, whilst the ventral cord is darkened. 

 I have already hinted that the rule is not invariable, and that in 

 many larvae both cephalic ganglia and ventral cord are visible, but even 

 then there is a difference in the depth of colour, and the darker of 

 the two is the one which, by being uppermost, is the most exposed to 

 the light. Such, then, I venture to suggest, has been the action of 

 light upon the tissues ; equally interesting has been the action of the 

 tissues upon the light, for it is here, I believe, we shall find the reason 

 for the high colouring of the nerve centres in so many of the yellow 

 larvfe, and its remarkable absence in the bright green ones (the ventral 

 cord, so far as my experience goes, is always quite imperceptible, and 

 the cephalic ganglia are only faintly discernible in one or two of them). 

 The tissues, as the colours of the animals testify, break up the light 

 into its component parts, retaining and appropriating some of the rays, 

 and rejecting and throwing off the others. In the green larva? it is 

 the actinic or chemical rays that are rejected, whilst they are the ones 

 that arc retained in the yellow larvrc. Consequently, it seems to mo 



