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Mr. E. B. Poulton. Essential Nature of the [Apr. 23, 



sequent transparency ; but the changes are slight compared with 

 those shown by green larvae, and are evidently due to the hypodermis 

 pigment being in amount or in character insufficient to render the 

 surface entirely opaque. In the instances mentioned above as being 

 identical in appearance with the living larvae the pigment is entirely 

 opaque. In such larvae some of the pigment is often deposited in the 

 cuticle, and is therefore visible in the latter after being thrown off at 

 ecdysis. As a rule the cast off cuticle is transparent and colourless, 

 except the last one thrown off at pupation, which is generally dis- 

 coloured and brown, because it is moistened by the fluid which covers 

 the pupa, and which hardens and darkens into the protective layer of 

 the latter. The effect in spirit specimens is even more striking than 

 in the other method of preservation, for the green colour is not merely 

 changed, but has entirely disappeared, leaving the surfaces colourless. 



This is very obvious with larval colouration into which both factors 

 prominently enter. Thus in Dicranura Y inula (kept for a year and a 

 half in spirit) the green ground colour has entirely faded, and the 

 spirit is yellow with dissolved xanthophyll, while the large purple 

 area on the back with its white marginal line is entirely normal. 



4. Colours due to Larval Tissues independently of Pigment. 



There is one sentence in Dr. Weismann's essay (on the markings 

 of caterpillars) which shows that he attributes importance to the 

 deeply placed substances in producing larval markings. Speaking of 

 the second stage of the larva of Macroglossa Stellatarum (p. 246, 

 English translation), he says: — "A dark green dorsal line, which, 

 however, does not arise from the deposition of pigment, as is gene- 

 rally the case, but from a division in the folds of the fatty 

 tissue along this position." According to my experience the dorsal 

 line is generally formed in this way when its colour is similar 

 to that of the ground colour, except for a deeper tint. This divi- 

 sion in the folds of fat is occupied by the dorsal vessel, and the 

 latter can be distinctly seen contracting; its margins being thus 

 shown to be coextensive with the dorsal line, which is therefore 

 really due to the derived pigment in the blood. (This is true of 

 8. Ocellatus, Sphinx Ligustri, &c.) But the superficial colours can 

 be caused by an accumulation of internal tissues immediately beneath 

 the transparent skin quite independently of pigment. Weismann 

 implies in the above-quoted sentence that the fat exercises a modifying 

 influence upon colour. But it may do more than this ; it may be the 

 only factor in certain markings. I have already described (" Trans. 

 Ent. Soc," April, 1884) a Phytophagous Hymenopterous larva 

 Nematus (Curtispina) in which the dark-green dorsal line was due to 

 the blood of the dorsal vessel, while on each side of this line there was 



