1885.] 



Colouring of Phytophagous JLarvce. 



313 



of the former was always greener, and gave the principal band of 

 chlorophyll more distinctly when equal thicknesses were compared. 

 It therefore appears that the blood and subcuticular pigment are of 

 similar tints in this larva. In this case also the influence of the food- 

 plant extends to the true larval pigments, for the purple stripes are 

 much brighter in the privet forms. 



There is very little doubt that the green and brown varieties of 

 Noctua larvsB could be produced by a proper arrangement of surround- 

 ings, but as the experiment has not yet been tried, it must not at 

 present be assumed that the food-plant could produce so great an 

 effect as the removal of the derived pigments from the blood. Ob- 

 viously the common experience that brown larvae rest on the brown 

 parts of the plant or on the earth is not a sufficient argument, as the 

 habit may have followed the change of colour. But a change in the 

 relative proportion of the derived pigments >passed through the walls 

 of the digestive tract into the blood seems to result from a more 

 complex influence than that which would lead to the entire removal 

 of pigment from the blood ; and yet we have proved that the food- 

 plant exerts the former influence. 



22. Summary ; and Conclusion as to the Nature of the Influence of the 



Food-plant. 



It has been shown that the influence of the food-plant is not 

 uniform, that it must act during a large proportion of the whole 

 larval life in order to produce an effect, that effects of surface coloura- 

 tion due to consistence may be imitated in colour, and it has been 

 rendered extremely probable that the effects accumulate during 

 successive generations. It has been shown that the effects are 

 partially due to pigment which is proper to the larva, and which has 

 no immediate relation to the food-plant, while the changes produced 

 in the derived pigments are even more complicated, and due to the 

 predominance of one or other of the vegetal colouring matters in the 

 tissues and blood, and before this in the materials which traverse the 

 walls of the digestive tract (for the hypothesis that certain pigments 

 are continuously destroyed after passing the digestive tract, until a 

 certain colour is produced, is even more complicated). 



Such effects are entirely inexplicable by the simple theory of 

 phytophagic influence with which I was strongly prepossessed on 

 approaching this inquiry. So impossible does it seem that the effect 

 could be produced by the direct influence of the material which is 

 eaten, that it would be wiser to abandon the term "phytophagic," at 

 any rate in the sense of producing these changes. The term still holds 

 good for the broad fact that pigments derived from the food-plant 

 play a most important part in larval colouration, and further than 



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