1885.] Colouring of Phytophagous Larva?. 307 



Tt is, therefore, hardly conceivable that this small proportion can 

 directly modify the whole larva into its own likeness ; — that the under 

 surface eaten by the larva should cause the latter to resemble the 

 surface itself. And yet it is only this part of the food which is 

 common to the various food-plants producing similar effects. In 

 everything except whiteness and pubescence of the underside, the 

 leaves of the apple must differ more widely from those of S. Viminalis, 

 Ferruginea, and Smithiana than these do from S. Rubra. And yet the 

 effects of the latter plant are very different from the three former 

 species of sallow, of which the tendencies incline towards those of 

 apple. And further, the resulting larvae are far better protected by 

 such effects produced in common by apple and certain sallows than if 

 all the sallows had tended in the same direction. Furthermore, the 

 appearance of the undersides is quite as much due to texture as 

 colour, for the pubescence produces a large part of the effect of dead 

 whiteness. But the larva gains the same appearance by colour, for 

 its hairs are microscopic rudiments (only discovered by myself last 

 summer, 1884). Again, there is no reason for supposing that the 

 chlorophyll from the leaves of the food-plants would yield solutions 

 corresponding in colour with the larvae; in fact, it is certain that this 

 is not the case, for the colour of the upper sides of the leaves do not 

 afford an indication of the effects on the larvae, and yet the true 

 colour of the leaf is far more distinct here than it is on the under 

 surface. 



These conclusions are also confirmed by considering the effects of 

 the experiments. While the existence of an influence of the food- 

 plant upon larval colouration is undeniably proved, the simple 

 theory of its manner of action is entirely refuted, for such a theory 

 does not allow for any powerful tendency on the part of the larva 

 which may even counterbalance that of the food. Thus in the experi- 

 ments the effects produced were not according to the power of the 

 food- plant (as proved by observation), when the tendency was in the 

 direction of yellowish, while the effects were very powerful when the 

 tendency was in the other direction. And so also with the observa- 

 tions in the field, as was proved by the very different colours on the 

 same tree, and the strongly yellowish variety upon apple. It cannot 

 be argued that the irregularities observed are due to the effects of 

 the food-plant varying in intensity upon larvae of different constitu- 

 tions. They can only be explained by the possession of an opposing 

 force in the larva itself, which may not only exactly counterbalance 

 the tendencies of the food, but may produce the result of the strongest 

 opposite tendencies. The former explanation would account for all 

 varieties from intermediate to whitish upon apple, and from inter- 

 mediate to yellowish upon S. Rubra, but it fails to account for 

 anything on the whitish side of intermediate upon S. Rubra or on 



