1 62 Mr. E. B. Poulton. Colour-relation between the [Feb. 4, 



upper as tlie under side. Hence the effect of the narrow and small- 

 leaved 8. viminalis in producing yellow larvae, and the exceedingly 

 white-leaved (under sides) Salix linearis in producing a yellowish 

 intermediate larva. The influence of 8. cinerea (in the direction 

 of yellow) is probably in a great measure due to the same facts, 

 for its small leaves are often downy underneath, and are always 

 much whiter than those of S. rubra, &c. The much stronger in- 

 fluence of S. Smithiana towards white is probably due to its much 

 larger but very similar leaves. It is likely that some of the 

 irregular results referred to other factors may be explained in this 

 way. Thus it was very obvious that the leaves of 8. ferruginea (?), 

 upon which very differently coloured larvae were found (p. 301 

 of the former paper), varied very much in size, those on the lower 

 branches looking like rather narrow leaves of 8. cinerea, the upper 

 ones being exactly like those of 8. Smithiana ; but in such an isolated 

 case it is not possible to determine certainly which of the factors 

 caused the exception, or whether it was due to a combination of 

 causes. I think it is unlikely that any great difference could be 

 caused by a slight variation of habit in larvae, i.e., in the period at 

 which different individuals would retire to the stem from leaves of the 

 same size. It is probable that the habit is very uniform, and always 

 leads the larvae to remain on the leaves as long as the size and 

 strength of the latter will permit them to do so. In Salix alba the 

 question is complicated by the fact that after the larva retires (early 

 in this case) to the stem, the whiteness of the environment will 

 partially depend upon nearness to the apices of the twigs, for the 

 upper sides of the young leaves are white as well as the under sides. 

 The exceedingly strong influence of apple is readily explicable by the 

 considerations advanced above ; for the leaves are large, broad, and 

 strong, and will take the weight of a larva advanced in the last stage 

 without bending. The single larva found upon apple in 1885 

 (August 30th) was resting before changing its skin for the last time 

 on the under side of the leaf, and I have often before found the large 

 larvae in the same position. Af ter the larva retires to the stem the apple 

 leaves form broad curved white surfaces, which everywhere environ 

 the (presumably) sentient part of its body, which is always directed 

 during rest towards the apex of the twig. Upon all the varieties of 

 food-plant, and especially upon apple, the larva tends to rests upon 

 the young and vigorous twigs which stand out from the trees and 

 bear fewer larger leaves at wider intervals, and with more regular 

 arrangement than those upon the older wood below. Thus the larva 

 gets the maximum effect from the under sides of the leaves after it 

 has retired to the stem. This explanation also helps towards clearing 

 up the difficulty about the irregular effect of crab. Although the 

 under sides of the leaves are smooth and green, they are generally of a 



