318 Studies in the Theory of Descent. 



just the same importance as longitudinal stripes. 

 The former serve to render the caterpillar diffi- 

 cult of detection, by making it resemble, as far as 

 possible, a leaf ; they are imitations of the leaf-veins. 



Nobody who is in the habit of searching for 

 caterpillars will doubt that, in cases where the 

 oblique stripes are simply white or greenish-white, 

 it is extremely difficult to see the insect on its 

 food-plant, e.g. S. Ocellatus on Saiix ; not only 

 because it possesses the colour of the leaves, but 

 no less because its large body does not present 

 an unbroken green surface, which would bring it 

 into strong contrast with the leaves, and thus 

 arrest the attention. In the case of the species 

 named, the coloured area of the body is divided 

 by oblique parallel stripes, just in the same 

 manner as a willow leaf. In such instances of 

 course we have not presented to us any special 

 imitation of a leaf with all its details there is not 

 a perfect resemblance of the insect to a leaf, but 

 only an arrangement of lines and interspaces which 

 does not greatly differ from the division of a leaf 

 by its ribs. 



That this view is correct is shown by the occur- 

 rence of this form of marking. It is on the whole 

 rare, being found, besides in many Sphingidcz, in 

 isolated cases in various families, but is always con- 

 fined to those larvae which live on ribbed leaves, and 

 never occurring in species which feed on grasses 

 or on trees with needle-shaped leaves. This has 



