48 ON PLANTS AND INSECTS. [lect. 



tudinal streaks only occur on caterpillars which live on 

 or among narrow-leaved plants. As the insect grows, 

 these lines often disappear on certain segments, and are 

 replaced by diagonal lines. These diagonal lines (Fig. 

 39) occur in a great many caterpillars, belonging to the 

 most distinct families of butterflies and moths. They 

 come off just at the same angle as the ribs of leaves, and 

 resemble them very much in general effect. They occur 

 also especially in. species which feed on large-leaved 

 plants, and I believe I may say that though a great 

 many species of caterpillars present these lines, they 



Fig. 39. — Smervnthus ocellatus. 



rarely, if ever, occur in species which live on grass ; 

 while, on the contrary, they are very frequent in those 

 species which live on large-leaved plants. It might at 

 first be objected to this view that there are many cases, 

 as in the elephant hawk-moth, in which caterpillars have 

 both. A little consideration, however, will explain this. 

 In small caterpillars these oblique lines would be useless, 

 because they must have some relation, not only in colour, 

 but in their distance apart, to the ribs of the leaves. 

 Hence, while there are a great many species which have 

 longitudinal lines when young, and diagonal ones when 

 they are older and larger, there is not, I believe, a single 



