48 ON PLANTS AND INSECTS. [LECT; 



no relation to anything which occurs in plants, and would 

 make the creature more conspicuous. When, therefore, 

 the diagonal lines are developed, the longitudinal ones 

 often disappear. There is one other point in connec- 

 tion with these diagonal lines to which I must call 

 your attention. In many species they are white, but 

 in some cases, as for instance in the beautiful green 

 caterpillar of the privet hawk-moth, the white streak 

 is accompanied by a colored one in that case lilac. 

 At first we might think that this would be a dis- 

 advantage, as tending to make the caterpillar more 

 conspicuous ; and in fact, if we put one in full view, 

 for instance, out on a table, and focus the eye on 

 it, the colored lines are very striking. But we must 

 remember that the habit of the insect is to sit on the 

 lower side of the leaf, generally near the midrib, and in 

 the subdued light of such a situation, especially if the 

 eye be not looking exactly at them, the colored lines 

 beautifully simulate a line of soft shadow, such as must 

 always accompany a strong rib ; and I need not tell any 

 artist that the shadows of yellowish green must be 

 purplish. Moreover, any one who has ever found one 

 of these large caterpillars will, I am sure, agree with me 

 that it is surprising, when we consider their size and 

 conspicuous coloring, how difficult they are to see. 



But though the prevailing color of caterpillars is 

 green, there are numerous exceptions. In one great 

 family of moths (the Geometridce) the prevailing color is 

 brown. These caterpillars, however, escape observation 

 by their great similarity to brown twigs, a resemblance 

 which is heightened by their peculiar attitudes, and in 

 many cases by the existence of warts or protuberances, 



