ii.] WHITE AND COLOURED STREAKS. 49 



one which begins with diagonal lines, and then replaces 

 them with longitudinal ones. The disappearance of the 

 longitudinal lines on those segments which have diagonal 

 ones, is striking, where the lines are marked. It is an 

 advantage, because white lines crossing one another at 

 such an angle have no relation to anything which occurs 

 in plants, and would make the creature more conspi- 

 cuous. When, therefore, the diagonal lines are deve- 

 loped, the longitudinal ones often disappear. There is 

 one other point in connection 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 coloured one in that 

 case lilac. At first we might think that this would be a 

 disadvantage, 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 

 coloured 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 coloured 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 colouring, how difficult they are to see. 



But though the prevailing colour of caterpillars is 

 green, there are numerous exceptions. In one great 



E 



