The cunning caterpillar which finally develops into the Privet Hawk-moth has a slightly more evolved mode of purely theatrical frightening. You see him in No. 6, a full-fed specimen, just ready to turn at once into a chrysalis. This grub feeds usually on the vivid leaves of the privet; he is therefore protectively coloured a bright green, like that of the foliage about him. "But why those great purple stripes on his sides?" you will ask. "Surely they must make him an easy mark for birds?" Not at all: please notice that they run obliquely. There is method in that obliquity. When the caterpillar is smaller, he lurks unseen on the under-side of the leaves, and this pattern of oblique purplish lines exactly imitates the general effect of the shadows cast by the ribs--so much so, that if you look for him on a privet-tree in spring, I doubt whether you will find him till I point him out to you. Even when he waxes fat and full fed, the purple stripes still aid him more or less by breaking up the large green surface into smaller areas, as Professor Poulton has well noticed. He harmonizes better so with the broken masses of the leaves about him. Then again, when the time arrives for him to turn into a chrysalis, he descends to the ground, which, under a thickly-leaved privet bush, is most often brown. So, just as he is coming of age and reaching the proper moment for migration, his back all at once begins to turn brown, in order that he may be less observed as he walks about on the stem; while by the time he is quite ready to take to the earth he has grown brown all over, thus matching the soil in which he has next to bury himself. You could hardly have a better example of the sort of colour-change which often accompanies altered habits of living. [Illustration: 6.--FULL-GROWN CATERPILLAR OF THE PRIVET HAWK-MOTH, SIMILARLY OCCUPIED.] In the illustration, however, you see this really harmless and undefended grub in the act of trying to pretend he is poisonous. He is now mature, and the stripes on his sides stand out conspicuously as he walks on the stem. A sparrow threatens him. He retorts by showing fight--fallaciously and deceptively, for he has nothing to fight with. He lifts his head with an aggressive air, and throws himself about from side to side, as if he knew he could bite, and meant to do it. He also lashes his tail in pretended anger--"I would have you to know, Sir Bird, I am not to be trifled with!" The empty demonstration usually succeeds: the sparrow gets alarmed and believes he means it. This policy is, in essence, that commonly known as "spirited": it consists in trying to frighten your enemy instead of fighting him.