These caterpillars and the locusts devour the leaves bodily, but there are others which live inside them. These so-called "leaf-miner" caterpillars make white irregularly-winding tunnels between the upper and the lower skin of the leaf. The tunnel increases or widens because the caterpillar itself grows fatter as it eats its tunnel. They can be seen on a great many leaves, and can be at once recognized by this peculiarity. Plants cannot run away from their enemies like animals, and it would seem at first sight that their case was very hopeless. But it is not so, for there is a vast, active, keen-eyed, and eager army of helpers always ready for eggs and caterpillars. It is birds that are of the greatest importance. A titmouse will eat 200,000 insects in a season. A starling has been seen to fetch food for its young ones from a grass paddock 100 yards away no less than eighteen times in a quarter of an hour. All the following are excellent birds, and without a stain upon their characters: the plover, partridge, robin, wagtail, starling. Crows and wood-pigeons are under suspicion, for though the latter do good in devouring the seeds of weeds, and the former in destroying wireworms, both are fond of corn and take large quantities of it. Thrushes, mavises, and blackbirds are amongst the most persevering and useful of our friends, but they are certainly fond of fruit. Yet the good which they do is very much more than any possible harm which an injudicious indulgence in the juicy fruits of summer might bring about. The sparrow cannot be given a character. Indeed, he is objectionable in every way, for he not only does no good himself, but he devours corn and drives away starlings and other valuable and interesting helpers. But it is very difficult to say what will happen if man interferes with the regular working of Nature. The starling has been a pest in Australia, though here it does nothing but good work. We are still grossly ignorant of many simple but very important facts. Even when we do know something, as for instance, that the peewit's or plover's whole life is occupied in clearing the ground of wireworm, daddy-long-legs grub, insects' eggs, and the like, that does not help the bird in the least. Plovers' eggs are regularly sold in enormous quantities. Every farm-labourer collects them, and the farmer never dreams of interfering. Man shoots down owls, kestrels, hawks, who prey upon mice, voles, and sparrows. Then, when some farmers are half ruined, he has Royal Commissions to find out why the voles have increased so much. There are one or two peculiar contrivances found in plants which are intended to keep off insects, and which may be noticed here. Thus, the importance of a moat (which almost always formed part of the defence of a medieval castle) had been already found out by one or two plants.