The leather-jackets, or grubs of the common crane-fly (Fig. 22, B), sometimes destroy hundreds of acres--even whole districts--of grassland in England and France by gnawing the young subterranean roots and shoots of the grass. They also destroy young wheat crops. The leather-jacket is regarded by agriculturists as an intractable pest, since it gets too deep into the turf to be destroyed by chemical poisons. Its thick skin also makes it very resistant to such treatment. When immersed in brine for twenty-four hours the grubs are not killed; prolonged immersion in water is equally ineffective; they may be frozen until they are brittle, and will yet recover; and when kept three weeks without food, still remain alive. Birds are their natural enemies, and rooks not only dig after the grubs, but swallow the flies at the rate of four a minute! Ploughing up the land in which the grubs abound is recommended as a means of destroying them, and also the application of gas-lime to the ground. Rolling the turf and pressing it down also kills the grubs, but the best chance of diminishing their ravages is found in draining wet land and in feeding up the young grass plants with "fertilizers," so that they may grow rapidly and resist the injurious effect of the leather-jackets' nibbling. Before leaving this subject it will be found interesting to contrast the "leather-jacket" with the true "wire-worms," which are the grubs of a remarkable kind of beetle (there are half a dozen British species) called the click-beetle (Fig. 22, C). They belong to a great family of beetles (Coleoptera), known as the Elaterids or Elaters, of which 7000 species are known, sixty being British. Some of the most brilliant light-giving or phosphorescent insects (not, however, the common glow-worms) belong here. The click-beetles are so called because when one is laid on its back it regains its proper pose, with the legs beneath it, by a spring or "skip," accompanied by a sharp click. The grubs of the click-beetles, known as "wire-worms" (the name is also applied to centipedes), are more threadlike, that is to say narrower, than the leather-jackets. They are not legless "maggots," but have three pairs of small legs (Fig. 22, D). They destroy corn and grass, and do not change into the adult condition in a few months, as do the leather-jackets, but remain for three, and in some cases five, years in the ground feeding on the roots of the corn and grass plants, doing much destruction before they finally change into beetles. CHAPTER XXV THE MOTH AND THE CANDLE