It is curious—gratifying, too, if one allows oneself to give way to a natural, though unreasonable feeling—to learn that some of these very ichneumons themselves become, in a similar manner, the victims of others of their own species. Thus from two corpses, on the slowly dying bodies of both of which it has directly, or indirectly, fed, the third life in death emerges, like some ghoul from a double tomb. Could the caterpillar know that the being which so remorselessly preyed upon its tissues and juices had a similar parasite within its own body, doing it to death in the same horrid way, how relieved and almost happy it might feel! But Nature, though she often brings in her revenges, seldom grants to her suffering children the proverbial sweetness of revenge. Caterpillars, however, do not always submit to the machinations of the ichneumon-fly without a struggle, and in some cases they may be successful—how often it is not easy to say—in guarding themselves against their attacks. The puss-moth, for instance, which is especially liable to them, is furnished, no doubt for that very reason, with a special weapon for its defence. The end of its body is forked, and each fork is prolonged into a sort of tail, from which a red filament can be extruded and waved about at the will of the creature. In this way, and by its violent contortions, it may sometimes succeed in whipping off, as it were, the ichneumon that is attacking it, but it has another and more efficacious means of defending itself. An aperture in the skin behind the head communicates with a gland containing a clear fluid, forty per cent. of which is formic acid, and the rest water. This the caterpillar can eject with great force, and so pungent is it that a few drops falling on an unwary ichneumon-fly are sufficient to incapacitate, if not actually to kill it. Lizards, indeed, and monkeys, as has been experimentally ascertained, are affected by this powerful irritant, nor, as far as we know, is there any other animal secretion which contains so large a proportion of strong acid. It is probable that the caterpillar of the puss-moth is not the only one which has this power of spurting a noxious fluid over its enemies, whilst many are protected in other ways: some by their hairiness, others by being coloured like the leaves they eat, or resembling, when at rest, a twig of the plant on which they sit immovable. By these latter means they certainly avoid being eaten by birds, and there seems no reason why they should not sometimes deceive the ichneumon-fly also. But in spite of all defences, whether consciously or unconsciously brought into play, a large proportion of most caterpillars yield to destiny, and are slowly eaten alive by the special parasite which Nature has provided for them.