That is not all, either. The Puss Moth caterpillar has a rapid trick of facing about abruptly in the direction of the enemy as if it meant to bite: and this trick is always most disconcerting. If ever so lightly touched, it instantly assumes the terrifying attitude, and presents its pretended face to the astonished aggressor. From a harmless caterpillar it becomes all at once a raging bulldog. Touch it on the other side, and it faces round like lightning in the opposite direction. Professor Poulton tried the effect of its grimace on a marmoset, and found the marmoset was afraid to touch the mysterious creature. We are not marmosets, but I notice that most human beings recoil instinctively from a Puss Moth caterpillar when it assumes its mask. Even if you _know_ it is harmless, there is something very alarming in its rapid twists and turns, and in the persistent way in which it grins and spits at you. [Illustration: 8.--THE SAME CATERPILLAR TERRIFYING AN ENEMY.] Really spits, too; for the insect has a gland in its head which ejects, at need, an irritating fluid. If this fluid gets into your eyes, they smart most unpleasantly. It contains formic acid, and is strong enough to be exceedingly stinging and painful. The discharge repels lizards, and probably also birds, who are among the chief enemies of this as of other caterpillars. The deadliest foe of the Puss Moth larva, however, is the ichneumon-fly, a parasitic creature, which lays its eggs in living caterpillars, and lets its grubs hatch out inside them, so as to devour the host from within in the most ruthless fashion. There are many kinds of ichneumon-fly, some of them very minute: the one which attacks the Puss Moth in its larval stage is a comparatively big one. The fly lays its eggs behind the caterpillar's head, where the victim is powerless to dislodge them. In all probability the defensive attitude and the shower of formic acid are chiefly of use against these parasitic foes: for when an ichneumon-fly appears, the caterpillar assumes his "terrifying" attitude the moment it touches him, and faces full round to the foe with his false mask inflated. A very small quantity of the formic acid Professor Poulton found sufficient to kill an ichneumon: and there can be little doubt that this is its main object. [Illustration: 9.--CATERPILLARS OF THE LOBSTER MOTH DEMONSTRATING IN FORCE BEFORE THE HOSTILE BATTALIONS.]