Little Paul has not failed, with my aid, to rear the pretty bearer of the tufts and brushes. How many times, with his sensitive finger, has he not stroked the creature’s furry costume? He found it softer than velvet. And yet, enlarged under the microscope, the caterpillar’s hairs are horrible barbed spears, no less menacing than those of the Processionary. The resemblance goes no farther: handled without precautions, the tufted caterpillar does not provoke even a simple rash. Nothing could be more harmless than his coat. It is evident, then, that the cause of the irritation lies elsewhere than in the barbs. If the barbed bristles were enough to poison the fingers, most hairy caterpillars would be dangerous, for nearly all have spiny bristles. We find, on the contrary, that virulence is bestowed upon a very small number, which are not distinguished from the rest by any special structure of the hair. That the barbs have a part to play, that of fixing the irritant atom upon the epidermis, of keeping it anchored in its place, is, after all, possible; but the shooting pains cannot by any means be caused by the mere prick of so delicate a harpoon. Much less slender, the hairs clustered into pads on the prickly pears are ferociously barbed. Woe to the fingers that handle this kind of velvet too confidently! At the least touch they are pierced with harpoons whose extraction involves a severe tax upon our patience. Other inconvenience there is little or none, for the action of the barb is in this case purely mechanical. Supposing—a very doubtful thing—that the Processionary’s hairs could penetrate our skin, they would act likewise, only with less effect, if they had merely their sharp points and their barbs. What then do they possess in addition? They must have, not inside them, like the hairs of the nettle, but outside, on the surface, an irritant agent; they must be coated with a poisonous mixture, which makes them act by simple contact. Let us remove this virus, by means of a solvent; and the Processionary’s darts, reduced to their insignificant mechanical action, will be harmless. The solvent, on the other hand, rid of all hairs by filtration, will be charged with the irritant element, which we shall be able to test without the agency of the hairs. Isolated and concentrated, the stinging element, far from losing by this treatment, ought to gain in virulence. So reflection tells us. The solvents tried are confined to three: water, spirits of wine and sulphuric ether. I employ the latter by preference, although the other two, spirits of wine especially, have yielded satisfactory results. To simplify the experiment, instead of submitting to the action of the solvent the entire caterpillar, who would complicate the extract with his fats and his nutritive juices, I prefer to employ the cast skin alone.