There have been occasions when I have rubbed myself with the whole caterpillar, or with his cast skin, or with the broken hairs gathered on a paint-brush, without producing any unpleasant results. The irritant dust seems to vary in quality according to certain circumstances which I have not been able to discover. From my various tests it is evident that the discomfort is caused by the delicate hairs which the lips of the dorsal mouths, gaping and closing again, never cease grinding, to the detriment of their beards and moustaches. The edges of these slits, as their bristles rub off, furnish the stinging dust. Having established this fact, let us proceed to more serious experiments. In the middle of March, when the Processionaries for the most part have migrated underground, I decide to open a few nests, as I wish to collect their last inhabitants for the purpose of my investigations. Without taking any precautions, my fingers tug at the silken dwelling, which is made of solid stuff; they tear it into shreds, search it through and through, turn it inside out and back again. Once more and this time in a more serious fashion, I am the victim of my unthinking enthusiasm. Hardly is the operation completed, when the tips of my fingers begin to hurt in good earnest, especially in the more delicate part protected by the edge of the nail. The feeling is like the sharp pain of a sore that is beginning to fester. All the rest of the day and all through the night, the pain persists, troublesome enough to rob me of my sleep. It does not quiet down until the following day, after twenty-four hours of petty torment. How did this new misadventure befall me? I had not handled the caterpillars: indeed, there were very few of them in the nest at the time. I had come upon no shed skins, for the moults do not take place inside the silken purse. When the moment has come to doff the second costume, that of the red mosaic, the caterpillars cluster outside, on the dome of their dwelling, and there leave in a single heap their old clothes entangled with bits of silk. What is left to explain the unpleasant consequences to which the handling of the nest exposes us? The broken red bristles are left, the fallen hairs forming a dust that is invisible without a very careful examination. For a long time the Processionaries crawl and swarm about the nest; they pass to and fro, penetrating the thickness of the wall when they go to the pastures and when they return to their dormitory. Whether motionless or on the move, they are constantly opening and closing their apparatus of information, the dorsal mouths. At the moment of closing, the lips of these slits, rolling on each other like the cylinders of a flattening-mill, catch hold of the fluff near them, tear it out and break it into fragments which the bottom of the pocket, presently reascending, shoots outside.