On this memorable day, therefore, the 13th of December, 1895, I institute the caterpillars’ meteorological observatory. I have at my disposal absolutely none of the apparatus dear to science, not even a modest thermometer, for my unlucky star continues in the ascendant, proving as unkind to-day as when I learnt chemistry with pipe-bowls for crucibles and bottles that once contained sweets for retorts. I confine myself to visiting nightly the Processionaries in the greenhouse and those in the garden. It is a hard task, especially as I have to go to the far end of the enclosure, often in weather when one would not turn a Dog out of doors. I set down the acts of the caterpillars, whether they come out or stay at home; I note the state of the sky during the day and at the moment of my evening examination. To this list I add the meteorological chart of Europe which the Temps publishes daily. If I want more precise data, I request the Normal School at Avignon to send me, on occasions of violent disturbances, the barometrical records of its observatory. These are the only documents at my disposal. Before we come to the results obtained, let me once more repeat that my caterpillars’ meteorological institute has two stations: one in the greenhouse and one in the open air, on the pines in the enclosure. The first, protected against the wind and rain, is that which I prefer: it provides more regular and more continuous information. In fact, the open-air caterpillars often enough refuse to come out, even though the general conditions be favourable. It is enough to keep them at home if there be too strong a wind shaking the boughs, or even a little moisture dripping on the web of the nests. Saved from these two perils, the greenhouse caterpillars have only to consider atmospheric incidents of a higher order. The small variations escape them; the great alone make an impression on them: a most useful point for the observer and going a long way towards solving the problem for him. The colonies under glass, therefore, provide most of the material for my notes; the colonies in the open air add their testimony, which is not always quite clear. Now what did they tell me, those greenhouse caterpillars who, on the 13th of December, refused to show themselves to my guest, the forest-ranger? The rain that was to fall that night could hardly have alarmed them: they were so well sheltered. The snow about to whiten Mont Ventoux was nothing to them: it was so far away. Moreover, it was neither snowing yet nor raining. Some extraordinary atmospheric event, profound and of vast extent, must have been occurring. The charts in the Temps and the bulletin of the Normal School told me as much.