During the whole winter, the Pine Caterpillars are active only at night. In the daytime, when the weather is fine, they readily repair to the dome of the nest and there remain motionless, gathered into heaps. It is the hour of the open-air siesta, under the pale December and January sun. As yet none leaves the home. It is quite late in the evening, towards nine o’clock, when they set out, marching in an irregular procession, to browse on the leaves of the branches hard by. Their grazing is a protracted affair. The flock returns [[97]]late, some time after midnight, when the temperature falls too low.
Secondly, it is in the heart of winter, during the roughest months, that the Processionary displays his full activity. Indefatigably at this time of year he spins, adding each night a new web to his silken tent; at this time, whenever the weather permits, he ventures abroad on the neighbouring boughs to feed, to grow and to renew his skein of silk.
By a very remarkable exception, the harsh season marked by inactivity and lethargic repose in other insects is for him the season of bustle and labour, on condition, of course, that the inclemencies of the weather do not exceed certain limits. If the north wind blow too violently, so that it is like to sweep the flock away; if the cold be too piercing, so that there is a risk of freezing to death; if it snow, or rain, or if the mist thicken into an icy drizzle, the caterpillars prudently stay at home, sheltering under their weatherproof tent.
It would be convenient to some extent to foresee these inclemencies. The caterpillar dreads them. A drop of rain sets him in a flutter; a snowflake exasperates him. To [[98]]start for the grazing-grounds at dark of night, in uncertain weather, would be dangerous, for the procession goes some distance and travels slowly. The flock would fare ill before regaining shelter did any sudden atmospheric trouble supervene, an event of some frequency in the bad season of the year. So that he may be informed in this particular during his nocturnal winter rambles, can the Pine Caterpillar be endowed with some sort of meteorological aptitudes? Let us describe how the suspicion occurred to me.
Divulged I know not how, my rearing of caterpillars under glass acquired a certain renown. It was talked about in the village. The forest-ranger, a sworn enemy to destructive insects, wanted to see the grazing of the famous caterpillars, of whom he had retained a too poignant memory ever since the day when he gathered and destroyed their nests in a pine-wood under his charge. It was arranged that he should call the same evening.
He arrives at the appointed hour, accompanied by a friend. For a moment we sit and chat in front of the fire; then, when the clock strikes nine, the lantern is lit and we [[99]]all three enter the greenhouse. The visitors are eager for the spectacle of which they have heard such wonderful things, while I am certain of satisfying their curiosity.
But, but … what is this? Not a caterpillar on the nests, not one on the fresh ration of branches! Last night and on the previous nights they came out in countless numbers; to-night not one reveals himself. Can it be that they are merely late in going to dinner? Can their habitual punctuality be at fault because appetite has not yet arrived? We must be patient.… Ten o’clock. Nothing. Eleven. Still nothing. Midnight was at hand when we abandoned our watch, convinced that it would be vain to prolong the sitting. You can imagine what an abject fool I looked at having thus to send my guests away.
Next day I thought that I dimly perceived the explanation of this disappointment. It rained in the night and again in the morning. Snow, not the earliest of the year, but so far the most abundant, whitened the brow of the Ventoux.[2] Had the caterpillars, more [[100]]sensitive than any of us to atmospheric changes, refused to venture forth because they anticipated what was about to happen? Had they foreseen the rain and the snow, which nothing seemed to announce, at all events to us? After all, why not? Let us continue to observe them and we shall see whether the coincidence is fortuitous or not.
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. [[101]]
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.