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 [[102]]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.
A cyclonic disturbance, coming from the British Isles, was passing over our district; an atmospheric depression the like of which the season had not as yet known, had spread in our direction, reaching us on the 13th and persisting, in a more or less accentuated form, until the 22nd. At Avignon the barometer suddenly fell half an inch, to 29.1 in., on the 13th and lower still, to 29 in., on the 19th. [[103]]
During this period of ten days, the garden caterpillars made no sortie on the pine-trees. True, the weather was changeable. There were a few showers of fine rain and some violent gusts of the mistral; but more frequently there were days and nights when the sky was superb and the temperature moderate. The prudent anchorites would not allow themselves to be caught. The low pressure persisted, menacing them; and so they stopped at home.
In the greenhouse things happen rather differently. Sorties take place, but the staying-in days are still more numerous. It looks as though the caterpillars, alarmed at first by the unexpected things happening overhead, had reassured themselves and resumed work, feeling nothing, in their shelter, of what they would have suffered out of doors—rain, snow and furious mistral blasts—and had then suspended their work again when the threats of bad weather increased.
There is, indeed, a fairly accurate agreement between the oscillations of the barometer and the decisions of the herd. When the column of mercury rises a little, they come out; when it falls they remain at home. Thus [[104]]on the 19th, the night of the lowest pressure, 29 in., not a caterpillar ventures outside.
As the wind and rain can have no effect on my colonies under glass, one is led to suppose that atmospheric pressure, with its physiological results, so difficult to define, is here the principal factor. As for the temperature, within moderate limits there is no need to discuss it. The Processionaries have a robust constitution, as behoves spinners who work in the open air in midwinter. However piercing the cold, so long as it does not freeze, when the hour comes for working or feeding they spin on the surface of the nest or browse on the neighbouring branches.
Another example. According to the meteorological chart in the Temps, a depression whose centre is near the Iles Sanguinaires, at the entrance of the Gulf of Ajaccio, reaches my neighbourhood, with a minimum of 29.2 in., on the 9th of January. A tempestuous wind gets up. For the first time this year there is a respectable frost. The ice on the large pond in the garden is two or three inches thick. This wild weather lasts for five days. Of course, the garden caterpillars do not [[105]]sally forth on the pine-trees while these are battered by such a gale.
The remarkable part of the business is that the greenhouse caterpillars do not venture out of their nests either. And yet for them there are no boughs dangerously shaken, no cold piercing beyond endurance, for it is not freezing under the glass. What keeps them in can be only the passage of that wave of depression. On the 15th the storm ceases; and the barometer remains between 29.6 and 30 in. for the rest of the month and a good part of February. During this long period there are magnificent sorties every evening, especially in the greenhouse.
On the 23rd and 24th of February, suddenly the Processionaries stop at home again, for no apparent reason. Of the six nests under cover, only two have a few rare caterpillars out on the pine-branches, while previously, in the case of all six, I used every night to see the leaves bending under the weight of an innumerable multitude. Warned by this forecast, I enter in my notes: