The protuberances that rise from those expanded cavities are formed of a soft, pale, hairless membrane, which gives the impression of a visceral hernia, as though the caterpillar were wounded and exposing its delicate entrails to the air. The sensitiveness just here is great. The lightest touch with the point of a hair-pencil causes the immediate indrawing of the protuberances and the closing of the containing lips. The touch of a solid object even is not essential. I pick up a tiny drop of water on the point of a pin and, without shaking it off, present this drop to the sensitive projection. At the moment when contact occurs the apparatus contracts and closes up. The recoil of the Snail’s horns, withdrawing the visual and olfactory organs into their sheaths, is no prompter. Everything seems to prove that these optional tumours, appearing and disappearing at the caterpillar’s will, are instruments of sensorial perception. The caterpillar exposes them to obtain information; he shelters them under his skin to preserve their delicate functions. Now what is it that they perceive? This is a difficult question, in which the habits of the Processionary alone can afford us a little guidance. 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 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.