This hairy erection produces a sudden modification in the caterpillar’s aspect. The red shiny bristles have disappeared, buried under the dark skin; the white hairs, now standing on end, form a hirsute mane; an ashy tinge has crept into the general colour of the costume.
When calm is restored, as soon happens, the slits open and yawn afresh; the sensitive protuberances emerge, quick to disappear once more should any cause for alarm occur. These alternate expansions and contractions are rapidly repeated. I provoke them at will in various ways. A slight puff of tobacco-smoke immediately causes the stomata to yawn and the protuberances to emerge. One would think that the insect was putting itself on its guard and displaying some special apparatus of information. Before long the protuberances go in again. A second puff of smoke brings them out once more. But, if the smoke is too abundant, too acrid, the caterpillar wriggles and writhes without opening his apparatus. [[93]]
Or else I touch one or other of these uncovered protuberances, very delicately, with a bit of straw. The pimple affected immediately contracts, draws into itself, like the horns of the Snail, and is replaced by a gaping mouth, which in its turn closes. Usually, but not always, the segment excited by the contact of my straw is imitated by the others, both front and back, which close their apparatus one by one.
When undisturbed and in repose, the caterpillar generally has his dorsal slits expanded; in moving, he sometimes opens and sometimes closes them. In either case expansion and contraction are frequently repeated. Constantly coming together and retreating under the skin, the lips of the mouth-like opening therefore end by losing their brittle moustaches of russet hairs, which break off. In this way a sort of dust collects at the bottom of the crater, a dust formed of broken hairs, which, thanks to their barbs, soon collect into little tufts. When the slit expands rather suddenly, the central projection shoots out on the insect’s sides its load of hairy remnants, which the least breath blows into a cloud of golden atoms highly disagreeable to the observer. [[94]]I shall have something to say presently of the itch to which he is at such times exposed.
Are these peculiar stomata designed merely to collect the adjoining bristles and to grind them to powder? Are these fine-skinned papillæ, which inflate and ascend from the depths of their hiding-place, intended to get rid of the accumulation of broken hairs? Or is it the sole function of this peculiar apparatus to prepare, at the expense of the caterpillar’s fleece, an irritant dust which shall act as a means of defence? Nothing tells us so.
Certainly the caterpillar is not armed against the enquirer who from time to time takes it into his head to come and examine him through a magnifying-glass. It is even very doubtful whether he troubles at all about those passionate caterpillar-lovers, Calosoma sycophanta[1] among insects and the Cuckoo among birds. Those who consume such fare have a stomach expressly fashioned for the purpose, a stomach that laughs at blistering hairs and possibly finds an appetizing stimulant in their sting. No, I do not see the motives that prompted the Processionary to [[95]]cleave his back with so many slits, if he merely strips himself of his hair to throw an irritating dust in our eyes. There must certainly be something else in question.
Réaumur mentions these openings, of which he made a brief study. He calls them stigmata and is inclined to take them for exceptional breathing-holes. That they are not, O my master; no insect contrives air-holes on its back! Moreover, the magnifying-glass reveals no channel of communication with the interior. Respiration plays no part here; the solution of the enigma must lie elsewhere.
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. [[96]]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.