The Life of the Caterpillar

by Jean-Henri Fabre

Let us remove this virus, by means of a solvent; and the Processionary’s darts, reduced to their insignificant mechanical action, will be harmless. The solvent, on the other hand, rid of all hairs by filtration, will be charged with the irritant element, which we shall be able to test without the agency of the hairs. Isolated and concentrated, the stinging element, far from losing by this treatment, [[140]]ought to gain in virulence. So reflection tells us.

The solvents tried are confined to three: water, spirits of wine and sulphuric ether. I employ the latter by preference, although the other two, spirits of wine especially, have yielded satisfactory results. To simplify the experiment, instead of submitting to the action of the solvent the entire caterpillar, who would complicate the extract with his fats and his nutritive juices, I prefer to employ the cast skin alone.

I therefore collect, on the one hand, the heap of dry skins which the moult of the second phase has left on the dome of the silken dwelling and, on the other hand, the skins which the caterpillars have rejected in their cocoons before becoming chrysalids; and I leave the two lots to infuse, separately, in sulphuric ether for twenty-four hours. The infusion is colourless. The liquid, carefully filtered, is exposed to spontaneous evaporation; and the skins are rinsed with ether in the filter, several times over.

There are now two tests to be made: one with the skins and one with the product of maceration. The first is as conclusive as can [[141]]be. Hairy as in the normal state and perfectly dried, the skins of both lots, drained by the ether, produce not the slightest effect, although I rub myself with them, without the least caution, at the juncture of the fingers, a spot very sensitive to stinging.

The hairs are the same as before the action of the solvent: they have lost none of their barbs, of their javelin-points; and yet they are ineffectual. They produce no pain or inconvenience whatever. Deprived of their toxic smearing, these thousands of darts become so much harmless velvet. The Hedgehog Caterpillar and the Brush Caterpillar are not more inoffensive.

The second test is more positive and so conclusive in its painful effects that one hardly likes to try it a second time. When the ethereal infusion is reduced by spontaneous evaporation to a few drops, I soak in it a slip of blotting-paper folded in four, so as to form a square measuring something over an inch. Too unsuspecting of my product, I do things on a lavish scale, both as regards the superficial area of my poor epidermis and the quantity of the virus. To any one who might wish to renew the investigation I should recommend [[142]]a less generous dose. Lastly, the square of paper, that novel sort of mustard-plaster, is applied to the under surface of the fore-arm. A thin waterproof sheeting covers it, to prevent it from drying too rapidly; and a bandage holds it in place.

For the space of ten hours, I feel nothing; then I experience an increasing itch and a burning sensation acute enough to keep me awake for the greater part of the night. Next day, after twenty-four hours of contact, the poultice is removed. A red mark, slightly swollen and very clearly outlined, occupies the square which the poisoned paper covered.

The skin feels sore, as though it had been cauterized, and looks as rough as shagreen. From each of its tiny pustules trickles a drop of serous fluid, which hardens into a substance similar in colour to gum-arabic. This oozing continues for a couple of days and more. Then the inflammation abates; the pain, hitherto very trying, quiets down; the skin dries and comes off in little flakes. All is over, except the red mark, which remains for a long time, so tenacious in its effects is this extract of Processionary. Three weeks after the experiment, [[143]]the little square on the fore-arm subjected to the poison is still discoloured.

For thus branding one’s self, does one at least obtain some small reward? Yes. A little truth is the balm spread upon the wound; and indeed truth is a sovran balm. It will come presently to solace us for much greater sufferings.

For the moment, this painful experiment shows us that the irritation has not as its primary cause the hairiness of the Processionary. Here is no hair, no barb, no dart. All of that has been retained by the filter. We have nothing now but a poisonous agent extracted by the solvent, the ether. This irritant element recalls, to a certain extent, that of cantharides, which acts by simple contact. My square of poisoned blotting-paper was a sort of plaster, which, instead of raising the epidermis in great blisters, makes it bristle with tiny pustules.