Now consider the Hedgehog Caterpillar. Why is he harmless, despite his fierce and hirsute aspect? Because he lives in isolation and is always on the move. His mane, apt though it be to collect and retain irritant particles, will never give us the itch, for the simple reason that the caterpillar does not lie on his excretions. Distributed all over the fields and far from numerous, owing to the caterpillar’s solitary habits, the droppings, though poisonous, cannot transfer their properties to a fleece which does not come into contact with them. If the Hedgehog lived in a community, in a nest serving as a cesspit, he would be the foremost of our stinging caterpillars.
At first sight, the barrack-rooms of the Silkworm-nurseries seem to fulfil the conditions necessary to the surface venom of the worms. Each change of litter results in the removal of basketfuls of droppings from the trays. Over this heaped-up ordure the Silkworms swarm. How is it that they do not acquire the poisonous properties of their own excrement? [[176]]
I see two reasons. In the first place, they are hairless; and a brushlike coat may well be indispensable to the collection of the virus. In the second place, far from lying in the filth, they live above the soiled stratum, being largely separated from it by the bed of leaves, which is renewed several times a day. Despite crowding, the population of a tray has nothing that can be compared with the ordinary habits of the Processionary; and so it remains harmless, in spite of its stercoral toxin.
These first enquiries lead us to conclusions which themselves are very remarkable. All caterpillars excrete an urticating matter, which is identical throughout the series. But, if the poison is to manifest itself and to cause us that characteristic itching, it is indispensable that the caterpillar shall dwell in a community, spending long periods in the nest, a silken bag laden with droppings. These furnish the virus; the caterpillar’s hairs collect it and transfer it to us.
The time has come to tackle the problem from another point of view. Is this formidable matter which always accompanies the excretions a digestive residuum? Is it not rather one of those waste substances which [[177]]the organism engenders while at work, waste substances designated by the general appellation of urinary products?
To isolate these products, to collect them separately, would scarcely be practicable, if we did not have recourse to what follows on the metamorphosis. Every Moth, on emerging from her chrysalis, rejects a copious mixture of uric acid and various humours of which very little is as yet known. It may be compared with the broken plaster of a building rebuilt on a new plan and represents the by-products of the mighty labours accomplished in the transfigured insect. These remains are essentially urinary products, with no admixture of digested foodstuffs.
To what insect shall I apply for this residuum? Chance does many things. I collect, from the old elm-tree in the garden, about a hundred curious caterpillars. They have seven rows of prickles of an amber yellow, a sort of bush with four or five branches. I shall learn from the Butterfly that they belong to the Great Tortoiseshell (Vanessa polychloros, Lin.).
Reared on elm-leaves under a wire-gauze cover, my caterpillars undergo their transformation [[178]]towards the end of May. Their chrysalids are specked with brown on a whitish ground and display on the under surface six radiant silvery spots, a sort of decorative tinsel, like so many mirrors. Fixed by the tail with a silken pad, they hang from the top of the dome, swinging at the least movement and emitting vivid flashes of light from their reflectors. My children are amazed at this living chandelier. It is a treat for them when I allow them to come and admire it in my animal studio.
Another surprise awaits them, this time a tragic one, however. A fortnight later, the Butterflies emerge. I have placed under the cover a large sheet of white paper, which will receive the desired products. I call the children. What do they see on the paper?
Large spots of blood. Under their very eyes, from up there, at the top of the dome, a butterfly lets fall a great red drop: plop! No joy for the children to-day; anxiety rather, almost fear.