Chapter VIII. An Insect Virus

Jean-Henri Fabre The Life of the Caterpillar

pine processionary caterpillar

Pine Processionary Caterpillar

ONE step forward has been taken, but only a very little one as yet, in the problem of the stinging caterpillars. The drenching with ether teaches us that hairiness plays a very secondary part in the matter. With its dust of broken bristles, which the least breath wafts in all directions, it bothers us by depositing and fixing its irritant coating upon us; but this virus does not originate in the creature's fleece; it comes from elsewhere. What is the source of it?

I will enter into a few details. Perhaps, in so doing, I shall be of service to the novice. The subject, which is very simple and sharply defined, will show us how one question gives rise to another; how experimental tests confirm or upset hypotheses, which are, as it were, a temporary scaffolding; and, lastly, how logic, that severe examiner, leads us by degrees to generalities which are far more important than anything that we were led to anticipate at the outset.

And, first of all, does the Pine Processionary possess a special glandular structure which elaborates the virus, as do, for instance, the poison-glands of the Wasps and Bees? By no means. Anatomy shows that the internal structure of the stinging caterpillar is similar to that of the harmless one. There is nothing more and nothing less.

The poisonous product, of unlocalized origin, results, therefore, from a general process in which the entire organism is brought into play. It should, in consequence be found in the blood, after the manner of urea in higher animals. This is a suggestion of grave import, but after all quite valueless without the conclusive verdict of actual experiment.

Five or six Processionaries, pricked with the point of a needle, furnish me with a few drops of blood. I allow these to soak into a small square of blotting-paper, which I then apply to my fore-arm with a waterproof bandage. It is not without a certain anxiety that I await the outcome of the experiment. The result will show whether the conclusions already forming in my mind will receive a solid basis or vanish into thin air.

At a late hour of the night, the pain wakes me, a pain which this time is an intellectual joy. My anticipations were correct. The blood does indeed contain the venomous substance. It causes itching, swelling, a burning sensation, an exudation of serum and, lastly, a shedding of the skin. I learn more than I had hoped to learn. The test is more valuable than that of mere contact with the caterpillar could have been. Instead of treating myself with the small quantity of poison with which the hairs are smeared, I have gone to the source of the irritant substance and I thereby gain an increase of discomfort.

Very happy in my suffering, which sets me on a safe path, I continue my enquiry by arguing thus. the virus in the blood cannot be a living substance, one that takes part in the working of the organism; it is rather, like urea, a form of decay, an offthrow of the vital process, a waste product which is expelled as and when it forms. If this be the case, I ought to find it in the caterpillar's drop­pings, which are made up of both the digestive and the urinary residues.

Let us describe the new experiment, which is no less positive than the last. I leave a few pinches of very dry droppings, such as are found in abundance in the old nests, to soak for two days in sulphuric ether. The liquid, coloured as it is with the chlorophyll of the caterpillar's food, turns a dirty green. Then I repeat precisely the process which I mentioned when I wanted to prove the innocuousness of the hairs deprived of their poisonous varnish. I refer to it a second time in order thoroughly to explain the method pursued and to save repetition in the various experiments undertaken.

The infusion is filtered, spontaneously evaporated and reduced to a few drops, with which I soak my stinger. This consists of a small piece of blotting-paper, folded in four to increase the thickness of the pad and to give it a greater power of absorption. An area of a square inch or less suffices; in some cases it is even too much. A novice in this kind of research-work, I was too lavish with the liniment; and in return for my generosity I had such a bad time that I make a point of warning any reader desirous of repeating the experiment upon his own person.

Fully soaked, the square of paper is applied to the fore-arm, on the inner surface, where the skin is more tender. A sheet of rubber covers it and, being waterproof, guards against the loss of the poison. Finally, a linen bandage keeps the whole in place.

On the afternoon of the 4th of June 1897, a memorable date for me, I test, as I have just said, the etheric extract of the Processionary's drop­pings. All night long, I feel a violent itching, a burning sensation and shooting pains. On the following day, after twenty hours of contact, I remove the dressing.

The venomous liquid, too lavishly employed in my fear of failure, has considerably overflowed the limits of the square of paper. The parts which it has touched and still more the portion covered by the pad are swollen and very red; moreover, in the latter case, the skin is ridged, wrinkled and mortified. It smarts a little and itches; and that is all.

On the following day, the swelling becomes more pronounced and goes deep into the muscles, which, when touched with the finger, throb like an inflamed cheek. The colour is a bright carmine and extends all round the spot which the paper covered. This is due to the escape of some of the liquid. There is a plentiful discharge of serum, oozing from the sore in tiny drops. The smarting and itching increase and become so intense, especially during the night, that, to get a little sleep, I am driven to employ a palliative, vaseline with borax and a lint dressing.

In five days' time, it has developed into a hideous ulcer, which looks more painful than it really is. The red, swollen flesh, quivering and denuded of its epidermis, provokes commiseration. The person who night and morning renews my dressing of lint and vaseline is almost sick at the sight.

„One would think,“ she says, „that the dogs had been gnawing your arm. I do hope you won't try any more of those horrible decoctions.“

I allow my sympathetic nurse to talk away and am already meditating further experiments, some of which will be equally painful. O sacred truth, what can rival thy power over us mortals! Thou turnest my petty torment into contentment; thou makest me rejoice in my flayed arm! What shall I gain by it all? I shall know why a wretched caterpillar sets us scratching ourselves. Nothing more; and that is enough for me.

Three weeks later, new skin is forming, but is covered all over with painful little pimples.

The swelling diminishes; the redness persists and is still very marked. The effect of the infernal paper lasts a long time. At the end of a month, I still feel an itching, a burning irritation, which is intensified by the warmth of the bed-clothes. At last, a fortnight later, all has disappeared but the redness, of which I shall retain the marks for a long time yet, though it grows gradually fainter and fainter. It will take three months or more to vanish altogether.

We now have some light on the problem: the Processionary's vi­rus is certainly an off-throw of the organic factory, a waste product of the living edifice. The caterpillar discards it with his excrement. But the material of the droppings has a twofold origin: the greater part represents the digestive residuum; the rest, in a much smaller proportion, is composed of the urinary products. To which of the two does the virus belong? Before going farther, let us permit ourselves a digression which will assist us in our subsequent enquiries. Let us ask what advantages the Processionary derives from his urticating product.

I already hear the answer:

„It is a means of protection, of defence. With his poisoned mane, he repels the enemy.“

I do not clearly perceive the bearing of this explanation. I think of the creature's re­cognized enemies: of the larva of Calosoma sycophanta,, which lives in the nests of the Processionary of the Oak and gobbles up the inhabitants with never a thought of their burning fleece; of the Cuckoo, another mighty consumer, so we are told, of the same caterpillars, who gorges on them to the point of implanting in his gizzard a bristling coat of their hairs.

I am not aware if the Processionary of the Pine pays a like tribute. I do know of at least one of his exploiters. This is a Dermestes, Bacon-beetle.--Translator's Note. establishes himself in the silken city and feeds upon the remains of the defunct caterpillars. This ghoul assures us of the existence of other consumers, all furnished with stomachs expressly fashioned for such highly-seasoned fare. For every harvest of living creatures there is always a harvester.

No, the theory of a special virus, expressly prepared to defend the Processionary and his emulators in urtication, is not the last word on the subject. I should find it difficult to believe in such a prerogative. Why have these caterpillars, more than others, need of protection? What reasons would make of them a caste apart, endowed with an exceptional defensive venom? The part which they play in the entomological world does not differ from that of other caterpillars, hairy or smooth. It is the naked caterpillars who, in default of a mane capable of striking awe into the assailant, ought, one would think, to arm themselves against danger and impregnate themselves with corrosives, instead of remaining a meek and easy prey. Is it likely that the shaggy, bristling caterpillar should anoint his fleece with a formidable cosmetic and his smooth-coated kinsman be unfamiliar with the chemical properties of the poison beneath his satin skin! These contradictions do not inspire confidence.

Have we not here, rather, a property common to all caterpillars, smooth-skinned or hairy? Among the latter, there might be some, just a few, who, under certain special conditions which will need to be defined, would be quick to reveal by urtication the venomous nature of their organic refuse; the others, the vast majority, living outside these conditions, even though endowed with the necessary product, would be inexpert at the stinging business and would not produce irritation by contact. In all, the same virus is to be found, resulting from an identical vital process. Sometimes it is brought into prominence by the itching which it produces; sometimes, indeed most often, it remains latent, unrecognized, if our artifices do not intervene.

What shall these artifices be? Something very simple. I address myself to the Silkworm. If there be an inoffensive caterpillar in the world, it is certainly he. Women and children take him up by the handful in our Silkworm-nurseries; and their delicate fingers are none the worse for it. The satin-skinned caterpillar is perfectly innocuous to a skin almost as tender as his own.

But this lack of caustic venom is only apparent. I treat with ether the excretions of the Silkworm; and the infusion, concentrated into a few drops, is tested according to the usual method. The result is wonderfully definite. A smarting sore on the arm, similar in its mode of appearance and in its effects to that produced by the droppings of the Processionary, assures me that logic was right.

Yes, the virus which makes one scratch so much, which blisters and eats away the skin, is not a defensive product vested in only a few caterpillars. I recognize it, with its invariable properties, even in a caterpillar which at first sight appears as though it could not possess anything of the kind.

The Silkworm's virus, besides, is not unknown in my village. The casual observation of the peasant-woman has outstripped the precise observation of the man of science. The women and girls entrusted with the rearing of the Silkworm--the magnanarelles as they are called--complain of certain tribulations caused, they say, by lou venn di magnan, the Silkworms' poison. This trouble consists of a violent itching of the eyelids, which become red and swollen. In the case of the more susceptible, there is a rash and the skin peels off the fore-arm, which the turned-up sleeves fail to protect during work.

I now know the cause of this little trouble, my plucky magnanarelles. It is not contact with the worm that afflicts you; you need have no fear of handling him. It is only the litter that you need distrust. There, jumbled up with the remains of the mulberry-leaves, is a copious mass of droppings, impregnated with the substance which has just so painfully eaten into my skin; there and there only is lou verin, as you call it.

It is a relief merely to know the cause of one 's trouble; but I will provide you with another consolation. When you remove the litter and renew the leaves, you should raise the irritant dust as little as possible; you should avoid lifting your hands to your face, above all to your eyes; and it is just as well to turn down your sleeves in order to protect your arms. If you take these precautions, you will suffer no unpleasantness.

The successful result obtained with the Silkworm caused me to foresee a similar success with any caterpillar that I might come across. The facts fully confirmed my expectations. I tested the stercoral pellets of various caterpillars, not selected, but just as the hazard of collecting provided them: the Great Tortoiseshell, the Heath Fritillary, the Large Cabbage Butterfly, the Spurge Hawk-moth, the Great Peacock Moth, the Death's-head Moth, the Puss-moth, the Tiger-moth and the Arbutus Liparis. All my tests, with not a single exception, brought about stinging, of various degrees of violence, it is true. I attribute these differences in the result to the greater or lesser quantities of the virus employed, for it is impossible to measure the dose.

So the urticating excretion is common to all the caterpillars. By a very unexpected reversion of the usual order of things, the popular repugnance is well-founded; prejudice becomes truth: all caterpillars are venomous. We must draw a distinction, however: with the same venomous properties, some are inoffensive and others, far less numerous, are to be feared. Whence comes this difference?

I note that the caterpillars marked out as stinging live in communities and weave themselves dwellings of silk, in which they stay for long periods. Moreover, they are furry. Of this number are the Pine Processionary, the Oak Processionary and the caterpillars of various Lipares.

Let us consider the first-named in particular. His nest, a voluminous bag spun at the tip of a branch, is magnificent in its silky whiteness, on the outside; inside, it is a disgusting cesspit. The colony remains in it all day and for the greater part of the night. It sallies forth in procession only in the late hours of twilight, to browse upon the adjacent foliage. This long internment leads to a considerable accumulation of droppings in the heart of the dwelling.

From all the threads of this labyrinth hang chaplets of these droppings; the walls are upholstered with them in all the corridors; the little narrow chambers are encumbered with them. From a nest the size of a man's head I have obtained, with a sieve, over three-quarters of a pint of stercoral pellets.

Now it is in the midst of this ordure that the caterpillars live and have their being; in the midst of it they move, swarm and sleep. The results of this utter contempt for the rules of cleanliness are obvious. Certainly, the Processionary does not soil his coat by contact with those dry pellets; he leaves his home with his costume neat and glossy, suggesting not a suspicion of uncleanliness. No matter: by constantly rubbing against the droppings, his bristles are inevitably smeared with virus and their barbs poisoned. The caterpillar becomes irritant, because his manner of life subjects him to prolonged contact with his own ordure.

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 Silk-worms swarm. How is it that they do not acquire the poisonous properties of their own excrement?

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 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 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.

I send them away, saying to them:

„Be sure and remember, kiddies, what you have just seen; and, if ever any one talks to you about showers of blood, don't be silly and frightened. A pretty Butterfly is the cause of those blood-red stains, which have been known to terrify country-folk. The moment she is born, she casts out, in the form of a red liquid, the remains of her old caterpillar body, a body remodelled and reborn in a beautiful shape. That is the whole secret.“

When my artless visitors have departed, I resume my examination of the rain of blood falling under the cover. Still clinging to the shell of its chrysalis, each Tortoiseshell ejects and sheds upon the paper a great red drop, which, if left standing, deposits a powdery pink sediment, composed of urates. The liquid is now a deep crimson.

When the whole thing is perfectly dry, I cut out of the spotted paper some of the richer stains and steep the bits in ether. The spots on the paper remain as red as at the outset; and the liquid assumes a light lemon tint. When reduced by evaporation to a few drops, this liquid provides me with what I require to soak my square of blotting-paper.

What shall I say to avoid repeating myself? The effects of the new caustic are precisely the same as those which I experienced when I used the droppings of the Processionary. The same itching, the same burning, the same swelling with the flesh throbbing and inflamed, the same serous exudation, the same peeling of the skin, the same persistent redness, which lingers for three or four months, long after the ulceration itself has disappeared.

Without being very painful, the sore is so irksome and above all looks so ugly that I swear never to let myself in for it again Henceforth, without waiting for the thing to eat into my flesh, I shall remove the caterpillar plaster as soon as I feel a conclusive itching.

In the course of these painful experiences, friends upbraid me with not having recourse to the assistance of some animal, such as the Guinea-pig, that stock victim of the physiologists. I take no note of their reproaches The animal is a stoic. It says nothing of its sufferings. If, the torture being a little too intense, it complains, I am in no position to interpret its cries exactly or to attribute them to a definite impression. The Guinea-pig will not say:

„It smarts, it itches, it burns.“

He will simply say:

„That hurts.“

As I want to know the details of the sensations experienced, the best thing is to resort to my own skin, the only witness on whose evidence I can rely implicitly.

At the risk of provoking a smile, I will venture on another confession. As I begin to see into the matter more clearly, I hesitate to torture or destroy a single creature in God's great community. The life of the least of these is a thing to be respected. We can take it away, but we cannot give it. Peace to those innocents, so little interested in our investigations! What does our restless curiosity matter to their calm and sacred ignorance? If we wish to know, let us pay the price ourselves as far as possible. The acquisition of an idea is well worth the sacrifice of a bit of skin.

The Elm Tortoiseshell, with her rain of blood, may leave us to a certain extent in doubt. Might not this strange red substance, with its unusual appearance, contain a poison which is likewise exceptional? I address myself therefore to the Mulberry Bombyx, to the Pine Bombyx and to the Great Peacock.

I collect the uric excretions ejected by the newly hatched Moths.

This time, the liquid is whitish, sullied here and there with uncertain tints. There is no blood-red colouration; but the result is the same. The virulent energy manifests itself in the most definite manner. Therefore the Processionary's vi­rus exists equally in all caterpillars, in all Butterflies and Moths emerging from the chrysalis; and this virus is a byproduct of the organism, a urinary product.

The curiosity of our minds is insatiable. The moment a reply is obtained, a fresh question arises. Why should the Lepidoptera alone be endowed in this manner? The organic labours accomplished within them cannot differ greatly, as to the nature of the materials, from those presiding over the maintenance of life in other insects. There. fore these others also elaborate a by-product which has stinging powers. This can be verified-and that forthwith-with the elements at my disposal.

The first reply is furnished by Cetonia floricola, of which Beetle I collect half a dozen chrysalids from a heap of leaves half-converted into mould. A box receives my find, laid on a sheet of white paper, on which the urinary fluid of the perfect insect will fall as soon as the caskets are broken. The weather is favourable and I have not long to wait. The thing is done: the matter rejected is white, the usual colour of these residua, in the great majority of insects, at the moment of the metamorphosis. Though by no means abundant, it nevertheless provokes on my fore-arm a violent itching, together with mortification of the skin, which comes off in flakes. The reason why it does not display a more distinct sore is that I judged it prudent to end the experiment. The burning and itching tell me enough as to the results of a contact unduly prolonged.

Now to the Hymenoptera. I have not in my possession, I regret to say, any of those with whom my rearing-chambers used formerly to provide me, whether Honey-bee or Hunting Wasps. I have only a Green Saw-fly, whose larva lives in numerous families on the leaves of the alder. Reared under cover, this larva provides me with enough tiny black droppings to fill a thimble. That is sufficient: the urtication is quite definite.

I take next the insects with incomplete transformations. My recent rearings have given me quite a collection of excretions emanating from the Orthoptera. I consult those of the Vine Ephippiger species of Grasshopper.--Translator's Note. and the Great Grey Locust. Both sting to a degree which once more makes me regret my lavish hand.

We will be satisfied with this; indeed my arms demand as much, for, tattooed with, red squares, they refuse to make room for fresh brandings. The examples are sufficiently varied to impose the following conclusion: the Processionary's vi­rus is found in a host of other insects, apparently even in the entire series. It is a urinary product inherent in the entomological organism.

The dejections of insects, especially those evacuated at the end of the metamorphosis, contain or are even almost entirely composed of urates. Can the stinging material be the inevitable associate of uric acid? It should then form part of the excrement of the bird and the reptile, which in both cases is very rich in urates. Here again is a suspicion worthy of verification by experiment.

For the moment it is impossible for me to question the reptile; it is easy, on the other hand, to interrogate the bird, whose reply will suffice. I accept what is offered by chance: an insectivorous bird, the Swallow, and a graminivorous bird, the Goldfinch. Well, their urinary dejections, when carefully separated from the digestive residua, have not the slightest stinging effect. The virus that causes itching s independent therefore of uric acid. It accompanies it in the insect class, without being its invariable concomitant every elsewhere.

A last step remains for us to take, namely, to isolate the stinging element and to obtain it in quantities permitting of precise enquiries into its nature and properties. It seems to me that medical science might turn to account a material whose energy rivals that of cantharides, if it does not exceed it. The question appeals to me. I would gladly return to my beloved chemistry; but I should want reagents, apparatus, a laboratory, a whole costly arsenal of which I must not dream, afflicted as I am with a terrible ailment: impecuniosity, the searcher's ha­bitual lot.


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