AN INSECT VIRUS
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 offthrow 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, [23]
who 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 verin
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 Silkworms 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 by-product 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. Therefore
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 [24] 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 is
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.