To purge the earth of death's impurities and cause deceased animal
matter to be once more numbered among the treasures of life there
are hosts of sausage queens, including, in our part of the world, the
bluebottle (Calliphora vomitaria, LIN.) and the checkered flesh fly
(Sarcophaga carnaria, LIN.). Every one knows the first, the big,
dark-blue fly who, after effecting her designs in the ill-watched meat
safe, settles on our window panes and keeps up a solemn buzzing, anxious
to be off in the sun and ripen a fresh emission of germs. How does
she lay her eggs, the origin of the loathsome maggot that battens
poisonously on our provisions, whether of game or butcher's meat? What
are her stratagems and how can we foil them? This is what I propose to
investigate.
The bluebottle frequents our homes during autumn and a part of winter,
until the cold becomes severe; but her appearance in the fields dates
back much earlier. On the first fine day in February, we shall see her
warming herself, chillily, against the sunny walls. In April, I notice
her in considerable numbers on the laurestinus. It is here that she
seems to pair, while sipping the sugary exudations of the small white
flowers. The whole of the summer season is spent out of doors, in brief
flights from one refreshment bar to the next. When autumn comes, with
its game, she makes her way into our houses and remains until the hard
frosts.
This suits my stay-at-home habits and especially my legs, which are
bending under the weight of years. I need not run after the subjects of
my present study; they call on me. Besides, I have vigilant assistants.
The household knows of my plans. Every one brings me, in a little screw
of paper, the noisy visitor just captured against the panes.
Thus do I fill my vivarium, which consists of a large, bell-shaped
cage of wire gauze, standing in an earthenware pan full of sand. A
mug containing honey is the dining room of the establishment. Here the
captives come to recruit themselves in their hours of leisure. To
occupy their maternal cares, I employ small birds--chaffinches, linnets,
sparrows--brought down, in the enclosure, by my son's gun.
I have just served up a Linnet shot two days ago. I next place in the
cage a bluebottle, one only, to avoid confusion. Her fat belly proclaims
the advent of a laying time. An hour later, when the excitement of being
put in prison is allayed, my captive is in labor. With eager, jerky
steps, she explores the morsel of game, goes from the head to the tail,
returns from the tail to the head, repeats the action several times and
at last settles near an eye, a dimmed eye sunk into its socket.
The ovipositor bends at a right angle and dives into the junction of the
beak, straight down to the root. Then the eggs are emitted for nearly
half an hour. The layer, utterly absorbed in her serious business,
remains stationary and impassive and is easily observed through my lens.
A movement on my part would doubtless scare her; but my restful presence
gives her no anxiety. I am nothing to her.
The discharge does not go on continuously until the ovaries are
exhausted; it is intermittent and performed in so many packets. Several
times over, the fly leaves the bird's beak and comes to take a rest upon
the wire gauze, where she brushes her hind legs one against the other.
In particular, before using it again, she cleans, smoothes and polishes
her laying tool, the probe that places the eggs. Then, feeling her womb
still teeming, she returns to the same spot at the joint of the beak.
The delivery is resumed, to cease presently and then begin anew. A
couple of hours are thus spent in alternate standing near the eye and
resting on the wire gauze.
At last, it is over. The fly does not go back to the bird, a proof
that her ovaries are exhausted. The next day, she is dead. The eggs are
dabbed in a continuous layer, at the entrance to the throat, at the
root of the tongue, on the membrane of the palate. Their number appears
considerable; the whole inside of the gullet is white with them. I fix
a little wooden prop between the two mandibles of the beak, to keep them
open and enable me to see what happens.
I learn in this way that the hatching takes place in a couple of days.
As soon as they are born, the young vermin, a swarming mass, leave the
place where they are and disappear down the throat. To inquire further
into the work is useless for the moment. We shall learn more about it
later, under conditions that make examination easier.
The beak of the bird invaded was closed at the start, as far as the
natural contact of the mandibles allowed. There remained a narrow slit
at the base, sufficient at most to admit the passage of a horsehair.
It was through this that the laying was performed. Lengthening her
ovipositor like a telescope, the mother inserted the point of her
implement, a point slightly hardened with a horny armor. The fineness
of the probe equals the fineness of the aperture. But, if the beak were
entirely closed, where would the eggs be laid then?
With a tied thread, I keep the two mandibles in absolute contact; and
I place a second bluebottle in the presence of the linnet, which the
colonists have already entered by the beak. This time, the laying
takes place on one of the eyes, between the lid and the eyeball. At
the hatching, which again occurs a couple of days later, the grubs make
their way into the fleshy depths of the socket. The eyes and the beak,
therefore, form the two chief entrances into feathered game.
There are others; and these are the wounds. I cover the linnet's head
with a paper hood which will prevent invasion through the beak and eyes.
I serve it, under the wire gauze bell, to a third egg layer. The bird
has been struck by a shot in the breast, but the sore is not bleeding:
no outer stain marks the injured spot. Moreover, I am careful to arrange
the feathers, to smooth them with a hair pencil, so that the bird looks
quite smart and has every appearance of being untouched.
The fly is soon there. She inspects the linnet from end to end; with
her front tarsi she fumbles at the breast and belly. It is a sort of
auscultation by sense of touch. The insect becomes aware of what is
under the feathers by the manner in which these react. If scent comes
to her assistance, it can only be very slightly, for the game is not yet
high. The wound is soon found. No drop of blood is near it, for it is
closed by a plug of down rammed into it by the shot. The fly takes up
her position without separating the feathers or uncovering the wound.
She remains here for two hours without stirring, motionless, with her
abdomen concealed beneath the plumage. My eager curiosity does not
distract her from her business for a moment.
When she has finished, I take her place. There is nothing either on the
skin or at the mouth of the wound. I have to withdraw the downy plug
and dig to some depth before discovering the eggs. The ovipositor has
therefore lengthened its extensible tube and pushed beyond the feather
stopper driven in by the lead. The eggs are in one packet; they number
about three hundred.
When the beak and eyes are rendered inaccessible, when the body,
moreover, has no wounds, the laying still takes place, but, this time,
in a hesitating and niggardly fashion. I pluck the bird completely, the
better to watch what happens; also, I cover the head with a paper hood
to close the usual means of access. For a long time, with jerky steps,
the mother explores the body in every direction; she takes her stand by
preference on the head, which she sounds by tapping on it with her front
tarsi. She knows that the openings which she needs are there, under
the paper; but she also knows how frail are her grubs, how powerless to
pierce their way through the strange obstacle which stops her as well
and interferes with the work of her ovipositor. The cowl inspires her
with profound distrust. Despite the tempting bait of the veiled head,
not an egg is laid on the wrapper, slight though it may be.
Weary of vain attempts to compass this obstacle, the Fly at last decides
in favor of other points, but not on the breast, belly or back, where
the hide would seem too tough and the light too intrusive. She needs
dark hiding places, corners where the skin is very delicate. The spots
chosen are the cavity of the axilla, corresponding with our armpit,
and the crease where the thigh joins the belly. Eggs are laid in both
places, but not many, showing that the groin and the axilla are adopted
only reluctantly and for lack of a better spot.
With an unplucked bird, also hooded, the same experiment failed: the
feathers prevent the fly from slipping into those deep places. Let us
add, in conclusion, that, on a skinned bird, or simply on a piece of
butcher's meat, the laying is effected on any part whatever, provided
that it be dark. The gloomiest corners are the favorite ones.
It follows from all this that, to lay the eggs, the Bluebottle picks out
either naked wounds or else the mucous membranes of the mouth or eyes,
which are not protected by a skin of any thickness. She also needs
darkness. We shall see the reasons for her preference later on.
The perfect efficiency of the paper bag, which prevents the inroads
of the worms through the eye sockets or the beak, suggests a similar
experiment with the whole bird. It is a matter of wrapping the body in a
sort of artificial skin which will be as discouraging to the fly as the
natural skin. Linnets, some with deep wounds, others almost intact,
are placed one by one in paper envelopes similar to those in which the
nursery gardener keeps his seeds, envelopes just folded, without being
stuck. The paper is quite ordinary and of average thickness. Torn pieces
of newspaper serve the purpose.
These sheaths with the corpses inside them are freely exposed to the
air, on the table in my study, where they are visited, according to the
time of day, in dense shade and in bright sunlight. Attracted by the
effluvia from the dead meat, the bluebottles haunt my laboratory, the
windows of which are always open. I see them daily alighting on the
envelopes and very busily exploring them, apprised of the contents by
the gamy smell. Their incessant coming and going is a sign of intense
cupidity; and yet none of them decides to lay on the bags. They do not
even attempt to slide their ovipositor through the slits of the folds.
The favorable season passes and not an egg is laid on the tempting
wrappers. All the mothers abstain, judging the slender obstacle of the
paper to be more than the vermin will be able to overcome.
This caution on the fly's part does not at all surprise me: motherhood
everywhere has gleams of great perspicacity. What does astonish me is
the following result. The parcels containing the linnets are left for a
whole year uncovered on the table; they remain there for a second year
and a third. I inspect the contents from time to time. The little birds
are intact, with unrumpled feathers, free from smell, dry and light,
like mummies. They have become not decomposed, but mummified.
I expected to see them putrefying, running into sanies, like corpses
left to rot in the open air. On the contrary, the birds have dried and
hardened, without undergoing any change. What did they want for their
putrefaction? Simply the intervention of the fly. The maggot, therefore,
is the primary cause of dissolution after death; it is, above all, the
putrefactive chemist.
A conclusion not devoid of value may be drawn from my paper game bags.
In our markets, especially in those of the South, the game is hung
unprotected from the hooks on the stalls. Larks strung up by the dozen
with a wire through their nostrils, thrushes, plovers, teal, partridges,
snipe, in short, all the glories of the spit which the autumn migration
brings us, remain for days and weeks at the mercy of the flies. The
buyer allows himself to be tempted by a goodly exterior; he makes his
purchase and, back at home, just when the bird is being prepared for
roasting, he discovers that the promised dainty is alive with worms. O
horror! There is nothing for it but to throw the loathsome, verminous
thing away.
The bluebottle is the culprit here. Everybody knows it; and nobody
thinks of seriously shaking off her tyranny: not the retailer, nor the
wholesale dealer, nor the killer of the game. What is wanted to keep the
maggots out? Hardly anything: to slip each bird into a paper sheath. If
this precaution were taken at the start, before the flies arrive, any
game would be safe and could be left indefinitely to attain the degree
of ripeness required by the epicure's palate.
Stuffed with olives and myrtle berries, the Corsican blackbirds are
exquisite eating. We sometimes receive them at Orange, layers of them,
packed in baskets through which the air circulates freely and
each contained in a paper wrapper. They are in a state of perfect
preservation, complying with the most exacting demands of the kitchen.
I congratulate the nameless shipper who conceived the bright idea of
clothing his blackbirds in paper. Will his example find imitators? I
doubt it.
There is, of course, a serious objection to this method of preservation.
In its paper shroud, the article is invisible; it is not enticing; it
does not inform the passer by of its nature and qualities. There is one
resource left which would leave the bird uncovered: simply to case the
head in a paper cap. The head being the part most threatened, because of
the mucus membrane of the throat and eyes, it would be sufficient, as a
rule, to protect the head, in order to keep off the Flies and to thwart
their attempts.
Let us continue to study the bluebottle, while varying our means
of information. A tin, about four inches deep, contains a piece of
butcher's meat. The lid is not put in quite straight and leaves a
narrow slit at one point of its circumference, allowing, at most, of the
passage of a fine needle. When the bait begins to give off a gamy scent,
the mothers come. Singly or in numbers. They are attracted by the odor
which, transmitted through a thin crevice, hardly reaches my nostrils.
They explore the metal receptacle for some time, seeking an entrance.
Finding naught that enables them to reach the coveted morsel, they
decide to lay their eggs on the tin, just beside the aperture.
Sometimes, when the width of the passage allows of it, they insert the
ovipositor into the tin and lay the eggs inside, on the very edges of
the slit. Whether outside or in, the eggs are dabbed down in a fairly
regular and absolutely white layer. I as it were shovel them up with a
little paper scoop. I thus obtain all the germs that I require for
my experiments, eggs bearing no trace of the stains which would be
inevitable if I had to collect them on tainted meat.
We have seen the bluebottle refusing to lay her eggs on the paper bag,
notwithstanding the carrion fumes of the Linnet enclosed; yet now,
without hesitation, she lays them on a sheet of metal. Can the nature of
the floor make any difference to her? I replace the tin lid by a paper
cover stretched and pasted over the orifice. With the point of my knife,
I make a narrow slit in this new lid. That is quite enough: the parent
accepts the paper.
What determined her, therefore, is not simply the smell, which can
easily be perceived even through the uncut paper, but, above all, the
crevice, which will provide an entrance for the vermin, hatched outside,
near the narrow passage. The maggots' mother has her own logic, her
prudent foresight. She knows how feeble her wee grubs will be, how
powerless to cut their way through an obstacle of any resistance; and
so, despite the temptation of the smell, she refrains from laying so
long as she finds no entrance through which the newborn worms can slip
unaided.
I wanted to know whether the color, the shininess, the degree of
hardness and other qualities of the obstacle would influence the
decision of a mother obliged to lay her eggs under exceptional
conditions. With this object in view, I employed small jars, each baited
with a bit of butcher's meat. The respective lids were made of different
colored paper, of oilskin, or of some of that tinfoil, with its gold or
coppery sheen, which is used for sealing liqueur bottles. On not one
of these covers did the mothers stop, with any desire to deposit their
eggs; but, from the moment that the knife had made the narrow slit,
all the lids were, sooner or later, visited and all of them, sooner or
later, received the white shower somewhere near the gash. The look of
the obstacle, therefore, does not count; dull or brilliant, drab or
colored: these are details of no importance; the thing that matters is
that there should be a passage to allow the grubs to enter.
Though hatched outside, at a distance from the coveted morsel, the
newborn worms are well able to find their refectory. As they release
themselves from the egg, without hesitation, so accurate is their scent,
they slip beneath the edge of the ill-joined lid, or through the passage
cut by the knife. Behold them entering upon their promised land, their
reeking paradise.
Eager to arrive, do they drop from the top of the wall? Not they! Slowly
creeping, they make their way down the side of the jar; they use their
fore part, ever in quest of information, as a crutch and grapnel in one.
They reach the meat and at once install themselves upon it.
Let us continue our investigation, varying the conditions. A large
test-tube, measuring nine inches high, is baited at the bottom with a
lump of butcher's meat. It is closed with wire gauze, whose meshes, two
millimeters wide, do not permit of the fly's passage. The bluebottle
comes to my apparatus, guided by scent rather than sight. She hastens to
the test tube whose contents are veiled under an opaque cover with the
same alacrity as to the open tube. The invisible attracts her quite as
much as the visible.
She stays a while on the lattice of the mouth, inspects it attentively;
but, whether because circumstances have failed to serve me, or because
the wire network inspires her with distrust, I never saw her dab her
eggs upon it for certain. As her evidence was doubtful, I had recourse
to the flesh fly (Sarcophaga carnaria).
This fly is less finicky in her preparations, she has more faith in the
strength of her worms, which are born ready-formed and vigorous, and
easily shows me what I wish to see. She explores the trellis-work,
chooses a mesh through which she inserts the tip of her abdomen and,
undisturbed by my presence, emits, one after the other, a certain number
of grubs, about ten or so. True, her visits will be repeated, increasing
the family at a rate of which I am ignorant.
The newborn worms, thanks to a slight viscidity, cling for a moment to
the wire gauze; they swarm, wriggle, release themselves and leap into
the chasm. It is a nine inch drop at least. When this is done, the
mother makes off, knowing for a certainty that her offspring will shift
for themselves. If they fall on the meat, well and good; if they fall
elsewhere, they can reach the morsel by crawling.
This confidence in the unknown factor of the precipice, with no
indication but that of smell, deserves fuller, investigation. From
what height will the flesh fly dare to let her children drop? I top the
test-tube with another tube, the width of the neck of a claret bottle.
The mouth is closed either with wire gauze, or with a paper cover with a
slight cut in it. Altogether, the apparatus measures twenty-five inches
in height. No matter: the fall is not serious for the lithe backs of the
young grubs; and, in a few days, the test-tube is filled with larvae,
in which it is easy to recognize the flesh fly's family by the fringed
coronet that opens and shuts at the maggot's stern like the petals of
a little flower. I did not see the mother operating: I was not there at
the time; but there is no doubt possible of her coming nor of the great
dive taken by the family: the contents of the test-tube furnish me with
a duly authenticated certificate.
I admire the leap and, to obtain one better still, I replace the tube
by another, so that the apparatus now stands forty-six inches high. The
column is erected at a spot frequented by flies, in a dim light. Its
mouth, closed with a wire gauze cover, reaches the level of various
other appliances, test-tubes and jars, which are already stocked or
awaiting their colony of vermin. When the position is well known to the
flies, I remove the other tubes and leave the column, lest the visitors
should turn aside to easier ground.
From time to time, the bluebottle and the flesh fly perch on the
trellis-work, make a short investigation and then decamp. Throughout the
summer season, for three whole months, the apparatus remains where it
is, without the least result: never a worm. What is the reason? Does
the stench of the meat not spread, coming from that depth? Certainly it
spreads: it is unmistakable to my dulled nostrils and still more so to
the nostrils of my children, whom I call to bear witness. Then why does
the flesh fly, who but now was dropping her grubs from a goodly height,
refuse to let them fall from the top of a column twice as high? Does
she fear lest her worms should be bruised by an excessive drop? There
is nothing about her to point to anxiety aroused by the length of the
shaft. I never see her explore the tube or take its size. She stands on
the trellised orifice; and there the matter ends. Can she be apprised
of the depth of the chasm by the comparative faintness of the offensive
odors that arise from it? Can the sense of smell measure the distance
and judge whether it be acceptable or not? Perhaps.
The fact remains that, despite the attraction of the scent, the flesh
fly does not expose her worms to disproportionate falls. Can she know
beforehand that, when the chrysalides break, her winged family, knocking
with a sudden flight against the sides of a tall chimney, will be unable
to get out? This foresight would be in agreement with the rules which
order maternal instinct according to future needs.
But when the fall does not exceed a certain depth, the budding worms of
the flesh fly are dropped without a qualm, as all our experiments show.
This principle has a practical application which is not without its
value in matters of domestic economy. It is as well that the wonders of
entomology should sometimes give us a hint of commonplace utility.
The usual meat safe is a sort of large cage with a top and bottom
of wood and four wire gauze sides. Hooks fixed into the top are used
whereby to hang pieces which we wish to protect from the flies. Often,
so as to employ the space to the best advantage, these pieces are simply
laid on the floor on the cage. With these arrangements, are we sure of
warding off the fly and her vermin?
Not at all. We may protect ourselves against the Bluebottle, who is not
much inclined to lay her eggs at a distance from the meat; but there is
still the flesh fly, who is more venturesome and goes more briskly to
work and who will slip the grubs through a hole in the meshes and drop
them inside the safe. Agile as they are and well able to crawl, the
worms will easily reach anything on the floor; the only things secure
from their attacks will be the pieces hanging from the ceiling. It is
not in the nature of maggots to explore the heights, especially if this
implies climbing down a string in addition.
People also use wire gauze dish covers. The trellised dome protects the
contents even less than does the meat safe. The flesh fly takes no heed
of it. She can drop her worms through the meshes on the covered joint.
Then what are we to do? Nothing could be simpler. We need only wrap
the birds which we wish to preserve--thrushes, partridges, snipe and so
on--in separate paper envelopes; and the same with our beef and mutton.
This defensive armor alone, while leaving ample room for the air to
circulate, makes any invasion by the worms impossible, even without a
cover or a meat safe: not that paper possesses any special preservative
virtues, but solely because it forms an impenetrable barrier. The
Bluebottle carefully refrains from laying her eggs upon it and the flesh
fly from bringing forth her offspring, both of them knowing that their
newborn young are incapable of piercing the obstacle.
Paper is equally successful in our strife against the Moths, those
plagues of our furs and clothes. To keep away these wholesale ravages,
people generally use camphor, naphthalene, tobacco, bunches of lavender
and other strong-scented remedies. Without wishing to malign those
preservatives, we are bound to admit that the means employed are none
too effective. The smell does very little to prevent the havoc of the
moths.
I would therefore counsel our housewives, instead of all this chemist's
stuff, to use newspapers of a suitable shape and size. Take whatever you
wish to protect--your furs, your flannel or your clothes--and pack each
article carefully in a newspaper, joining the edges with a double fold,
well pinned. If this joining is properly done, the Moth will never get
inside. Since my advice has been taken and this method employed in my
household, the old damage has never been repeated.
To return to the fly. A piece of meat is hidden in a jar under a layer
of fine, dry sand, a finger's-breadth thick. The jar has a wide mouth
and is left quite open. Let whoever come that will, attracted by the
smell. The Bluebottles are not long in inspecting what I have prepared
for them: they enter the jar, go out and come back again, inquiring into
the invisible thing revealed by its fragrance. A diligent watch enables
me to see them fussing about, exploring the sandy expanse, tapping it
with their feet, sounding it with their proboscis. I leave the visitors
undisturbed for a fortnight or three weeks. None of them lays any eggs.
This is a repetition of what the paper bag, with its dead bird, showed
me. The flies refuse to lay on the sand, apparently for the same
reasons. The paper was considered an obstacle which the frail vermin
would not be able to overcome. With sand, the case is worse. Its
grittiness would hurt the newborn weaklings, its dryness would absorb
the moisture indispensable to their movements. Later, when preparing for
the metamorphosis, when their strength has come to them, the grubs will
dig the earth quite well and be able to descend; but, at the start,
that would be very dangerous for them. Knowing these difficulties, the
mothers, however greatly tempted by the smell, abstain from breeding. As
a matter of fact, after long waiting, fearing lest some packets of eggs
may have escaped my attention, I inspect the contents of the jar from
top to bottom. Meat and sand contain neither larvae nor pupae: the whole
is absolutely deserted.
The layer of sand being only a finger's-breadth thick, this experiment
requires certain precautions. The meat may expand a little, in going
bad, and protrude in one or two places. However small the fleshy eyots
that show above the surface, the flies come to them and breed. Sometimes
also the juices oozing from the putrid meat soak a small extent of the
sandy floor. That is enough for the maggot's first establishment. These
causes of failure are avoided with a layer of sand about an inch thick.
Then the bluebottle, the flesh fly and other flies whose grubs batten on
dead bodies are kept at a proper distance.
In the hope of awakening us to a proper sense of our insignificance,
pulpit orators sometimes make an unfair use of the grave and its worms.
Let us put no faith in their doleful rhetoric. The chemistry of man's
final dissolution is eloquent enough of our emptiness: there is no need
to add imaginary horrors. The worm of the sepulchre is an invention of
cantankerous minds, incapable of seeing things as they are. Covered by
but a few inches of earth, the dead can sleep their quiet sleep: no fly
will ever come to take advantage of them.
At the surface of the soil, exposed to the air, the hideous invasion
is possible; ay, it is the invariable rule. For the melting down and
remolding of matter, man is no better, corpse for corpse, than the
lowest of the brutes. Then the fly exercises her rights and deals with
us as she does with any ordinary animal refuse. Nature treats us with
magnificent indifference in her great regenerating factory: placed in
her crucibles, animals and men, beggars and kings are one and all alike.
There you have true equality, the only equality in this world of ours:
equality in the presence of the maggot.