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iStudy Flies - THE GREY FLESH FLY (1913) by Jean-Henri Fabre
THE GREY FLESH FLY (1913) by Jean-Henri Fabre
The following extract is from The Life of the Fly [48] by Jean-Henri Fabre (1823-1915). Fabre was a French entomologist, who is considered by many the father
of modern entomology (the scientific study of insects). In it, Chapter 10 "The
Grey Flesh Fly," Fabre traces the unique life cycle of the grey flesh fly --
of the family Sarcophagidae -- whose larvae (maggots) consume dead carcasses. Maggots,
remember, serve many useful purposes, e.g., forensic entomologists use the age of
the oldest larvae present in a dead body to extrapolate backwards the time of death
of the decomposing corpse. Notice Fabre's description of how the adult female grey
flesh fly deposits its offspring, i.e., "her abdomen touches the meat; and the
thing is done: a group of vermin wriggles out, releases itself and disperses
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Grey Flesh Flies
Image source: Pictures of Flies & Other Observations
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so nimbly that I have no time to take my lens and count them accurately."
Listen, as Fabre helps us think through the stages of fly development -- the larva
stage is followed by the pupa stage. "When duly fattened, the grubs [larvae]
of the flesh flies go underground to transform themselves into pupae. The burial
is intended, obviously, to give the worm the tranquillity necessary for the metamorphosis."
Jean-Henri Fabre, with the authority of a scientific naturalist, spoke in an approachable
voice to his thoughtful, but less formally trained audience. Again listen, as Fabre
speaks with great animation of the emergence of the adult grey flesh fly from the
pupa. "Enclosed in her pupa, the nascent fly begins by bursting the lid of her
casket with a hernia which comes between her two eyes and doubles or trebles the
size of her head." Read the rest of the interesting context of Fabre's Chapter
10 "The Grey Flesh Fly":
- Here the costume changes, not the manner of life. We find the
same frequenting of dead bodies, the same capacity for the speedy liquefaction of
the fleshy matter. I am speaking of an ash-gray fly, the greenbottle's superior in
size, with brown streaks on her back and silver gleams on her abdomen. Note also
the blood-red eyes, with the hard look of the knacker in them. The language of science
knows her as Sarcophaga, the flesh eater; in the vulgar tongue she is the grey flesh
fly, or simply the flesh fly.
Let not these expressions, however accurate, mislead us into believing for a moment
that the Sarcophagae are the bold company of master tainters who haunt our dwellings,
more particularly in autumn, and plant their vermin in our ill-guarded viands. The
author of those offences is Calliphora vomitoria, the bluebottle, who is of a stouter
build and arrayed in darkest blue. It is she who buzzes against our windowpanes,
who craftily besieges the meat safe and who lies in wait in the darkness for an opportunity
to outwit our vigilance. The other, the grey fly, works jointly with the greenbottles,
who do not venture inside our houses and who work in the sunlight. Less timid, however,
than they, should the outdoor yield be small, she will sometimes come indoors to
perpetrate her villainies. When her business is done, she makes off as fast as she
can, for she does not feel at home with us.
At this moment, my study, a very modest extension of my open air establishments,
has become something of a charnel house. The grey fly pays me a visit. If I lay a
piece of butcher's meat on the windowsill, she hastens up, works her will on it and
retires. No hiding place escapes her notice among the jars, cups, glasses and receptacles
of every kind with which my shelves are crowded.
With a view to certain experiments, I collected a heap of wasp grubs, asphyxiated
in their underground nests. Stealthily she arrives, discovers the fat pile and, hailing
as treasure trove this provender whereof her race perhaps has never made use before,
entrusts to it an installment of her family. I have left at the bottom of a glass
the best part of a hard-boiled egg from which I have taken a few bits of white intended
for the greenbottle maggots. The grey fly takes possession of the remains, recks
not of their novelty and colonizes them. Everything suits her that falls within the
category of albuminous matters: everything, down to dead silkworms; everything, down
to a mess of kidney-beans and chick-peas.
Nevertheless, her preference is for the corpse: furred beast and feathered beast,
reptile and fish, indifferently. Together with the greenbottles, she is sedulous
in her attendance on my pans. Daily she visits my snakes, takes note of the condition
of each of them, savors them with her proboscis, goes away, comes back, takes her
time and at last proceeds to business. Still, it is not here, amid the tumult of
callers, that I propose to follow her operations. A lump of butcher's meat laid on
the window sill, in front of my writing table, will be less offensive to the eye
and will facilitate my observations.
Two flies of the genus Sarcophaga frequent my slaughter yard: Sarcophaga carnaria
and Sarcophaga haemorrhoidalis, whose abdomen ends in a red speck. The first species,
which is a little larger than the second, is more numerous and does the best part
of the work in the open air shambles of the pans. It is this fly also who, at intervals
and nearly always alone, hastens to the bait exposed on the windowsill.
She comes up suddenly, timidly. Soon she calms herself and no longer thinks of fleeing
when I draw near, for the dish suits her. She is surprisingly quick about her work.
Twice over--buzz! Buzz!- -the tip of her abdomen touches the meat; and the thing
is done: a group of vermin wriggles out, releases itself and disperses so nimbly
that I have no time to take my lens and count them accurately. As seen by the naked
eye, there were a dozen of them. What has become of them? One would think that they
had gone into the flesh, at the very spot where they were laid, so quickly have they
disappeared. But that dive into a substance of some consistency is impossible to
these newborn weaklings. Where are they? I find them more or less everywhere in the
creases of the meat; singly and already groping with their mouths. To collect them
in order to number them is not practicable, for I do not want to damage them. Let
us be satisfied with the estimate made at a rapid glance: there are a dozen or so,
brought into the world in one discharge of almost inappreciable length.
Those live grubs, taking the place of the usual eggs, have long been known. Everybody
is aware that the flesh flies bring forth living maggots, instead of laying eggs.
They have so much to do and their work is so urgent! To them, the instruments of
the transformation of dead matter, a day means a day, a long space of time which
it is all important to utilize. The greenbottle's eggs, though these are of very
rapid development, take twenty-four hours to yield their grubs. The flesh flies save
all this time. From their matrix, laborers flow straightway and set to work the moment
they are born. With these ardent pioneers of sanitation, there is no rest attendant
upon the hatching, there is not a minute lost.
The gang, it is true, is not a numerous one; but how often can it not be renewed!
Read Reaumur's description of the wonderful procreating machinery boasted by the
Flesh flies. It is a spiral ribbon, a velvety scroll whose nap is a sort of fleece
of maggots set closely together and each cased in a sheath. The patient biographer
counted the host: it numbers, he tells us, nearly twenty thousand. You are seized
with stupefaction at this anatomical fact.
How does the gray fly find the time to settle a family of such dimensions, especially
in small packets, as she has just done on my window sill? What a number of dead dogs,
moles and snakes must she not visit before exhausting her womb! Will she find them?
Corpses of much size do not abound to that extent in the country. As everything suits
her, she will alight on other remains of minor importance. Should the prize be a
rich one, she will return to it tomorrow, the day after and later still, over and
over again. In the course of the season, by dint of packets of grubs deposited here,
there and everywhere, she will perhaps end by housing her entire brood. But then,
if all things prosper, what a glut, for there are several families born during the
year! We feel it instinctively: there must be a check to these generative enormities.
Let us first consider the grub. It is a sturdy maggot, easy to distinguish from the
greenbottle's by its larger girth and especially by the way in which its body terminates
behind. There is here a sudden breaking off, hollowed into a deep cup. At the bottom
of this crater are two breathing holes, two stigmata with amber-red tips. The edge
of the cavity is fringed with half a score of pointed, fleshy festoons, which diverge
like the spikes of a coronet. The creature can close or open this diadem at will
by bringing the denticulations together or by spreading them out wide. This protects
the air holes which might otherwise be choked up when the maggot disappears in the
sea of broth. Asphyxia would supervene, if the two breathing holes at the back became
obstructed. During the immersion, the festooned coronet shuts like a flower closing
its petals and the liquid is not admitted to the cavity.
Next follows the emergence. The hind part reappears in the air, but appears alone,
just at the level of the fluid. Then the coronet spreads out afresh, the cup gapes
and assumes the aspect of a tiny flower, with the white denticulations for petals
and the two bright red dots, the stigmata at the bottom, for stamens. When the grubs,
pressed one against the other, with their heads downwards in the fetid soup, make
an unbroken shoal, the sight of those breathing cups incessantly opening and closing,
with a little clack like a valve, almost makes one forget the horrors of the charnel
yard. It suggests a carpet of tiny Sea anemones. The maggot has its beauties after
all.
It is obvious, if there be any logic in things, that a grub so well-protected against
asphyxiation by drowning must frequent liquid surroundings. One does not encircle
one's hindquarters with a coronet for the sole satisfaction of displaying it. With
its apparatus of spokes, the Grey Fly's grub informs us of the dangerous nature of
its functions: when working upon a corpse, it runs the risk of drowning. How is that?
Remember the grubs of the greenbottle, fed on hard-boiled white of egg. The dish
suits them; only, by the action of their pepsin, it becomes so fluid that they die
submerged. Because of their hinder stigmata, which are actually on the skin and devoid
of any defensive machinery, they perish when they find no support apart from the
liquid.
The flesh fly's maggots, though incomparable liquefiers, know nothing of this peril,
even in a puddle of carrion broth. Their bulky hind part serves as a float and keeps
the air holes above the surface. When, for further investigation, they must needs
go under completely, the anemone at the back shuts and protects the stigmata. The
grubs of the gray fly are endowed with a life buoy because they are first class liquefiers,
ready to incur the danger of a ducking at any moment.
When high and dry on the sheet of cardboard where I place them to observe them at
my ease, they move about actively, with their breathing rose widespread and their
stigmata rising and falling as a support. The cardboard is on my table, at three
steps from an open window, and lit at this time of day only by the soft light of
the sky. Well, the maggots, one and all of them, turn in the opposite direction to
the window; they hastily, madly take to flight.
I turn the cardboard round, without touching the runaways. This action makes the
creatures face the light again. Forthwith, the troop stops, hesitates, takes a half
turn and once more retreats towards the darkness. Before the end of the racecourse
is reached, I again turn the cardboard. For the second time, the maggots veer round
and retrace their steps. Repeat the experiment as often as I will, each time the
squad wheels about in the opposite direction to the window and persists in avoiding
the trap of the revolving cardboard.
The track is only a short one: the cardboard measures three hand's breadths in length.
Let us give more space. I settle the grubs on the floor of the room; with a hair
pencil, I turn them with their heads pointing towards the lighted aperture. The moment
they are free, they turn and run from the light. With all the speed whereof their
cripple's shuffle allows, they cover the tiled floor of the study and go and knock
their heads against the wall, twelve feet off, skirting it afterwards, some to the
right and some to the left. They never feel far enough away from that hateful illuminated
opening.
What they are escaping from is evidently the light, for, if I make it dark with a
screen, the troop does not change its direction when I turn the cardboard. It then
progresses quite readily towards the window; but, when I remove the screen, it turns
tail at once.
That a grub destined to live in the darkness, under the shelter of a corpse, should
avoid the light is only natural; the strange part is its very perception. The maggot
is blind. Its pointed fore part, which we hesitate to call a head, bears absolutely
no trace of any optical apparatus; and the same with every other part of the body.
There is nothing but one bare, smooth, white skin. And this sightless creature, deprived
of any special nervous points served by ocular power, is extremely sensitive to the
light. Its whole skin is a sort of retina, incapable of seeing, of course, but able,
at any rate, to distinguish between light and darkness. Under the direct rays of
a searching sun, the grub's distress could be easily explained. We ourselves; with
our coarse skin, in comparison with that of the maggot, can distinguish between sunshine
and shadow without the help of the eyes. But, in the present case, the problem becomes
singularly complicated. The subjects of my experiment receive only the diffused light
of the sky, entering my study through an open window; yet this tempered light frightens
them out of their senses. They flee the painful apparition; they are bent upon escaping
at all costs.
Now what do the fugitives feel? Are they physically hurt by the chemical radiations?
Are they exasperated by other radiations, known or unknown? Light still keeps many
a secret hidden from us and perhaps our optical science, by studying the maggot,
might become the richer by some valuable information. I would gladly have gone farther
into the question, had I possessed the necessary apparatus. But I have not, I never
have had and of course I never shall have the resources which are so useful to the
seeker. These are reserved for the clever people who care more for lucrative posts
than for fair truths. Let us continue, however, within the measure which the poverty
of my means permits.
When duly fattened, the grubs of the flesh flies go underground to transform themselves
into pupae. The burial is intended, obviously, to give the worm the tranquillity
necessary for the metamorphosis. Let us add that another object of the descent is
to avoid the importunities of the light. The maggot isolates itself to the best of
its power and withdraws from the garish day before contracting into a little keg.
In ordinary conditions, with a loose soil, it goes hardly lower than a hand's breadth
down, for provision has to be made for the difficulties of the return to the surface
when the insect, now full grown, is impeded by its delicate fly wings. The grub,
therefore, deems itself suitably isolated at a moderate depth. Sideways, the layer
that shields it from the light is of indefinite thickness; upwards, it measures about
four inches. Behind this screen reigns utter darkness, the buried one's delight.
This is capital.
What would happen if, by an artifice, the sideward layer were nowhere thick enough
to satisfy the grub? Now, this time, I have the wherewithal to solve the problem,
in the shape of a big glass tube, open at both ends, about three feet long and less
than an inch wide. I use it to blow the flame of hydrogen in the little chemistry
lessons which I give my children.
I close one end with a cork and fill the tube with fine, dry, sifted sand. On the
surface of this long column, suspended perpendicularly in a corner of my study, I
install some twenty Sarcophaga grubs, feeding them with meat. A similar preparation
is repeated in a wider jar, with a mouth as broad as one's hand. When they are big
enough, the grubs in either apparatus will go down to the depth that suits them.
There is no more to be done but to leave them to their own devices.
The worms at last bury themselves and harden into pupae. This is the moment to consult
the two apparatus. The jar gives me the answer which I should have obtained in the
open fields. Four inches down, or thereabouts, the worms have found a quiet lodging,
protected above by the layer through which they have passed and on every side by
the thickness of the vessel's contents. Satisfied with the site, they have stopped
there.
It is a very different matter in the tube. The least buried of the pupae are half
a yard down. Others are lower still; most of them even have reached the bottom of
the tube and are touching the cork stopper, an insuperable barrier. These last, we
can see, would have gone yet deeper if the apparatus had allowed them. Not one of
the score of grubs has settled at the customary halting place; all have traveled
farther down the column, until their strength gave way. In their anxious flight,
they have dug deeper and ever deeper.
What were they flying from? The light. Above them, the column traversed forms a more
than sufficient shelter; but, at the sides, the irksome sensation is still felt through
a coat of earth half an inch thick if the descent is made perpendicularly. To escape
the disturbing impression, the grub therefore goes deeper and deeper, hoping to obtain
lower down the rest which is denied it above. It only ceases to move when worn out
with the effort or stopped by an obstacle.
Now, in a soft diffused light, what can be the radiations capable of acting upon
this lover of darkness? They are certainly not the simple luminous rays, for a screen
of fine, heaped up earth, nearly half an inch in thickness, is perfectly opaque.
Then, to alarm the grub, to warn it of the over proximity of the exterior and send
it to mad depths in search of isolation, other radiations, known or unknown, must
be required, radiations capable of penetrating a screen against which ordinary radiations
are powerless. Who knows what vistas the natural philosophy of the maggot might open
out to us? For lack of apparatus, I confine myself to suspicions.
To go underground to a yard's depth--and farther if my tube had allowed it--is on
the part of the Flesh fly's grub a vagary provoked by unkind experiment: never would
it bury itself so low down, if left to its own wisdom. A hand's breadth thickness
is quite enough, is even a great deal when, after completing the transformation,
it has to climb back to the surface, a laborious operation absolutely resembling
the task of an entombed well sinker. It will have to fight against the sand that
slips and gradually fills up the small amount of empty space obtained; it will perhaps,
without crowbar or pickaxe, have to cut itself a gallery through something tantamount
to tufa, that is to say, through earth which a shower has rendered compact. For the
descent, the grub has its fangs; for the assent, the fly has nothing. Only that moment
come into existence, she is a weakling, with tissues still devoid of any firmness.
How does she manage to get out? We shall know by watching a few pupae placed at the
bottom of a test-tube filled with earth. The method of the Flesh flies will teach
us that of the greenbottles and the other Flies, all of whom make use of the same
means.
Enclosed in her pupa, the nascent fly begins by bursting the lid of her casket with
a hernia which comes between her two eyes and doubles or trebles the size of her
head. This cephalic blister throbs: it swells and subsides by turns, owing to the
alternate flux and reflux of the blood. It is like the piston of an hydraulic press
opening and forcing back the front part of the keg.
The head makes its appearance. The hydrocephalous monster continues the play of her
forehead, while herself remaining stationary. Inside the pupa, a delicate work is
being performed: the casting of the white nymphal tunic. All through this operation,
the hernia is still projecting. The head is not the head of a fly, but a queer, enormous
mitre, spreading at the base into two red skull caps, which are the eyes. To split
her cranium in the middle, shunt the two halves to the right and left and send surging
through the gap a tumor which staves the barrel with its pressure: this constitutes
the Fly's eccentric method.
For what reason does the hernia, once the keg is staved, continue swollen and projecting?
I take it to be a waste pocket into which the insect momentarily forces back its
reserves of blood in order to diminish the bulk of the body to that extent and to
extract it more easily from the nymphal slough and afterwards from the narrow channel
of the shell. As long as the operation of the release lasts, it pushes outside all
that it is able to inject of its accumulated humors; it makes itself small inside
the pupa and swells into a bloated deformity without. Two hours and more are spent
in this laborious stripping.
At last, the fly comes into view. The wings, mere scanty stumps, hardly reach the
middle of the abdomen. On the outer edge, they have a deep notch similar to the waist
of a violin. This diminishes by just so much the surface and the length, an excellent
device for decreasing the friction along the earthy column which has next to be scaled.
The hydrocephalous one resumes her performance more vigorously than ever; she inflates
and deflates her frontal knob. The pounded sand rustles down the insect's sides.
The legs play but a secondary part. Stretched behind, motionless, when the piston
stroke is delivered, they furnish a support. As the sand descends, they pile it and
nimbly push it back, after which they drag along lifelessly until the next avalanche.
The head advances each time by a length equal to that of the sand displaced. Each
stroke of the frontal swelling means a step forward. In a dry, loose soil, things
go pretty fast. A column six inches high is traversed in less than a quarter of an
hour.
As soon as it reaches the surface, the insect, covered with dust, proceeds to make
its toilet. It thrusts out the blister of its forehead for the last time and brushes
it carefully with its front tarsi. It is important that the little pounding engine
should be carefully dusted before it is taken inside to form a forehead that will
open no more: this lest any grit should lodge in the head. The wings are carefully
brushed and polished; they lose their curved notches; they lengthen and spread. Then,
motionless on the surface of the sand, the fly matures fully. Let us set her at liberty.
She will go and join the others on the Snakes in my pans.
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