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THE GREY FLESH FLIES




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 then
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.





Next: THE BUMBLEBEE FLY

Previous: THE GREENBOTTLES



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