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.