By J.W. SLATER.
The two-winged flies, in their behavior to man, stand in a marked
contrast to all the other orders of insects. The Lepidoptera, the
Coleoptera, the Neuroptera, the Hymenoptera no doubt occasion, in some
of their forms at least, much damage to our crops. But none of them are
parasitic in or upon our bodies; none of them persistently intrude into
our dwellings, hover around us in our walks, and harass us with noise
and constant attempts to bite, or at least to crawl upon us. Even the
ants, except in a few tropical districts, rarely act upon the offensive.
The Hemiptera contain one semi-parasitic species which has attained a
"world-wide circulation," and one degraded, purely parasitic group.
But the Diptera, among which the fleas are now generally included as a
degenerated type, comprise more forms personally annoying to man than
all the remaining insect orders put together. These hostile species are,
further, incalculably numerous, and occur in every part of the globe.
Mosquitoes swarm not merely in the swampy forests of the Orinoco or the
Irrawaddy, but in the Tundras of Siberia, en the storm-beaten rocks of
the Loffodens, and are even encountered by voyagers in quest of the
North Pole. The common house fly was probably at one time peculiar to
the Eastern Continent, but it followed the footsteps of the Pilgrim
Fathers, and is now as great a nuisance in the United Slates and the
Dominion as in any part of Europe. It is curious, but distressing, to
note the tendency of evils to become international. We have communicated
to America the house-fly and the Hessian fly, the "cabbage-white,"
the small pox, and the cholera. She, in return, has given us the
_Phylloxera_, a few visitations of yellow fever, the _Blatta gigantea_,
and, climate allowing, may perhaps throw in the Colorado beetle as a
make-weight. In this department, at least, free trade reigns undisputed.
It is a singular thing that no beautiful, useful, or even harmless
species of bird or insect seems capable of acclimatizing itself as do
those characterized by ugliness and noisomeness.
But, returning from this digression, we find in the Diptera the habit of
obtrusion and intrusion, of coming in actual contact with our food and
our persons, combined with another propensity--that of feeding upon
carrion, excrement, blood, pus, and morbid matter of all kinds. This
is a combination far more serious than is generally imagined. If the
fly--which may at any moment settle upon our lips, our eyes, or upon
an abraded part of our skin--were cleanly in its habits, we need feel
little annoyance at its visits. Or if it were the most eager carrion
devourer, but did not, after having dined, think it necessary to
seek our company, we might hold it, as is done too hastily by some
naturalists, a valuable scavenger. I fear, however, that I have already
made too great a concession. So long as very many persons are suffering
from disease--so long as many diseases are capable of being transmitted
from the sick to the healthy--so long must any creature which is in the
habit of flying about, and touching first one person and then another,
be a possible medium of infection and death.
Let us take the following case, by no means imaginary, but a
generalization from occurrences far too frequent: A healthy man, sitting
in his house or walking in the fields, especially in countries where the
insectivorous birds have been shot down, suddenly feels a sharp prick on
his neck or his cheek. Putting his hand to the place he perhaps crushes,
perhaps merely brushes away, a fly which has bitten him so as to draw
blood. The man thinks little of so trifling a hurt, but the next morning
he finds the puncture exceedingly painful. An inflamed pimple forms,
which quickly gets worse, while constitutional symptoms of a feverish
kind come on. In alarm he seeks medical advice. The doctor tells him
that it is a malignant pustule, and takes at once the most active
measures. In spite of all possible skill and care the patient too often
succumbs to the bite of a _mouche charbonneuse_, or carbuncle-fly. But
has any kind of fly the property of producing malignant pustule by
some specific inherent power of its own? Surely not. The antecedent
circumstances are these: A sheep or heifer is attacked with the disease
known in France as _charbon_, in Germany as _milz-brand_, and in England
as _splenic fever_. Its blood on examination would be found plentifully
peopled with bacteria. If a lancet were plunged into the body of the
animal, and were then used to slightly scratch or cut the skin of a man,
he would be inoculated with "charbon." The bite of the fly is precisely
similar in its action. Its rostrum has been smeared with the poisoned
blood, an infinitesimal particle of which is sufficient to inclose
several of the disease "germs," and these are then transferred to the
blood of the next man or animal which the fly happens to bite. The
disease is reproduced as simply and certainly as the spores of some
species of fern give rise to their like if scattered upon soil suitable
for their growth. But flies which do not bite may transfer infection.
Every one must know that if blood be spilt upon the ground a crowd of
flies will settle upon and eagerly absorb it. Animals suffering from
splenic fever in the later stages of the disease sometimes emit bloody
urine. Often they are shot or slaughtered by way of stamping out the
plague, and their carcasses are buried deep in the ground. But some loss
of blood is sure to happen, and this will mostly be left to soak into
the ground. Here again the flies will come, and their feet and mouth
will become charged with the contagion. Such a fly, settling upon
another animal or a man, and selecting--as it will do by preference, if
such exist--a wound, or a place where the skin is broken, will convey
the disease.
Again, M. Pasteur has thoughtfully pointed out that if an animal has
died of splenic fever, and has been carefully buried, the earth-worms
may bring up portions of infectious matter to the surface, so that sheep
grazing, or merely being folded over the spot in question, may take the
plague and die. Hence be wisely counsels that the bodies of such animals
should be buried in sandy or calcareous soils where earth-worms are not
numerous. But it is perfectly legitimate to go a step farther. If such
worm-borings retain the slightest savor of animal matter, flies will
settle upon them and will convey the infectious dust to the most
unexpected places, giving wings to the plague.
Now it is very true that no one has seen a fly feasting upon the blood
of a heifer or sheep dying or just dead of splenic fever, has then
watched it settle upon and bite some person, and has traced the
following stages of the disease. But it is positively known that a
person has been bitten by a fly, and has then exhibited all the symptoms
of charbon, the place of the bite being the primary seat of the
infection. We know also, beyond all doubt, the eagerness with which
flies will suck up blood, and we likewise know the strange persistence
of the disease "germs."
Again, the avidity of flies for purulent matter is not a thing of mere
possibility. In Egypt, where ophthalmia is common, and where the "plague
of flies" seems never to have been removed, it is reported as almost
impossible to keep these insects away from the eyes of the sufferers.
The infection which they thus take up they convey to the eyes of persons
still healthy, and thus the scourge is continually multiplied.
A third case which seems established beyond question is the agency of
mosquitoes in spreading elephantiasis. These so-called sanitary agents
suck from the blood of one person the Filariae, the direct cause of the
disease, and transfer them to another. The manner in which this process
is effected will appear simple enough if we reflect that the mosquito
begins operations by injecting a few drops of fluid into its victim, so
as to dilute the blood and make it easier to be sucked.
So much being established it becomes in the highest degree probable that
every infectious disease may be, and actually is, at times propagated
by the agency of flies. Attention turned to this much neglected quarter
will very probably go far to explain obscure phenomena connected with
the distribution of epidemics and their sudden outbreaks in unexpected
quarters. I have seen it stated that in former outbreaks of pestilence
flies were remarkably numerous, and although mediaeval observations on
Entomology are not to be taken without a grain of salt, the tradition
is suggestive. Perhaps the Diptera have their seasons of unusual
multiplication and emigration. A wave of the common flea appears to have
passed over Maidstone in August, 1880.
We now see the way to some practical conclusions not without importance.
Recognizing a very considerable part of the order of Diptera, or
two-winged flies, as agents in spreading disease, it surely follows
that man should wage war against them in a much more systematic and
consistent manner than at present. The destruction of the common
house-fly by "_papier Moure_," by decoctions of quassia, by various
traps, and by the so-called "catch 'em alive," is tried here and there,
now and then, by some grocer, confectioner, or housewife angry at the
spoliation and defilement caused by these little marauders. But there
is no concerted continuous action--which after all would be neither
difficult nor expensive--and consequently no marked success. Experiments
with a view of finding out new modes of fly-killing are few and far
between.
Every one must occasionally have seen, in autumn, flies as if cemented
to the window-pane, and surrounded with a whitish halo. That in some
seasons numbers of flies thus perish--that the phenomenon is due to a
kind of fungus, the spores of which readily transfer the disease from
one fly to another--we know. But here our knowledge is at fault. We
have not learnt why this fly-epidemic is more rife in some seasons than
others. We are ignorant concerning the methods of multiplying this
fungus at will, and of launching it against our enemies. We cannot tell
whether it is capable of destroying _Stomoxys calcitram_, the blowflies,
gadflies, gnats, mosquitoes, etc. Experiment on these points is rendered
difficult by the circumstance that the fungus is rarely procurable
except in autumn, when some of the species we most need to destroy are
not to be found. Another question is whether the fungus, if largely
multiplied and widely spread, might not prove fatal to other than
Dipterous insects, especially to the Hymenoptera, so many of which,
in their character of plant-fertilizers, are highly useful, or rather
essential to man.
Another fungus, the so-called "green muscardine" (_Isaria destructor_),
has been found so deadly to insects that Prof. Metschnikoff, who is
experimenting upon it, hopes to extirpate the _Phylloxera_, the Colorado
beetle, etc., by its agency.
Coming to better known and still undervalued fly-destroyers, we have
interfered most unwisely with the balance of nature. The substitution of
wire and railings for live fences in so many fields has greatly lessened
the cover both for insectivorous birds and for spiders. The war waged
against the latter in our houses is plainly carried too far. Whatever
may be the case at the Cape, in Australia, or even in Southern Europe,
no British species is venomous enough to cause danger to human beings.
Though cobwebs are not ornamental, save to the eye of the naturalist,
there are parts of our houses where they might be judiciously tolerated:
their scarcity in large towns, even where their prey abounds, is
somewhat remarkable.
But perhaps the most effectual phase of man's war against the flies will
be negative rather than positive, turning not so much on putting to
death the mature individuals as in destroying the matter in which the
larvae are nourished. Or if, from other considerations, we cannot
destroy all organic refuse, we may and should render it unfit for the
multiplication of these vermin. We have, indeed, in most of our large
towns and in their suburbs, abolished cesspools, which are admirable
breeding-places for many kinds of Diptera, and which sometimes presented
one wriggling mass of larvae. We have drained many marshes, ditches,
and unclean pools, rich in decomposing vegetable matter, and have thus
notably checked the propagation of gnats and midges. I know an instance
of a country mansion, situate in one of the best wooded parts of the
home counties, which twenty years ago was almost uninhabitable, owing to
the swarms of gnats which penetrated into every room. But the present
proprietor, being the reverse of pachydermatous, has substituted covered
drains for stagnant ditches, filled up a number of slimy ponds as
neither useful nor ornamental, and now in most seasons the gnats no
longer occasion any annoyance.
But if we have to some extent done away with cesspools and ditches, and
have reaped very distinct benefit by so doing, there is still a grievous
amount of organic matter allowed to putrefy in the very heart of our
cities. The dust bins--a necessary accompaniment of the water-carriage
system of disposing of sewage--are theoretically supposed to be
receptacles mainly for organic refuse, such as coal-ashes, broken
crockery, and at worst the sweepings from the floors. In sober fact
they are largely mixed with the rinds, shells, etc., of fruits and
vegetables, the bones and heads of fish, egg-shells, the sweepings out
of dog-kennels and henhouses, forming thus, in short, a mixture of evil
odor, and well adapted for the breeding-place of not a few Diptera.
The uses to which this "dust" is put when ultimately fetched away are
surprising: without being freed from its organic refuse it is used to
fill up hollows in building-ground, and even for the repair of roads. A
few weeks ago I passed along a road which was being treated according
to the iniquity of Macadam. Over the broken stones had been shot, to
consolidate them, a complex of ashes, cabbage-leaves, egg and periwinkle
shells, straw, potato-parings, a dead kitten (over which a few
carrion-flies were hovering), and other promiscuous nuisances. The road
in question, be it remarked, is highly "respectable," if not actually
fashionable. The houses facing upon it are severely rated, and are
inhabited chiefly by "carriage people." What, then, may not be expected
in lower districts?
Much attention has lately been drawn to the fish trade of London. It has
_not_, however, come out in evidence that the fish retailers, if they
find a quantity of their perishable wares entering into decomposition,
send out late in the evening a messenger, who, watching his opportunity,
throws his burden down in some plot of building land, or over a fence.
When I say that I have seen in one place, close alongside a public
thoroughfare, a heap of about fifty herrings, in most active
putrefaction and buzzing with flies, and some days afterward, in another
place, some twenty soles, it will be understood that such nuisances
can only be occasioned by dealers. To get rid of, or at least greatly
diminish, carrion-flies, house-flies, and the whole class of winged
travelers in disease, it will be, before all things, essential to
abolish such loathsome malpractices. The dustbins must cease being made
the receptacle for putrescent and putrescible matter, the destruction of
which by fire should be insisted upon.
The banishment of slaughter-houses to some truly rural situation, where
the blood and offal could be at once utilized, would be another step
toward depriving flies of their pabulum in the larva state. An equally
important movement would be the substitution of steam or electricity for
horsepower in propelling tram-cars and other passenger carriages, with a
view to minimize the number of horses kept within greater London. Every
large stable is a focus of flies--_Journal of Science_.
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