different parts of the world. The writer has been assured by an old
inhabitant of Colombo that the mosquitoes have distinctly diminished in
number in parts of that town since the custom of storing water near the
houses was abandoned. During the summer of 1900 the authorities at
Sassari in Sardinia claim to have ‘practically exterminated the
mosquitoes ... by killing the larvæ in the swamps with petroleum, and
the flies with chlorine and other destructive chemicals.’[6]
The extinction of malaria in England is a kind of by-product of the
draining operations which restored to the agriculturist large tracts of
land in the fen districts and elsewhere. The breeding-places of the
mosquitoes were dried up and their numbers materially lessened; at the
same time the parasite was killed in an increasing number of patients.
Thus the mosquitoes which survived had fewer opportunities of infecting
themselves, and as time went on the parasite was ultimately eliminated.
_Anopheles_, though in diminished numbers, is still with us, and is
especially to be found in those parts of England once infested with the
malaria; but the parasite has disappeared.
‘INFINITE TORMENT OF FLIES’
_Where the water is stopped in a stagnant pond,
Danced over by the midge._
R. BROWNING: ‘By the Fireside.’
The last few years of the nineteenth and the first few years of the
present century are marked in the annals of medicine by a great increase
in our knowledge of certain parasitic diseases, and, above all, in our
knowledge of the agency by which the parasites causing the diseases are
conveyed from host to host.
Chief among these agencies in carrying the disease-causing organisms
from infected to uninfected animals are the insects, and, amongst the
insects, above all the flies. Flies--_e.g._, the common house-fly
(_Musca domestica_)--can carry about with them the bacillus of anthrax,
and, if brought into contact with a wounded surface, may thus set up an
outbreak of woolsorter’s disease. Flies, ants, and other even more
objectionable insects, are not only capable of disseminating the plague
bacillus from man to man, and from rat to man, but they themselves fall
victims to the disease, and perish in great numbers. They are active
agents in the spread of cholera, and the histories of the South African
and Cuban wars definitely show that flies play a large part in carrying
the bacilli of enteric fever from sources of infection to the food of
man, thus spreading the disease. They are also accused of conveying the
inflammatory matter of Egyptian ophthalmia, and of the ‘sore-eye,’ so
common in Florida, from one human being to another.
The diseases already mentioned are caused by bacteria. But flies also
play a part in the conveyance of a large number of organisms which are
not bacteria, but which, nevertheless, cause disease, and cause it on
the largest scale.
Of all the twenty-two orders into which the modern entomologist divides
the class Insecta, that of the Diptera, or true flies, is, perhaps, the
easiest to recognize, for it is characterized by one very obvious
feature, the presence of the fore-wings only. The hind-wings are
replaced by a pair of small-stalked, club-shaped ‘balancers,’ which are
readily visible in some kinds of fly--_e.g._, the daddy-long-legs--but
in others are by no means conspicuous. Thus it is an easy matter to
determine whether an insect be a fly or not. To determine what
particular kind of fly it be is, however, a very different affair. At
present some forty thousand species of Diptera are known, and have been
more or less completely described or figured; and Mr. D. Sharp estimates
that this number is ‘only a tithe of what are still unknown to science.’
Further, the group has been rather neglected. Flies, speaking generally,
are neither attractive in their appearance nor engaging in their habits,
and it is a cause for no astonishment that entomologists have preferred
to work at other groups.
In considering the part played by flies in disseminating diseases not
caused by bacteria, we can neglect all but a very few families, those
flies which suck blood having alone any interest in this connexion.
From the point of view of the physician, by far the most important of
these families is the Culicidæ, with over three hundred described
species and five sub-families, of which two, the Culicina and the
Anophelina, interest us in relation to disease. The gnats or
mosquitoes--the name is indifferently used, and has no scientific
application--are amongst the most graceful and most beautiful insects
that we know, but they have been judged by their works, and undoubtedly
are unpopular, and we shall see that this unpopularity is well deserved.
Gnats belong both to the genus _Culex_ and to the genus _Anopheles_. The
genus _Culex_, from which the order takes its name, includes not only
our commonest gnat, often seen in swarms on summer evenings, but some
hundred and thirty other species. Members of this genus convey from man
to man the _Filaria nocturna_, one of the causes of the widely-spread
disease filariasis, one variety of which is the elephantiasis, so common
in parts of the tropics. In patients suffering from this disease minute
embryonic round-worms swarm in the bloodvessels of the skin during the
hours of darkness. Between six and seven in the evening they begin to
appear in the superficial bloodvessels, and they increase in number till
midnight, when they may occur in such numbers that five or six hundred
may be counted in a single drop of blood. After midnight the swarms
begin to lessen, and by breakfast-time, about eight or nine in the
morning, except for a few strayed revellers, they have disappeared from
the superficial circulation, and are hidden away in the larger
bloodvessels and in the lungs.
In spite of their incredible number--some authorities place it at thirty
to forty millions in one man--these minute larval organisms, shaped
something like a needle pointed at each end, seem to cause little harm.
It might be thought that they would traverse the walls of the
bloodvessels and cause trouble in the surrounding tissues; but this is
prevented by a curious device. It is well known that, like insects,
round-worms from time to time cast their skins, and the young larvæ in
the blood cast theirs, but do not escape from the inside of this
winding-sheet; and thus, though they actively wriggle and coil and
uncoil their bodies, their progress is as small and their struggles as
little effective as are those of a man in a strait-waistcoat.
The causes of the periodicity of the appearance of these round-worms in
the superficial bloodvessels are not completely understood, but they
appear to have more relation with the usual sleeping hours of humanity
than with day and night. In individuals who sleep by day and work by
night the _Filaria nocturna_ is found in the bloodvessels of the skin
during the day. Thus, whilst between 5 p.m. and 7 or 8 a.m. the vessels
of the skin of Cox the Hatter would be well peopled by the round-worms,
they would only come to the surface in Box the Printer during the
daytime, whilst he was sleeping in the lodgings of Mrs. Bouncer.
One reason of the normal appearance of the creatures in the blood at
night is undoubtedly connected with the habits of its second host, the
gnat or mosquito. Two species are accused of carrying the _Filaria_ from
man to man--_Culex fatigans_ and _Anopheles nigerrimus_. Sucked up with
the blood, the round-worms pass into the stomach of the insect. Here
they appear to become violently excited, and rush from one end to the
other of their enveloping sheath, until they succeed in breaking through
it. When free, they pierce the walls of the stomach of the mosquito, and
come to rest in the great thoracic muscles. Here the _Filarias_ rest for
some two or three weeks, growing considerably, and developing a mouth
and alimentary canal; thence, when they are sufficiently developed, they
make their way to the proboscis of the mosquito. Here they lie in
couples, and it would be interesting to determine whether these couples
are male and female. Exactly how they effect their exit from the
mosquito and their entrance into man has not yet been accurately
observed, but presumably it is during the process of biting. Only
inside man they work their way to the lymphatics, and very soon the
female begins to pour into the lymph a stream of young embryos, which
reach the bloodvessels through the thoracic duct. It is, however, the
adults which are the source of all the trouble. They are of considerable
size, three or four inches in length, and their presence, by blocking
the channels of the lymphatics, gives rise to a wide range of disease,
of which elephantiasis is the most pronounced form. We can consider
later how the disease can be averted by keeping down the number of gnats
and by preventing their access to infected patients.
We now pass to the second of the diseases carried by gnats, that of
malaria.
The parasite which causes malaria is a much more lowly organized animal
than the _Filaria_. It is named _Hæmamœba_, and it, too, is conveyed
by an insect, and, so far as we know, by one genus of mosquito only, the
_Anopheles_. Hence, from the point of view of malaria, it is important
to know whether a district is infected with _Culex_ or _Anopheles_. The
former is rather humpbacked, and keeps its body parallel with the
surface it is biting, and its larva hangs at an angle below the surface
of the water, by means of a respiratory tube. _Anopheles_, on the other
hand, carries its body at a sharp angle with the surface upon which it
rests, and its larva lies flat below the surface-film and parallel with
it. The malarial parasite lives in the blood-cells of man, but at a
certain period it breaks up into spores, which escape into the fluid of
the blood, and it is at this moment that the sufferer feels the access
of fever. The presence and growth within the blood-cells result in the
destruction of the latter, a very serious thing to the patient if the
organisms be at all numerous. If the spores be sucked up by an
_Anopheles_, they undergo a complex change, and ultimately reproduce an
incredible number of minute spores or ‘blasts,’ each capable of
infecting man again if it can but win entrance into his body.
Under normal circumstances, for each _Filaria_ larva which enters a
mosquito, one _Filaria_ issues forth, longer, it is true, and more
highly developed, but not much changed. The malaria-parasite undergoes,
in its passage through the body of the _Anopheles_, many and varied
phases of its life-history. As the Frenchman said of the pork, which
goes into one end of the machine in the Chicago meat factories as live
pig, and comes out at the other in the form of sausages, ‘Il est
diablement changé en route.’ The mosquito is as truly a host of the
malarial parasite as man, and is as necessary for its full development
as is man. Judging by the number and extent of the lesions in the
insect’s body, it must suffer far more than man, and it is undoubtedly
killed at times, and perhaps fairly frequently, by the parasite.
Whoever has watched under a lens the process of ‘biting’ as carried on
by a mosquito, must have observed the fleshy proboscis (_labium_)
terminating in a couple of lobes. The labium is grooved like a gutter,
and in the groove lie five piercing stylets, and a second groove, or
_labrum_. It is along this labrum that the blood is sucked. Between the
paired lobes of the labium, and guided by them (as a billiard cue may be
guided by two fingers), a bundle of five extremely fine stylets sinks
slowly through the epidermis, cutting into the skin as easily as a
paper-knife into a soft cheese. Four of these stylets are toothed, but
the single median one is shaped like a two-edged sword. Along its
centre, where it is thickest, runs an extremely minute groove, only
visible under a high power of the microscope. Down this groove flows the
saliva, charged with the spores or blasts of the malaria-causing
parasite. Through this minute groove has flowed the fluid which, it is
no exaggeration to say, has changed the face of continents, and
profoundly affected the fate of nations.
It is an interesting fact that, amongst the Culicidæ, it is the female
alone that bites. The mouth-parts of the male are weaker, and seem
unable to pierce the skin. It has been suggested that a meal of blood is
necessary for the development of the eggs; but the evidence for this is
not conclusive. There must be millions and millions of mosquitoes in
sparsely inhabited or uninhabited districts, in Africa, in Finland, in
Northern Asia, and America, which never have a chance of sucking blood;
and it is impossible to believe that these millions do not lay eggs.
The female is undoubtedly greedy. If undisturbed, she simply gorges
herself until every joint of her chitinous armour is stretched to the
cracking-point. At times even, like Baron Munchausen’s horse after his
adventure with the portcullis, what she takes in at one end runs out at
the other. But she never ceases sucking. The great majority of
individuals, however, can never taste blood, and subsist mainly on
vegetable juices. In captivity they cannot last longer than five days
without food and drink; but they can be kept alive for weeks on a diet
of bananas, pineapples, and other juicy fruits.
_Anopheles_ is often conveyed great distances by the wind, or in railway
trains or ships; but of itself it does not fly far; about five or six
hundred yards--some authorities place it much lower--is its limit.
Beyond this distance they do not voluntarily stray from their
breeding-places. Both _Anopheles_ and _Culex_ lay their eggs, as is well
known, in standing water, and here three out of the four stages in their
life-history--the egg, the larva, and the pupa--are passed through. The
larva and the pupa hang on to the surface-film of the water by means of
certain suspensory hairs, and by their breathing apparatus. Anything
which prevents the breathing tubes reaching the air ensures the death of
the larva and pupa. Hence the use of paraffin on the pools or
breeding-places. It, or any other oily fluid, spreads as a thin layer
over the surface of the pools and puddles, and clogs the respiratory
pores, and the larvæ or pupæ soon die of suffocation.
In Ismailia the disease has been reduced to an amazing extent, and quite
recently remarkable results have followed the use of these preventive
measures at Port Swettenham, in the Federated Malay States. Within two
months of the opening of the port in 1902, 41 out of 49 of the
Government quarters were infected, and 118 out of 196 Government
servants were ill. Now, after filling up all pools and cleaning the
jungle, no single officer has suffered from malaria since July, 1904,
and the number of cases amongst the children fell from 34·8 to 0·77 per
cent. The only melancholy feature about this wonderful alleviation of
suffering due to the untiring efforts of the District Surgeon, Dr.
Malcolm Watson, is that his fees for attending malarial cases have
dropped to zero.
Thus a considerable degree of success has attended the efforts of the
sanitary authorities, largely at the instigation of Major Ross, all over
the world, to diminish the mosquito plague. It is, of course, equally
important to try and destroy the parasite in man by means of quinine.
This is, however, a matter of very great difficulty. In Africa and in
the East nearly all native children are infected with malaria, though
they suffer little, and gradually acquire a high degree of immunity.
Still, they are always a source of infection; and Europeans living in
malarious districts should always place their dwellings to the windward
of the native settlements. Knowing the cause, we can now guard against
malaria; mosquito-nets and wire windows and doors are a sufficient
check on the access of _Anopheles_ to man. If they could only be kept
permanently apart, we might hope for the disappearance of the parasite
from our fauna. In relieving man from the pest, all lovers of animals
will rejoice that we are also relieving the probably far more acute
sufferings of one of the most delicate and beautiful insects that we
know.
Another elegant little gnat, _Stegomyia fasciata_, closely allied to
_Culex_, with which, until recently, it was placed, is the cause of the
spread of that most fatal of epidemic diseases, the yellow fever. Like
the _Culex_, but unlike the _Anopheles_, _Stegomyia_ has a humpbacked
outline, and its larva has a long respiratory tube at an angle to its
body, from which it hangs suspended from the surface-film of its watery
home. It is a very widely distributed creature; it girdles the earth
between the Tropics, and is said to live well on shipboard. It breeds in
almost any standing fresh water, provided it be not brackish. The female
is said to be most active during the warmer hours of the day, from noon
till three or so, and in some of the West Indies it is known as the
‘day-mosquito.’
The organism which causes yellow fever has yet to be found. It seems
that it is not a bacterium, and that it lives in the blood of man. It
evidently passes through a definite series of changes in the mosquito,
for freshly infected mosquitoes do not at once convey the disease. After
biting an infected person, it takes twelve days for the unknown organism
to develop in the _Stegomyia_ before it is ready for a change of host.
The mosquitoes are then capable of inoculating man with the disease for
nearly two months. The period during which a man may infect the
mosquito, should it bite him, is far shorter, and extends only over the
first three days of the illness.
Very careful search has hitherto failed to reveal the presence of the
parasite of yellow fever. By its works alone can it be judged. It seems
that, like the germ of rinderpest and of foot-and-mouth disease, it is
ultra-microscopic, and our highest lenses fail to resolve it. From the
course of the disease and the nature of its host, it will probably prove
to be something like the organism which causes malaria. The means of
warring against _Anopheles_ and _Culex_ are equally applicable in the
case of _Stegomyia_, but, since the last-named flies by day, they are
more difficult to carry out, and more irksome to endure. By the
intelligent application of these preventive measures the Americans have
freed Havana for the first time from yellow fever, and have materially
reduced the amount of malaria, and they have been equally successful at
Panama.
King Solomon sent to Tarshish for gold and silver, ivory, and apes and
peacocks, and at the present day people mostly go to Africa for gold,
diamonds, ivory, and game. These are the baits that draw them in. Of the
great obstacles, however, which have for generations succeeded in
keeping that great continent, except at the fringes, comparatively free
from immigrants, three--and these by no means the least important--are
insignificant members of the order Diptera. We have considered the case
of _Culex_ and _Anopheles_; the third fly we have now to do with is the
tsetse fly (_Glossina_), which communicates fatal diseases to man and to
cattle and domesticated animals of all kinds.
There are at least seven species of the genus which received its name as
long ago as 1830, when Wiedemann first described it. Perhaps the best
known species is _Glossina morsitans_, which was named by Westwood.
The members of the genus _Glossina_ are unattractive insects, a little
larger than our common house-fly, with a sober brownish or
brownish-grey coloration. When at rest the two wings are completely
super-imposed, like the blades of a shut pair of scissors; and this
feature readily serves to distinguish the genus from that of all other
blood-sucking flies, and is of great use in discriminating between the
tsetse and the somewhat nearly allied _Stomoxys_ and _Hæmatopota_.
The tsetse flies rapidly and directly to the objects it seeks, and must
have a keen sense of smell or sight, or both, making straight for its
prey, and being most persistent in its attacks. The buzzing which it
produces when flying is peculiar, and easily recognized again when once
heard. After feeding, the fly emits a higher note, a fact recalling the
observation of Dr. Nuttall and the present writer on the note of
_Anopheles_, in which animal they observed that, ‘the larger the meal,
the higher the note.’ The tsetse does not settle lightly and
imperceptibly on the sufferer as the Culicidæ do, nor does it alight
slowly and circumspectly after the manner of the house-flies, but it
comes down with a bump, square on its legs. Like the mosquito, the
tsetse is greedy, and sucks voraciously. The abdomen becomes almost
spherical, and of a crimson red, and in the course of a few seconds the
fly has exchanged the meagre proportions of a Don Quixote for the ampler
circumference of a Sancho Panza. There is a good deal of discrepancy
between the reports of the various sufferers as to the pain of the bite.
No doubt different persons are very differently affected, and suffer to
very varying degrees. Unlike so many of the blood-sucking Diptera, in
which the habit is confined to the females, both sexes of _Glossina_
attack warm-blooded creatures.
The fly always seems to choose a very inaccessible portion of the body
to operate on--between the shoulders in man, or on the back and belly in
cattle and horses; even inside the nostrils in the latter, or on the
forehead in dogs. According to Lieutenant-Colonel D. Bruce, R.A.M.C., to
whom we owe so much of our knowledge of this fly and its evil work, the
female does not lay eggs, but is viviparous, and produces a large active
yellow larva, which immediately crawls away to some secluded crevice,
and straightway turns into a hard, black pupa, from which the imago
emerges in some six weeks. Thus two stages, the egg and the larva, both
peculiarly liable to destruction in the Culicidæ, are practically
skipped in the tsetse--at any rate, in some species. On the other hand,
this advantage is probably to a great extent counterbalanced by the
smallness of the number of the larvæ produced, compared with the number
of the eggs laid by the oviparous Diptera.
The genera of the Culicidæ which we have considered are found
practically all over the world, but the genus _Glossina_, except that it
just reaches Arabia, is fortunately confined to Africa. From the
admirable map of the geographical distribution of the fly compiled by
Mr. Austen we gather that its northern limit corresponds with a line
drawn from the Gambia, through Lake Chad to Somaliland, somewhere about
the 13th parallel of north latitude. Its southern limit is about on a
level with the northern limit of Zululand. The tsetse, of course, is not
found everywhere within this area, and, though it has probably escaped
observation in many districts, it seems clear that it is very
sporadically distributed. Mr. Austen further thinks that it may occur
outside the boundary above laid down, and suggests that the great
mortality amongst the horses in the Abyssinian campaign against King
Theodore may have been caused by it.
Even where the tsetse is found it is not uniformly distributed, but
occurs in certain localities only. These form the much dreaded
‘fly-belts.’ The normal prey of the fly is undoubtedly the big game of
Africa, including crocodiles, but they are not the only factor in its
distribution; the nature of the land also plays a part. There are the
usual discrepancies in the accounts of travellers, especially of African
travellers, as to the exact localities the _Glossina_ affects; but most
writers agree that the tsetse is not found in the open veld. It must
have cover. Warm, moist, steamy hollows, containing water and clothed
with forest growth, are the haunts chosen. Even within the fly-belt
there are oases, due, perhaps, to an absence of shrubs or trees, where
no flies are.
The tsetse fly belongs to the family Muscidæ, the true flies, a very
large family, which also includes our house-fly, blue-bottle fly, etc.
These flies, unlike _Anopheles_ and _Culex_, are day-flies, and begin to
disappear at or about sunset, a fact noted centuries ago by Dante: