MOSQUITOES AND PARASITE PROBLEMS.
There cannot be a doubt that some animals possess an instinctive
knowledge of their enemies--or, at all events, of some of their
enemies--though I do not believe that this faculty is so common as many
naturalists imagine. The most striking example I am acquainted with is
seen in gnats or mosquitoes, and in the minute South American sandflies
(Simulia), when a dragon-fly appears in a place where they are holding
their aerial pastimes. The sudden appearance of a ghost among human
revellers could not produce a greater panic. I have spoken in the last
chapter of periodical storms or waves of dragon-flies in the Plata
region, and mentioned incidentally that the appearance of these insects
is most welcome in oppressively hot weather, since they are known to
come just in advance of a rush of cool wind. In La Plata we also look
for the dragon-fly, and rejoice at its coming, for another reason. We
know that the presence of this noble insect will cause the clouds of
stinging gnats and flies, which make life a burden, to vanish like
smoke.
When a flight of dragon-flies passes over the country many remain along
the route, as I have said, sheltering themselves wherever trees occur;
and, after the storm blows over, these strangers and stragglers remain
for some days hawking for prey in the neighbourhood. It is curious to
note that they do not show any disposition to seek for watercourses. It
may be that they feel lost in a strange region, or that the panic they
have suffered, in their long flight before the wind, has unsettled their
instincts; for it is certain that they do not, like the dragon-fly in
Mrs. Browning's poem, "return to dream upon the river." They lead
instead a kind of vagabond existence, hanging about the plantations, and
roaming over the surrounding plains. It is then remarked that gnats and
sand-flies apparently cease to exist, even in places where they have
been most abundant. They have not been devoured by the dragon-flies,
which are perhaps very few in number; they have simply got out of the
way, and will remain in close concealment until their enemies take their
departure, or have all been devoured by martins, tyrant birds, and the
big robber-flies or devil's dykes--no name is bad enough for them--of
the family Asilidaa. During these peaceful gnatless days, if a person
thrusts himself into the bushes or herbage in some dark sheltered place,
he will soon begin to hear the thin familiar sounds, as of "horns of
elf-land faintly blowing"; and presently, from the ground and the under
surface of every leaf, the ghost-like withered little starvelings will
appear in scores and in hundreds to settle on him, fear not having
blunted their keen appetites.
When riding over the pampas on a hot still day, with a pertinacious
cloud of gnats or sandflies hovering just above my head and keeping me
company for miles, I have always devoutly wished for a stray dragon-fly
to show himself. Frequently the wish has been fulfilled, the dragon-fly,
apparently "sagacious of his quarry from afar," sweeping straight at his
prey, and instantly, as if by miracle, the stinging rain has ceased and
the noxious cloud vanished from overhead, to be re-formed no more. This
has always seemed very extraordinary to me; for in other matters gnats
do not appear to possess even that proverbial small dose of intellect
for which we give most insects credit. Before the advent of the
dragon-fly it has perhaps happened that I have been vigorously striking
at them, making it very unpleasant for them, and also killing and
disabling many hundreds--a larger number than the most voracious
dragon-fly could devour in the course of a whole day; and yet, after
brushing and beating them off until my arms have ached with the
exertion, they have continued to rush blindly on their fate, exhibiting
not the faintest symptom of fear. I suppose that for centuries
mosquitoes have, in this way, been brushed and beaten away with hands
and with tails, without learning caution. It is not in their knowledge
that there are hands and tails. A large animal is simply a field on
which they confidently settle to feed, sounding shrill flourishes on
their little trumpets to show how fearless they are. But the dragon-fly
is very ancient on the earth, and if, during the Devonian epoch, when it
existed, it preyed on some blood-sucking insect from which or Culicidae
have come, then these stupid little insects have certainly had ample
time in which to learn well at least one lesson.
There is not in all organic nature, to my mind, any instance of wasted
energy comparable in magnitude with the mosquito's thirst for blood, and
the instincts and elaborate blood-pumping apparatus with which it is
related. The amount of pollen given off by some wind-fertilized
trees--so great in some places that it covers hundreds of square miles
of earth and water with a film of yellow dust---strikes us as an amazing
waste of material on the part of nature; but in these cases we readily
see that this excessive prodigality is necessary to continue the
species, and that a sufficient number of flowers would not be
impregnated unless the entire trees were bathed for days in the
fertilizing cloud, in which only one out of many millions of floating
particles can ever hit the mark. The mosquito is able to procreate
without ever satisfying its ravenous appetite for blood. To swell its
grey thread-like abdomen to a coral bead is a delight to the insect, but
not necessary to its existence, like food and water to ours; it is the
great prize in the lottery of life, which few can ever succeed in
drawing. In a hot summer, when one has ridden perhaps for half a day
over a low-lying or wet district, through an atmosphere literally
obscured with a fog of mosquitoes, this fact strikes the mind very
forcibly, for in such places it frequently is the case that mammals do
not exist, or are exceedingly rare. In Europe it is different. There, as
Reaumur said, possibly one gnat in every hundred may be able to gratify
its appetite for blood; but of the gnats in many districts in South
America it would be nearer the mark to say that only one in a hundred
millions can ever do so.
Curtis discovered that only the female mosquito bites or sucks blood,
the male being without tongue or mandibles; and he asks, What, then,
does the male feed on? He conjectures that it feeds on flowers; but, had
he visited some swampy places in hot countries, where flowers are few
and the insects more numerous than the sands on the seashore, he would
most probably have said that the males subsist on decaying vegetable
matter and moisture of slime. It is, however, more important to know
what the female subsists on. We know that she thirsts for warm mammalian
blood, that she seeks it with avidity, and is provided with an admirable
organ for its extraction--only, unfortunately for her, she does not get
it, or, at all events, the few happy individuals that do get it are
swamped in the infinite multitude of those that are doomed by nature to
total abstinence.
I should like to know whether this belief of Curtis, shared by Westwood
and other distinguished entomologists, but originally put forward merely
as a conjecture, has ever been tested by careful observation and
experiment. If not, then it is strange that it should have crept into
many important works, where it is stated not as a mere guess, but as an
established fact. Thus, Van Beneden, in his work on parasites, while
classing female mosquitoes with his "miserable wretches," yet says, "If
blood fails them, they live, like the males, on the juices of flowers."
If this be so, it is quite certain that the juices fail to satisfy them;
and that, like Dr. Tanner, who was ravenously hungry during his forty
days' fast, in spite of his frequent sips of water, the mosquito still
craves for something better than a cool vegetarian diet. I cannot help
thinking, though the idea may seem fanciful, that mosquitoes feed on
nothing. We know that the ephemerae take no refreshment in the imago
state, the mouth being aborted or atrophied in these short-lived
creatures; but we also know that they belong to an exceedingly ancient
tribe, and possibly, after the earth had ceased to produce their proper
nourishment there came in their history a long hungry period, which did
not kill them, but lasted until their feeding instincts became obsolete,
the mouth lost its use, and their life in its perfect state dwindled to
its present length.
In any case, how unsatisfactory is the mosquitoes' existence, and what a
curious position they occupy in nature! Let us suppose that, owing to
some great change in the conditions of the earth, rapacious birds were
no longer able to capture prey, and that, by a corresponding change in
their organizations, they were able to subsist on the air they breathed,
with perhaps an occasional green leaf and a sip of water, and yet
retained the old craving for solid food, and the old predatory instincts
and powers undiminished; they would be in the position of mosquitoes in
the imago state. And if then fifty or a hundred individuals were to
succeed every year in capturing something and making one hearty meal,
these few fortunate diners would bear about the same proportion to all
the raptors on the globe as the mosquitoes that succeed in sucking blood
to their unsuccessful fellows. In the case of the hawks, the effect of
the few meals on the entire rapacious family or order would certainly be
_nil;_ and it is impossible to believe for a moment that the
comparatively infinitesimal amount of blood sucked by mosquitoes can.
serve to invigorate the species. The wonder is that the machinery, which
accomplishes nothing, should continue in such perfect working order.
When we consider the insect's delicate organ, so admirably fitted for
the purpose to which it is applied, it becomes difficult to believe that
it could have been so perfected except in a condition of things utterly
unlike the present. There must have been a time when mosquitoes found
their proper nourishment, and when warm mammalian blood was as necessary
to their existence as honey is to that of the bee, or insect food to the
dragon-fly.
This applies to many blood-sucking insects besides mosquitoes, and with
special force to the tick tribes (Ixodes), which swarm throughout
Central and South America; for in these degraded spiders the whole body
has been manifestly modified to fit it for a parasitical life; while the
habits of the insect during its blind, helpless, waiting existence on
trees, and its sudden great development when it succeeds in attaching
itself to an animal body, also point irresistibly to the same
conclusion. In the sunny uplands they act (writes Captain Burton) like
the mosquitoes of the hot, humid Beiramar. "The nuisance is general; it
seems to be in the air; every blade of grass has its colony; clusters of
hundreds adhere to the twigs; myriads are found in the bush clumps. Lean
and flat when growing to the leaves, the tick catches man or beast
brushing by, fattens rapidly, and, at the end-of a week's good living,
drops off, _plena cruoris."_ When on trees, Belt says, they
instinctively place themselves on the extreme tips of leaves and shoots,
with their hind legs stretching out, each foot armed with two hooks or
claws, with which to lay hold of any animal brushing by. During this
wretched, incom-plete existence (from which, in most cases, it is never
destined to emerge), its greatest length is about one-fourth of an inch;
but where it fastens itself to an animal the abdomen increases to a
globe as big as a medium-sized Barcelona nut. Being silvery-grey or
white in colour, it becomes, when thus distended, very conspicuous on
any dark surface. I have frequently seen black, smooth-haired dogs with
their coats, turned into a perfect garden of these white spider-flowers
or mushrooms. The white globe is leathery, and nothing can injure it;
and the poor beast cannot rub, bite, or scratch it off, as it is
anchored to his flesh by eight sets of hooks and a triangle of teeth.
The ticks inhabiting regions rich in bird and insect life, but with few
mammals, are in the same condition as mosquitoes, as far as the supply
of blood goes; and, like the mosquitoes, they are compelled and able to
exist without the nourishment best suited to them. They are nature's
miserable castaways, parasitical tribes lost in a great dry wilderness
where no blood is; and every marsh-born mosquito, piping of the hunger
gnawing its vitals, and every forest tick, blindly feeling with its
grappling-irons for the beast that never brushes by, seems to tell us of
a world peopled with gigantic forms, mammalian and reptilian, which once
afforded abundant pasture to the parasite, and which the parasite
perhaps assisted to overthrow.
It is almost necessary to transport oneself to the vast tick-infested
wilderness of the New World to appreciate the full significance of a
passage in Belt's _Naturalist in Nicaragua,_ in which it is suggested
that man's hairless condition was perhaps brought about by natural
selection in tropical regions, where he was greatly troubled with
parasites of this kind. It is certain that if in such a country as
Brazil he possessed a hairy coat, affording cover to the tick and
enabling it to get a footing on the body, his condition would be a very
sad one. Savages abhor hairs on the body, and even pluck them off their
faces. This seems like a survival of an ancient habit acquired when the
whole body was clothed with hair; and if primitive man ever possessed
such a habit, nature only followed his lead in giving him a hairless
offspring.
Is it not also probable that the small amount of mammalian life in South
America, and the aquatic habits of nearly all the large animals in the
warmer districts, is due to the persecutions of the tick?
The only way in which a large animal can rid itself of the pest is by
going into the water or wallowing in the mud; and this perhaps accounts
for the more or less aquatic habits of the jaguar, aguará-guazú, the
large Cervus paluclosus, tapir, capybara, and peccary. Monkeys, which
are most abundant, are a notable exception; but these animals have the
habit of attending to each other's skins, and spend a great deal of
their time in picking off the parasites. But how do birds escape the
ticks, since these parasites do not confine their attacks to any one
class of aninials, but attach themselves impartially to any living thing
coming within reach of their hooks, from snake to man? My own
observations bearing on this point refer less to the Ixodes than to the
minute bete-rouge, which is excessively abundant in the Plata district,
where it is known as _bicho colorado,_ and in size and habits resembles
the English Leptus autumnalis. It is so small that, notwithstanding its
bright scarlet colour, it can only be discerned by bringing the eye
close to it; and being, moreover, exceedingly active and abundant in all
shady places in summer--making life a misery to careless human
beings--it must be very much more dangerous to birds than the larger
sedentary Ixodes. The bete-rouge invariably lodges beneath the wings of
birds, where the loose scanty plumage affords easy access to the skin.
Domestic birds suffer a great deal from its persecutions, and their
young, if allowed to run about in shady places, die of the irritation.
Wild birds, however, seem to be very little troubled, and most of those
I have examined have been almost entirely free from parasites. Probably
they are much more sensitive than the domestic birds, and able to feel
and pick off the insects with their beaks before they have penetrated
into the skin. I believe they are also able to protect themselves in
another way, namely, by preventing the parasites from reaching their
bodies at all. I was out under the trees one day with a pet oven-bird
(Furnarius rufus), which had full liberty to range about at will, and
noticed that at short intervals it went through the motions of picking
something from its toes or legs, though I could see nothing on them. At
length I approached my eyes to within a few inches of the bird's feet,
and discovered that the large dry branch on which it stood was covered
with a multitude of parasites, all running rapidly about like foraging
ants, and whenever one came to the bird's feet it at once ran up the
leg. Every time this happened, so far as I could see, the bird felt it.
and quickly and deftly picked it off with the point of its bill. It
seemed very astonishing that the horny covering of the toes and legs
should be so exquisitely sensitive, for the insects are so small and
light that they cannot be felt on the hand, even when a score of them
are running over it; but the fact is as I have stated, and it is highly
probable, I think, that most wild birds keep themselves free from these
little torments in the same way.
Some observations of mine on a species of Orni-thomyia--a fly
parasitical on birds--might possibly be of use in considering the
question of the anomalous position in nature of insects possessing the
instincts and aptitudes of parasites, and organs manifestly modified to
suit a parasitical mode of life, yet compelled and able to exist free,
feeding, perhaps, on vegetable juices, or, like the ephemerae, on
nothing at all. For it must be borne in mind that I do not assert that
these "occasional" or "accidental" parasites, as some one calls them,
explaining nothing, do not feed on such juices. I do not know what they
feed on. I only know that the joyful alacrity with which gnats and
stinging flies of all kinds abandon the leaves, supposed to afford them
pasture, to attack a warm-blooded animal, serves to show how strong the
impulse is, and how ineradicable the instinct, which must have had an
origin. Perhaps the habits of the bird-fly I have mentioned will serve
to show how, in some cases, the free life of some blood-sucking flies
and other insects might have originated.
Kirby and Spence, in their _Introduction,_ mention that one or two
species of Ornithomyia have been observed flying about and alighting on
men; and in one case the fly extracted blood and was caught, the species
being thus placed beyond doubt. This circumstance led the authors to
believe that the insect, when the bird it is parasitical on dies,
takes to flight and migrates from body to body, occasionally tasting
blood until, coming to the right body--to wit, that of a bird, or of a
particular species of bird--it once more establishes itself permanently
in the plumage. I fancy that the insect sometimes leads a freer life and
ranges much more than the authors imagined; and I refer to Kirby and
Spence, with apologies to those who regard the _Introduction_ as out of
date, only because I am not aware that we have any later observations on
the subject.
There is in La Plata a small very common Dendrocolaptine bird--Anumbius
acuticaudatus--much infested by an Ornithomyia, a pretty, pale insect,
half the size of a house-fly, and elegantly striped with green. It is a
very large parasite for so small a bird, yet so cunning and alert is it,
and so swiftly is it able to swim through the plumage, that the bird is
unable to rid itself of so undesirable a companion. The bird lives with
its mate all the year round, much of the time with its grown-up young,
in its nest--a large structure, in which so much building-material is
used that the bird is called in the vernacular Leñatero, or
Firewood-gatherer. On warm bright days without wind, during the absence
of the birds, I have frequently seen a company of from half a dozen to a
dozen or fifteen of the parasitical fly wheeling about in the air above
the nest, hovering and gambolling together, just like house-flies in a
room in summer; but always on the appearance of the birds, returning
from their feeding-ground, they would instantly drop down and disappear
into the nest. How curious this instinct seems! The fly regards the
bird, which affords it the warmth and food essential to life, as its
only deadly enemy; and with an inherited wisdom, like that of the
mosquito with regard to the dragon-fly, or of the horse-fly with regard
to the Monedula wasp, vanishes like smoke from its presence, and only
approaches the bird secretly from a place of concealment.
The parasitical habit tends inevitably to degrade the species acquiring
it, dulling its senses and faculties, especially those of sight and
locomotion; but the Ornithomyia seems an exception, its dependent life
having had a contrary effect; the extreme sensitiveness, keenness of
sight, and quickness of the bird having reacted on the insect, giving it
a subtlety in its habits and motions almost without a parallel even
among free insects. A man with a blood-sucking flat-bodied flying
squirrel, concealing itself among his clothing and gliding and dodging
all over his body with so much artifice and rapidity as to defeat all
efforts made to capturo it or knock it off, would be a case parallel to
that of the bird-fly on the small bird. It might be supposed that the
Firewood-gatherer, like some ants that keep domestic pets, makes a pet
of the fly; for it is a very pretty insect, barred with green, and with
rainbow reflections on its wings--and birds are believed by some
theorists to possess aesthetic tastes; but the discomfort of having such
a vampire on the body would, I imagine, be too great to allow a kindly
instinct of that nature to grow up. Moreover, I have on several
occasions seen the bird making frantic efforts to capture one of the
flies, which had incautiously flown up from the nest at the wrong
moment. Bird and fly seem to know each other wonderfully well.
Here, then, we have a parasitical insect specialized in the highest
degree, yet retaining all its pristine faculties unimpaired, its love of
liberty, and of associating in numbers together for sportive exercises,
and well able to take care of itself during its free intervals. And
probably when thrown on the world, as when nests are blown down, or the
birds get killed, or change their quarters, as they often do, it is able
to exist for some time without avian blood. Let us then imagine some of
these orphaned colonies, unable to find birds, but through a slight
change in habits or organization able to exist in the imago state
without sucking blood until they laid their eggs; and succeeding
generations, still better able to stand the altered conditions of life
until they become practically independent (like gnats), multiplying
greatly, and disporting themselves in clouds over forests, yet still
retaining the old hunger for blood and the power to draw it, and ready
at any moment to return to the ancestral habit. It might be said that if
such a result were possible it would have occurred, but that we find no
insect like the Ornithomyia existing independently. With the bird-fly it
has not occurred, as far as we know; but in the past history of some
independent parasites it is possible that something similar to the
imaginary case I have sketched may have taken place. The bush-tick is a
more highly specialized, certainly a more degraded, creature than the
bird-fly, and the very fact of its existence seems to show that it is
possible for even the lowest of the fallen race of parasites to start
afresh in life under new conditions, and to reascend in the scale of
being, although still bearing about it the marks of former degeneracy.
The connection between the flea and the mammal it feeds on is even less
close than that which exists between the Ornithomyia and bird. The fact
that fleas are so common and universal--for in all lands we have them,
like the poor, always with us; and that they are found on all mammals,
from the king of beasts to the small modest mouse--seems to show a great
amount of variability and adaptiveness, as well as a very high
antiquity. It has often been reported that fleas have been found hopping
on the ground in desert places, where they could not have been dropped
by man or beast; and it has been assumed that these "independent" fleas
must, like gnats and ticks, subsist on vegetable juices. There is no
doubt that they are able to exist and propagate for one or two years
after being deprived of their proper aliment; houses shut up for a year
or longer are sometimes found infested with them; possibly in the
absence of "vegetable juices" they flourish on dust. I have never
detected them hopping on the ground in uninhabited places, although I
once found them in Patagonia, in a hamlet which had been attacked and
depopulated by the Indians about twenty months before my visit. On
entering one of the deserted huts I found the floor literally swarming
with fleas, and in less than ten seconds my legs, to the height of my
knees, were almost black with their numbers. This proves that they are
able toincrease greatly for a period without blood; but I doubt that
they can go on existing and increasing for an indefinite time; perhaps
their true position, with regard to the parasitical habit, is midway
between that of the strict parasite which never leaves the body, and
that of independent parasites like the Culex and the Ixodes, and all
those which are able to exist free for ever, and are parasitical only
when the opportunity offers.
Entomologists regard the flea as a degraded fly. Certainly it is very
much more degraded than the bird-borne Ornithomyia, with its subtle
motions and instinct, its power of flight and social pastimes. The poor
pulex has lost every trace of wings; nevertheless, in its fallen
condition it has developed some remarkable qualities and saltatory
powers, which give it a lower kind of glory; and, compared with another
parasite with which it shares the human species, it is almost a noble
insect. Darwin has some remarks about the smallness of the brain of an
ant, assuming that this insect possesses a very high intelligence, but I
doubt very much that the ant, which moves in a groove, is mentally the
superior of the unsocial flea. The last is certainly the most teachable;
and if fleas were generally domesticated and made pets of, probably
there would be as many stories about their marvellous intelligence and
fidelity to man as we now hear about our over-praised "friend" the dog.
With regard to size, the flea probably started on its downward course as
a comparatively large insect, probably larger than the Ornithomyia. That
insect has been able to maintain its existence, without dwindling like
the Leptus into a mere speck, through the great modification in organs
and instinct, which adapt it so beautifully to the feathery element in
which it moves. The bush-tick, wingless from the beginning, and
diverging in another direction, has probably been greatly increased in
size by its parasitical habit; this seems proven by the fact, that as
long as it is parasitical on nothing it remains small, but when able to
fasten itself to an animal it rapidly developes to a great size. Again,
the big globe of its abdomen is coriaceous and elastic, and is probably
as devoid of sensation as a ball of india-rubber. The insect, being made
fast by hooks and teeth to its victim, all efforts to remove it only
increase the pain it causes; and animals that know it well do not
attempt to rub, scratch, or bite it off, therefore the great size and
the conspicuous colour of the tick are positive advantages to it. The
flea, without the subtlety and highly-specialized organs of the
Ornithomyia, or the stick-fast powers and leathery body of the Ixodes,
can only escape its vigilant enemies by making itself invisible; hence
every variation, i.e. increase in jumping-power and diminished bulk,
tending towards this result, has been taken advantage of by natural
selection.