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The Naturalist in La Plata

W >> W. H. Hudson >> The Naturalist in La Plata




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.




CHAPTER XI

HUMBLE-BEES AND OTHER MATTERS.


Two humble-bees, Bombus thoracicus and B. violaceus, are found on the
pampas; the first, with a primrose yellow thorax, and the extremity of
the abdomen bright rufous, slightly resembles the English B. terrestris;
the rarer species, which is a trifle smaller than the first, is of a
uniform intense black, the body having the appearance of velvet, the
wings being of a deep violaceous blue.

A census of the humble-bees in any garden or field always shows that the
yellow bees outnumber the black in the proportion of about seven to one;
and I have also found their nests for many years in the same proportion;
about seven nests of the yellow to one nest of the black species. In
habits they are almost identical, and when two species so closely allied
are found inhabiting the same locality, it is only reasonable to infer
that one possesses some advantage over the other, and that the least
favoured species will eventually disappear. In this case, where one so
greatly outnumbers the other, it might be thought that the rarer species
is dying out, or that, on the contrary, it is a new-comer destined to
supplant the older more numerous species. Yet, during the twenty years I
have observed them, there has occurred no change in their relative
positions; though both have greatly increased in numbers during that
time, owing to the spread of cultivation. And yet it would scarcely be
too much to expect some marked change in a period so long as that, even
through the slow-working agency of natural selection; for it is not as
if there had been an exact balance of power between them. In the same
period of time I have seen several species, once common, almost or quite
disappear, while others, very low down as to numbers, have been exalted
to the first rank. In insect life especially, these changes have been
numerous, rapid, and widespread.

In the district where, as a boy, I chased and caught tinamous, and also
chased ostriches, but failed to catch them, the continued presence of
our two humble-bees, sucking the same flowers and making their nests in
the same situations, has remained a puzzle to my mind.

The site of the nest is usually a slight depression in the soil in the
shelter of a cardoon bush. The bees deepen the hollow by burrowing in
the earth; and when the spring foliage sheltering it withers up, they
construct a dome-shaped covering of small sticks, thorns, and leaves
bitten into extremely minute pieces. They sometimes take possession of a
small hole or cavity in the ground, and save themselves the labour of
excavation.

Their architecture closely resembles that of B. terrestris. They make
rudely-shaped oval honey-cells, varying from half an inch to an inch and
a half in length, the smaller ones being the first made; later in the
season the old cocoons are utilized for storing honey. The wax is
chocolate-coloured, and almost the only difference I can find in the
economy of the two species is that the black bee uses a large quantity
of wax in plastering the interior of its nest. The egg-cell of the
yellow bee always contains from twelve to sixteen eggs; that of the
black bee from ten to fourteen; and the eggs of this species are the
largest though the bee is smallest. At the entrance on the edge of the
mound one bee is usually stationed, and, when approached, it hums a
shrill challenge, and throws itself into a menacing attitude. The sting
is exceedingly painful.

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