CHAPTER X

.

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.

##