Chapter II: Diptera (2)

The country people are well aware of the nature and cause of these bots. They know that each one contains a worm, that this worm comes from a fly, and that later it will be transformed into a fly itself. Each of these bots has in its interior a cavity, occupied by a larva, which, as well as the bot, increases in size as the larva becomes developed.

It is generally on young cows or young bullocks--in fact, on cattle of from two to three years of age--that these tumours exist, and they are rarely to be seen on old animals. The fly, which by piercing the skin occasions these tumours, always chooses those whose skin offers little resistance. Each tumour is provided with a small opening, by which the larva breathes.

In order to examine the interior of the cavity, Réaumur opened some of these tumours, either with a razor or a pair of scissors. He found them in a most disgusting state. The larva is lodged in a regular festering wound, matter occupying the bottom of the cavity, and the head of the worm is continually, or almost continually, plunged in this liquid. "It is most likely very well off there," says Réaumur; and he adds that this matter appears to be the sole food of the larva.

"The position of a horned beast," observes the great naturalist, "which has thirty or forty of these bumps on its back, would be a very cruel one, and a terrible state of suffering, if his flesh were continually mangled by thirty or forty large worms. But it is probable they cause no suffering, or at least very little, to the large animal. Besides," continues Réaumur, "those cattle whose bodies are the most covered with bumps, not only show no signs of pain, but it does not appear that they are prejudicial to them in any way."

Réaumur tried to discover how the larva, when arrived at its full growth, succeeds in leaving its abode, as the opening is smaller than its own body.

"Nature," says Réaumur, "has taught this worm the surest, the gentlest, and the most simple of methods, the one to which surgeons often have recourse to hold wounds open, or to enlarge them. They press _tents_ into a wound they wish to enlarge. Two or three days before the worm wishes to come out, it commences to make use of its posterior part as a tent, to increase the size of its exit from its habitation. It thrusts it into the hole and draws it out again many times in the course of two or three days, and the oftener this is repeated, the longer it is able to retain its posterior end in the opening, as the hole becomes larger. On the day preceding that on which the worm is to come out, the posterior part is to be found almost continually in the hole. At last, it comes out backwards, and falls to the ground, when it gets under a stone, or buries itself in the turf; remaining quiet and preparing for its last transformation. Its skin hardens, the rings disappear, and it becomes black. Thenceforth the insect is detached from the outer skin, which forms a cocoon, or box. At the front and upper part of the cocoon is a triangular piece, which the fly gets rid of when it is in a fit state to come into the open air."

Fig. 47, taken from drawings in Réaumur's Memoirs, represents the imago of the _OEstrus_ leaving the cocoon.

The reader is, most likely, desirous to know with the aid of what instrument the _OEstrus_ is able to pierce the thick skin of the ox.

The female only is possessed of this instrument, which is situated in the posterior extremity of the body. It is of a shiny blackish brown colour, and as it were covered with scales. By pressing the abdomen of the fly between one's two fingers it is thrust out. Réaumur observed that it was formed of four tubes, which could be drawn the one into the other, like the tubes of a telescope (Fig. 48). The last of these appears to terminate in five small scaly knobs, which are not placed on the same line, but are the ends of five different parts. Three of these knobs are furnished with points, which form an instrument well fitted to operate upon a hard thick skin. United together, they form a cavity similar to that of an auger, and terminating in the form of a spoon.

The Gad-fly, or Breeze-fly of the sheep, _OEstrus_ (_Cephalemyia ovis_), has obtained notoriety on account of its attacking those animals.

Even at the sight of this insect the sheep feels the greatest terror. As soon as one of them appears, the flock becomes disturbed, the sheep that is attacked shakes its head when it feels the fly on its nostril, and at the same time strikes the ground violently with its fore-feet; it then commences to run here and there, holding its nose near the ground, smelling the grass, and looking about anxiously to see if it is still pursued.

It is to avoid the attacks of the _Cephalemyia_ that during the hot days of summer sheep lie down with their nostrils buried in dusty ruts, or stand up with their heads lowered between their fore-legs, and their noses nearly in contact with the ground. When these poor beasts are in the open country, they are observed assembled with their nostrils against each other and very near the ground, so that those which occupy the outside are alone exposed (PLATE III.). The _Cephalemyia ovis_ (Fig. 49) has a less hairy head, but larger in proportion to the size of its body than the Gad-fly (_Gasterophilus equi_). Its face is reddish; its forehead brown with purple bars; its eyes of a dark and changing green; its antennæ black, its thorax sometimes grey, sometimes brown, bristling with small black tubercles; the abdomen white, spotted with brown or black; and the wings hyaline.

The _Cephalemyia_ (_OEstrus_) _ovis_ is to be found in Europe, Arabia, Persia, and in the East Indies. It lays its eggs on the edges of the animal's nostrils, and the larva lives in the frontal and maxillary sinuses. It is a whitish worm, having a black transverse band on each of its segments. Its head is armed with two horny black hooks, parallel, and capable of being moved up and down and laterally. Underneath, each segment of the body has several rows of tubercles of nearly spherical form, surmounted by small bristles having reddish points, and all of them bent backwards. "These points," says M. Joly, "probably serve to facilitate the progress of the animal on the smooth and slippery surfaces of the mucous membranes to which it fixes itself to feed, and perhaps also to increase the secretion of these membranes by the irritation occasioned by the bristles with which they are furnished."[16]

[16] "Recherches sur les OEstrides en général, et
particulièrement sur les OEstres qui attaquent l'homme, le
cheval, le boeuf, et le mouton." Par N. Joly, Professeur à la
Faculté des Sciences de Toulouse. P. 63. Lyons, 1846.

Fixed by means of these hooks to the mucous membrane, which it perforates, the larva nourishes itself with mucus, and lives in this state, according to M. Joly, during nearly a whole year. At the end of this time it comes out, following the same course by which it entered, falls to the ground, and burying itself to the depth of a few inches, is transformed into a pupa. The cocoon is of a fine black colour. Thirty or forty days after its burial it emerges in the perfect state, and detaching the lid at the anterior end of the cocoon by the aid of its head, which has increased considerably in size, takes flight.

Notwithstanding the formidable appearance of their trunks, the habits of the perfect _Conopes_ (Fig. 50) are very quiet. In the adult state they are only to be seen on flowers, of which they suck the honeyed juice. But with their larvæ the case is otherwise. These latter live as parasites on the humble-bees (_Bombi_). Latreille saw the _Conops rufipes_ issue in the perfect state from the body of a humble-bee, through the intervals of the segments of the abdomen.

The _Mucides_ form that great tribe of Diptera commonly known as flies, and which are distributed in such abundance over the whole world. Faithful companions of plants, the flies follow them to the utmost limits of vegetation. At the same time they are called upon by Nature to hasten the dissolution of dead bodies. They place their eggs in the carcases of animals, and the larvæ prey upon the corrupt flesh, thus quickly ridding the earth of those fatal causes of infection to its inhabitants. The organs of these insects are also infinitely modified, in order to adapt them to their various functions.

M. Macquart divides the _Muscides_ into three sections--the _Creophili_, the _Anthomyzides_, and the _Acalyptera_.

The _Creophili_ have the strongest organisation; their movements and their flight are rapid. The greater part feed on the juices of flowers, some on the blood or the humours of animals. Some deposit their eggs on different kinds of insects, others on bodies in a state of decomposition, some again are viviparous. The insects of the genus _Echinomyia_, for instance (Fig. 51), derive their nourishment from flowers. They deposit their eggs on caterpillars, and the young larvæ on hatching penetrate their bodies and feed on their viscera. How surprised, sometimes, is the naturalist, who, after carefully preserving a chrysalis, and awaiting day by day the appearance of the beautiful butterfly of which it is the coarse and mysterious envelope, sees a cloud of flies emerge in place of it!

But there is another singular manoeuvre performed by some of the species of the Diptera with which we are at present occupied to prepare an abundant supply of provision for their larvæ as soon as they are hatched. The following are the means they employ. It is well known that certain fossorial Hymenoptera carry their prey--other insects which they have caught, weevils, flies, &c., and which they intend should serve as food for their own larvæ--into their subterranean abodes. These Diptera, spying a favourable moment, slip furtively into their retreats, and deposit their eggs on the very food which was intended for others. Their larvæ, which are soon hatched, make great havoc among the provisions gathered together in the cave, and cause the legitimate proprietors to die of starvation.

"This instinct," says M. Macquart, "is accompanied by the greatest agility, obstinacy, and audacity, which are necessary to carry on this brigandage; and, on the other hand, the Hymenoptera, seized with fear, or stupefied, offer no resistance to their enemies, and although they carry on a continual war against different insects, and particularly against different _Muscides_, they never seize those of whom they have so much to complain, and which, nevertheless, have no arms to oppose them with."

The _Sarcophagæ_ are a very common family of Diptera, and are chiefly to be found on flowers, from which they steal the juice. The females do not lay eggs, but are viviparous.

Réaumur, with his usual care, observed this remarkable instance of viviparism proved in a fly, which seeks those parts of our houses where meat is kept to deposit its larvæ. This fly is grey, its legs are black, and its eyes red.

When one of them is taken and held between the fingers, there may often be seen a small, oblong, whitish, cylindrical worm come out of the posterior part of the body, and shake itself in order to disengage itself thoroughly. It has no sooner freed itself than the head of another begins to show. Thirty or forty sometimes come out in this manner, and, on pressing the abdomen of the fly slightly, more than eighty of these larvæ may sometimes be made to come out in a short space of time. If a piece of meat be put near these worms, they quickly get into it, and eat greedily. They grow rapidly, attaining their full size in a few days, and make a cocoon of their skin, from which in a certain time the imago issues. If the body of one of these ovo-viviparous flies (for the eggs hatch within the parent) be opened, a sort of thick ribbon of spiral form is soon seen. This ribbon appears at first sight to be nothing but an assemblage of worms, placed alongside of and parallel to one another.

Each worm has a thin white membranous envelope, similar to those light spiders' webs which float about in autumn, which the French call _fils de la vierge_, and we denominate _gossamer_.

The fecundity of this fly is very great, for, in the length of a quarter of an inch, the envelope in which these small worms are enclosed contains 2,000 of them. Therefore this ribbon, being two inches and a half long, contains about 20,000 worms.

The members of the genus _Stomoxys_, though nearly related to the house-fly, differ from it very much in habits. They live on the blood of animals. The _Stomoxys calcitrans_ is very common in these climates. Its palpi are tawny yellow, antennæ black, thorax striped with black, abdomen spotted with brown, and its trunk hard, thin, and long. It deposits its eggs on the carcases of large animals.

The Golden Fly, _Lucilia Cæsar_, lays its eggs on cut-up meat, or on dead animals. It is only three or four lines in length, of a golden green, with the palpi ferruginous, antennæ brown, and feet black.

A species of this genus, the _Lucilia hominivorax_, has lately obtained a melancholy notoriety. We are indebted to M. Charles Coquerel, surgeon in the French Imperial navy, for the most exact information concerning this dangerous Dipteron, and the revelation of the dangers to which man is liable in certain parts of the globe. But let us first describe the insect, which is very pretty and of brilliant colours.

Fig. 52, taken from M. Charles Coquerel's Memoir, represents the larva and the perfect insect, as well as the horny mandibles with which the larva is provided. It is rather more than the third of an inch in length, the head is large, downy, and of a golden yellow. The thorax is dark blue and very brilliant, with reflections of purple, as is also the abdomen. The wings are transparent, and have rather the appearance of being smoked; their margins, as well as the feet, are black.

This beautiful insect is an assassin. M. Coquerel has informed us that it sometimes occasions the death of those wretched convicts whom human justice has transported to the distant penitentiary of Cayenne.

When one of these degraded beings, who live in a state of sordid filth, goes to sleep, a prey to intoxication, it happens sometimes that this fly gets into his mouth and nostrils; it lays its eggs there, and when they are changed into larvæ, the death of the victim generally follows.[17]

[17] "The majority of convicts attacked by the _Lucilia
hominivorax_," says M. F. Bouyer, captain of the frigate, in
"Un Voyage à la Guyane Française," "have succumbed, despite the
assistance of science. Cures have been the exception: in a
dozen cases three or four are reported."--_Tour du Monde, 1866,
1er Semestre_, p. 318.

These larvæ are of an opaque white colour, a little over half an inch in length, and have eleven segments. They are lodged in the interior of the nasal orifices and the frontal sinuses, and their mouths are armed with two very sharp horny mandibles. They have been known to reach the ball of the eye, and to gangrene the eyelids. They enter the mouth, corrode and devour the gums and the entrance of the throat, so as to transform those parts into a mass of putrid flesh, a heap of corruption.

Let us turn away from this horrible description, and observe that this hominivorous fly is not, properly speaking, a parasite of man, as it only attacks him accidentally, as it would attack any animal that was in a daily state of uncleanliness.

In many works on medicine may be found mentioned a circumstance which occurred twenty years ago, at the surgery of M. J. Cloquet. The story is perhaps not very agreeable, but is so interesting as regards the subject with which we are occupied, that we think it ought to be repeated here. One day a poor wretch, half dead, was brought to the Hôtel-Dieu. He was a beggar, who, having some tainted meat in his wallet, had gone to sleep in the sun under a tree. He must have slept long, as the flies had time enough to deposit their eggs on the tainted meat, and the larvæ time enough to be hatched, and to devour the beggar's meat. It seems that the larvæ enjoyed the repast, for they passed from the dead meat to the living flesh, and after devouring the meat they commenced to eat the owner. Awoke by the pain, the beggar was taken to the Hôtel-Dieu, where he expired.

Who would suppose that one of the causes which render the centre of Africa difficult to be explored is a fly not larger than the house-fly? The Tsetse fly (Fig. 53) is of brown colour, with a few transverse yellow stripes across the abdomen, and with wings longer than its body. It is not dangerous to man, to any wild animals, or to the pig, the mule, the ass, or the goat. But it stings mortally the ox, the horse, the sheep, and the dog, and renders the countries of Central Africa uninhabitable for those valuable animals. It seems to possess very sharp sight. "It darts from the top of a bush as quick as an arrow on the object it wishes to attack," writes a traveller, M. de Castelnau.

Mr. Chapman, one of the travellers who have advanced the farthest into the middle of Southern Africa, relates that he covered his body with the greatest care to avoid the bites of this nimble enemy; but if a thorn happened to make a nearly imperceptible hole in his clothing, he often saw the Tsetse, who appeared to know that it could not penetrate the cloth, dart forward and bite him on the uncovered part. The sucker of blood secretes--in a gland placed at the base of his trunk--so subtle a poison, that three or four flies are sufficient to kill an ox.

The _Glossina morsitans_ abounds on the banks of the African river, the Zambesi, frequenting the bushes and reeds that border it. It likes, indeed, all aquatic situations. The African cattle recognise at great distances the buzzing of this sanguinary enemy, and this fatal sound causes them to feel the greatest fear.

Livingstone, the celebrated traveller, in crossing those regions of Africa that are watered by the Zambesi, lost forty-three magnificent oxen by the bites of the Tsetse fly, notwithstanding that they were carefully watched, and had been very little bitten.

"A most remarkable feature in the bite of the Tsetse is its perfect harmlessness in man and wild animals, and even calves so long as they continue to suck the cows. We never experienced the slightest injury from them ourselves personally, although we lived two months in their habitat, which was in this case as sharply defined as in many others, for the south bank of the Chobe was infested by them, and the northern bank, where our cattle were placed, only fifty yards distant, contained not a single specimen. This was the more remarkable, as we often saw natives carrying over raw meat to the opposite bank with many Tsetses settled on it.

"The poison does not seem to be injected by a sting, or by ova placed beneath the skin, for, when one is allowed to feed freely on the hand, it is seen to insert the middle prong of three portions, into which the proboscis divides, somewhat deeply, into the true skin. It then draws it out a little way, and it assumes a crimson colour, as the mandibles come into brisk operation. The previously-shrunken belly swells out, and, if left undisturbed, the fly quietly departs when it is full. A slight itching irritation follows, but not more than in the bite of the mosquito. In the ox this same bite produces no more immediate effects than in man. It does not startle him, as the gad-fly does; but a few days afterwards the following symptoms supervene: the eyes and nose begin to run, the coat stares as if the animal were cold, a swelling appears under the jaw, and sometimes on the navel; and, though the animal continues to graze, emaciation commences, accompanied with a peculiar flaccidity of the muscles, and this proceeds unchecked until, perhaps months afterwards, purging comes on, and the animal, no longer able to graze, perishes in the state of extreme exhaustion. Those which are in good condition often perish, soon after the bite is inflicted, with staggering and blindness, as if the brain were affected by it. Sudden changes of temperature produced by falls of rain seem to hasten the progress of the complaint; but in general the emaciation goes on uninterruptedly for months, and, do what we will, the poor animals perish miserably.

"When opened, the cellular tissue on the surface of the body beneath the skin is seen to be injected with air, as if a quantity of soap bubbles were scattered over it, or a dishonest awkward butcher had been trying to make it look fat. The fat is of a greenish-yellow colour, and of an oily consistence. All the muscles are flabby, and the heart often so soft that the fingers may be made to meet through it. The lungs and liver partake of the disease. The stomach and bowels are pale and empty, and the gall-bladder is distended with bile. These symptoms seem to indicate, what is probably the case, a poison in the blood; the germ of which enters when the proboscis is inserted to draw blood. The poison-germ contained in a bulb at the root of the proboscis, seems capable, although very minute in quantity, of reproducing itself. The blood after death by Tsetse is very small in quantity, and scarcely stains the hands in dissection....

"The mule, ass, and goat enjoy the same immunity from the Tsetse as man and game. Many large tribes on the Zambesi can keep no domestic animals except the goat, in consequence of the scourge existing in their country. Our children were frequently bitten, yet suffered no harm; and we saw around us numbers of zebras, buffaloes, pigs, pallahs and other antelopes, feeding quietly in the very habitat of the Tsetse, yet as undisturbed by its bite as oxen are when they first receive the fatal poison. There is not so much difference in the natures of the horse and zebra, the buffalo and ox, the sheep and the antelope, as to afford any satisfactory explanation of the phenomenon. Is a man not as much a domestic animal as a dog?

"The curious feature in the case, that dogs perish though fed on milk, whereas the calves escape so long as they continue sucking, made us imagine that the mischief might be produced by some plant in the locality, and not by Tsetse; but Major Vardon, of the Madras army, settled that point by riding a horse up to a small hill infested by the insect, without allowing him time to graze, and though he only remained long enough to take a view of the country and catch some specimens of Tsetse on the animal, in ten days afterwards the horse was dead."[18]

[18] "Missionary Travels and Researches in South Africa," by
David Livingstone, LL.D., D.C.L. P. 81, _et seq._ London, John
Murray, 1857. (The extract in the original of this work is from
a French translation: "Explorations dans l'Intérieur de
l'Afrique australe, et voyages à travers le continent
Sainte-Paul de Loanda à l'Embouchure du Zambèze, de 1840 à
1846, traduit de l'Anglais." Pages 93-95. 8vo. Paris,
1859.--ED.)

The inhabitants of the Zambesi can, therefore, have no domestic animal but the goat. When herds of cattle driven by travellers or dealers are obliged to cross these regions, they only move them during the bright nights of the cool season, and are careful to smear them with dung mixed with milk; the Tsetse fly having an intense antipathy to the dung of animals, besides being in this season rendered dormant by the lowness of the temperature. It is only by such precautions that they are able to get through this dangerous stage of their journey.

The large blue Meat-fly, the familiar representative of the genus _Calliphora_, is known to all by its brilliant blue and white reflecting abdomen. This fly, which is common everywhere, is the _Calliphora vomitoria_ on which Réaumur has made many beautiful observations, which we will make known to our readers.

If we shut up a blue meat-fly in a glass vase, as Réaumur did, and place near the insect a piece of fresh meat, before half a day is passed, the fly will have deposited its eggs thereon one after the other, in irregular heaps, of various sizes. The whole of these heaps consist of about two hundred eggs, which are of an iridescent white colour, and four or five times as long as they are broad. In less than twenty-four hours after the egg is laid the larvæ is hatched. It is no sooner born than it thinks of feeding, and buries itself in the meat with the aid of the hooks and lancets with which it is provided.

These worms do not appear to discharge any solid excrement, but they produce a sticky liquid, which keeps the meat in a moist state and hastens its putrefaction. The larvæ eat voraciously and continually; so much so, that in four or five days they arrive at their full growth. They then take no more nourishment until they are transformed into flies. They are now about to assume the pupa state. In this condition it is no longer necessary for them to remain on the tainted meat, which has been alike their cradle and their larder, and where until now they were so well off. They therefore leave it and seek a retreat under ground.

The larva then assumes a globular form and reddish colour, loses all motion, and cannot any longer either lengthen or shorten, or dilate or contract itself. Life seems to have left it. "It would be considered a miracle," says Réaumur, "if we were told there was any kind of quadruped of the size of a bear, or of an ox, which at a certain time of the year, the beginning of winter for instance, disengages itself completely from its skin, of which it makes a box of an oval form; that it shuts itself up in this box; that it knows how to close it in every part, and besides that it knows how to strengthen it in such a manner as to preserve itself from the effects of the air and the attacks of other animals. This prodigy is presented to us, on a small scale, in the metamorphosis of our larva. It casts its skin to make itself a strong and well-closed dwelling."

If one opens these cocoons only twenty-four hours after the metamorphoses of the worms, no vestige of those parts appertaining to a pupa is to be found. But four or five days afterwards, the cocoon is occupied by a white pupa, provided with all the parts of a fly. The legs and wings, although enclosed in sheaths, are very distinct; these sheaths being so thin that they do not conceal them. The trunk of the fly rests on the thorax; one can discern its lips, and the case which encloses the lancet. The head is large and well formed, its large, compound eyes being very distinct. The wings appear still unformed, because they are folded, and, as it were, packed up. It is a fly, but an immovable and inanimate fly; it is like a mummy enveloped in its cloths.

Nevertheless, it is intended this mummy should awake, and when the time comes it will be strong and vigorous. Indeed, it has need of strength and vigour to accomplish the important work of its life. Although its coverings are thin, it is a considerable work for the insect to emerge, for each of its exterior parts is enclosed in them as in a case, much the same as a glove fits tightly to all the fingers of the hand. But that for which the most strength is necessary is the operation of forming the opening of the cocoon, in which as a mummy it is so tightly enclosed.

The fly always comes out at the same end of the cocoon, that is, at the end where its head is placed, and also where the head of the larva previously was. This end is composed of two parts--of two half cups placed one against the other. These can be detached from each other and from the rest of the cocoon. It is sufficient for the fly that one can be detached, and in order to effect this, it employs a most astonishing means. It expands and contracts its head alternately, as if by dilatation; and thus pushes the two half cups away from the end of the cocoon. This is not long able to resist the battering of the fly's head, and the insect at length comes out triumphant. This fly, which should be blue, is then grey; it, however, comes quickly to perfection, at the end of three hours attaining its ultimate colour; and in a very short space of time every part of the animal becomes of that firmness and consistency which characterises them. At the same time, the wings, which at the moment it came into the world were only stumps, extend and unfold themselves by degrees. The meat-fly is represented below (Fig. 55).

One of the features in the formation of this fly which most attracted the attention of Réaumur, and which is likely to excite the curiosity of all those who take an interest in insects, is the composition of its trunk. We will, therefore, with that illustrious observer, take a glimpse at the remarkable and complicated apparatus by the aid of which the fly can suck up liquids, and can even taste solid and crystalline substances, such as sugar.

It is no difficult matter to make a fly show its trunk extended to its full extent. One has only to press between the finger and thumb either the two sides of the upper and under part of the thorax. It is thus forced at once to put out its tongue.

The trunk appears to be composed of two parts joined together, and forming a more or less obtuse angle (Fig. 56). The first portion of the trunk, that which joins the head, is perfectly membranous and in the form of a funnel. We will call it the conical part, and show it separately (Fig. 57). The second portion terminates in a thick mass, in part cartilaginous or scaly, and of a shiny brown colour. Above the conical portion are two oblong antennæ, without joints, of chestnut colour, and furnished with hairs.

On ceasing to press the thorax, the membranous conical portion may be seen to draw itself back within its sheath (Fig. 58). The second portion is at the same time drawn into the cavity, but it raises itself by forming a more and more acute angle, so that when it reaches the opening of the cell it is parallel with, and its length is equal to that of the cell, which is quite large enough to receive it. The base lengthens and flattens a little, and conceals the trunk.

Let us cause the trunk to extend itself a second time, in order to observe its tip minutely. Here the opening is placed, which may be looked upon as the mouth of the insect, and is provided with two large thick lips (Fig. 59). These lips form a disc, perpendicular to the axis of the trunk; the disc is oval, and is divided into two equal and similar parts by a slit. The lips have each a considerable number of parallel channels situated perpendicularly to the slit. These channels are formed by a succession of vessels placed near each other. On pressing the trunk we see that these vessels are distended by a liquid. Réaumur, from whom we borrow these details, discovered a few of the uses to which this trunk is applied. He covered the interior of a transparent glass vase with a light coat of thick syrup. He then put in some flies, when it was easy to see some of them proceed to fix themselves to the sides of the vase, and regale themselves on the sugary liquid, of which they are very fond. He observed them carefully, and in his admirable work he recommends those who are curious to try the experiment, with which, like himself, they will certainly be satisfied.

While the body of the trunk is stationary its end is much agitated. It may be seen to move in different ways, and with an astonishing quickness; the lips acting in a hundred different ways, and always with great rapidity. The small diameter of the disc which they form lengthens and shortens alternately; the angle formed by the two lips varies every instant; they become successively flat and convex, either entirely or partly. All these movements, Réaumur remarks, give a high idea of the organisation of the part which performs them.

The object of all these movements is to draw the syrup into the interior of the trunk. If we observe the lips (Fig. 60) attentively, it will easily be seen that they touch each other about the centre of the disc, and leave two openings, one in front, the other at the back. The one in front is, one may say, the mouth of the fly, as it is to this opening that the liquid is brought, which is intended to be and is soon introduced into the trunk. Without occupying ourselves for the present with the channel through which it rises, we may first ask, whatever that channel may be, what is the power that forces the liquid into it?

It is nearly certain that suction is the principal cause of the liquid flowing up the trunk. It would thus be a sort of pump, into which the liquid is forced by the pressure of the external air. The fly exhausts the air from the tube of its trunk, and the drop of liquid which is at the opening penetrates and goes up this channel through the influence of the atmospheric pressure. To this physical phenomenon must be added the numerous and multiplied movements which take place in the trunk, and which are intended to cause sufficient pressure to drive the liquor which is introduced into the channel upwards.

Réaumur wished to know how it was that very thick syrups, and even solid sugar, can be sucked up by the soft trunk of the fly. What he saw is wonderful. If a fly meets with too thick a syrup, it can render it sufficiently liquid; if the sugar is too hard, it can dissolve small portions of it. In fact, there exists in its body a supply of liquid, of which it discharges a drop from the end of its trunk at will, and lets this fall on the sugar which it wishes to dissolve, or on the syrup it wishes to dilute. A fly, when held between the fingers, often shows at the end of its trunk a drop, very fluid and transparent, of this liquid. "The water poured on the syrup," says Réaumur, "would not always insinuate itself sufficiently quick into every part of it; the movement of the fly's lips hastens the operation; the lips turn over, work, and knead it, so that the water can quickly penetrate it, in the same way as one handles and kneads with one's hands a hard paste which it is wished to soften, by causing the water by which it is covered to mix with it. This, again, is the same means the fly employs with sugar. When the trunk is forced to act upon a grain of irregular and rugged form, on which it cannot easily fasten, its end distorts itself to seize and hold it. It is sometimes very amusing to see how the fly turns over the grain of sugar in different ways; it appears to play with it as a monkey would with an apple. It is, however, only that it may hold it well in order to moisten it more successfully, and afterwards to pump up the water which has partly dissolved it."

Réaumur often observed a drop of water at the end of the trunks of flies which were perfectly surfeited with food. This drop ascended the trunk, then re-descended to the end, and this many times in succession. It appeared to him that it was necessary for these insects, as for many quadrupeds, to chew the cud, as it were; that, in order the better to digest the liquid they had passed into their stomachs, they were obliged to bring it back into the trunk that it might return again better prepared.

In order to assure himself directly of the reality of his supposition, Réaumur tested the water which a fly, that he says "had got drunk on sugar," had brought back to the end of its trunk; he found this to be sugar and water. Also, having given a fly currant-jelly, he observed, after it had sufficiently gorged itself, several drops of red liquid in its trunk, and having tasted it, found it had the flavour which, from its appearance, he guessed it would have.

The illustrious observer, who had already made all these discoveries on the formation and functions of the trunks of insects, often reflected on the fact that the liquors of which flies are most fond are enclosed under the skin of certain fruits, such as pears, plums, grapes, &c., or even under the skin of some animals of which they suck the blood. In order that the trunk of a fly may act under such circumstances, it is necessary for it to pierce and open the skin. If this is the case, flies ought to be possessed of a lancet. He looked a long time for this lancet, and at last found it. It is situated on the upper side of the part of the trunk which is terminated by the lips; it is placed in a fleshy groove, and is enclosed in a case. It has a very fine point, and is of light colour (Fig. 61). The point is situated in the opening which is to be seen between the lips of the trunk, at its anterior end, through which liquids may pass. That is the only opening of the lips; and the sucker which takes up the liquid is the same part which we just now called the case of the lancet.

Réaumur is so interesting an author that it is difficult to cease quoting him; but we must continue our review of the principal kinds of Diptera.

The genus _Musca_ (fly), in which Linnæus comprised the immense series of Diptera, with the exception of the _Tipulidæ_, the _Tabanidæ_, the _Asilidæ_, the _Bombylidæ_, and the _Empidæ_, is now reduced to the House Fly and a few resembling it. The habits of these troublesome companions are in conformity with the two great principles of animal life, that is, eating and propagating their species.

Flies feed principally on fluids which exude from the bodies of animals; that is, sweat, saliva, and other secretions. They also seek vegetable juices; and they may be seen in our houses to feed eagerly on fruits and sweet substances.

The common flies deposit their eggs on vegetables, and particularly on fungi in a state of decomposition, on dung-heaps, cow dung, &c. They are essentially parasites, settling on both man and beast, to suck up the fluid substances which are diffused over the surface of their bodies. In our dwellings they eat anything that will serve to nourish them. Generation succeeds generation with the greatest rapidity.

The House Fly (_Musca domestica_, Fig. 62) is about three lines in length, ash coloured, with the face black, the sides of the head yellow, and the forehead yellow with black stripes; the thorax is marked with black lines; the abdomen is pale underneath, and a transparent yellow at the sides, in the males, and is speckled with black. The feet are black; the wings transparent, and yellowish at the base. This species is extremely plentiful throughout the whole of Europe. Every one knows how annoying it is towards the end of the summer, and especially so in the South of France during the hot season.

The Ox Fly (_Musca bovina_), a near relation of the house fly, is also very common. It settles on the nostrils, the eyes, and the wounds of animals.

The Executioner Fly (_Musca carnifex_), which is not rare in France, also attacks oxen. It is of a dark metallic green colour with a slight ash-coloured down. Its forehead is silvery at the front and sides; the abdomen is edged with black; the wings hyaline, and yellow at the base.

_Section of the Anthomyides._--The section of _Anthomyides_ comprises insects which appear to be _Creophili_ whose organisation has become weakened by almost insensible degrees. Their colours vary very much--black, grey, and iron-colour are everlastingly shaded and blended together. To that may be added reflections which are above the ground colour, and which change the hues of the little animal according to the incidence of the rays of light. The _Anthomyides_ resemble the genus _Musca_ very closely in their habits as well as in their organisation.

In this group of Diptera we will first say a few words about the _Anthomiæ_. These flies are to be found in most gardens, and on all flowers, particularly on the heads of Compositæ and Umbelliferæ. They often unite in numerous bands in the air, and indulge in the joyous dances to which love invites them. The females deposit their eggs in the ground, and their larvæ are there quickly developed. The latter suspend themselves to certain bodies, the same as some lepidopterous chrysalides, in order to transform themselves into pupæ.

The _Anthomyia pluvialis_ (Fig. 63) is from two to four lines in length, and of a whitish ash-colour. Its wings are hyaline, the thorax has five black spots, and the abdomen three rows of similar spots.

We will stop a moment with the _Pegomyiæ_, which are very interesting in the larva state, and which excited the interest and sagacity of Réaumur.

The cradle of these Diptera is the interior of leaves. They work as the miners of the vegetable world, in the parenchyma or cellular tissue of the leaf, between the two epidermal membranes. The henbane, the sorrel, and the thistle, especially nourish them. If one holds a leaf in which one of these miners has established itself against the light, one sees the workman boring the vegetable membrane. Its head is armed with a hook, formed of two horny pieces, and with this hook it digs into the parenchyma of the leaf. The effect of this digging is visible, as those places become by degrees transparent. Each blow detaches a small portion of the substance of the leaf. It is thus that these miners hollow out galleries for themselves, in which they find shelter, food, and security. Some are changed into pupæ in the gallery which they have hollowed out, others go out of the leaves when they are near their final transformation.

_Section of Acalyptera._--The _Acalyptera_, which are the last of the great tribe of _Muscidæ_, comprehend the greater number of these insects. Their constitution appears to be peculiar and slow. They live principally in the thickest part of woods, on grasses, and aquatic plants. Fearing the lustre and warmth of the sun, they never draw the nectar from flowers. Their flight is feeble, and they never indulge in those joyous ethereal dances which we have mentioned when speaking of the preceding groups. Their life is generally melancholy, obscure, and hidden. Some of them seek decomposed animal and vegetable substances, others living vegetables.

We shall only be able in this immense group of _Muscidæ_ to mention a few types which are interesting from various reasons, such as the _Helomyzæ_, the _Scatophagæ_, the _Ortalides_, the _Daci_, and the _Thyreophoræ_.

The _Helomyzæ_ (Fig. 64) live in the woods. Their larvæ are developed in the interior of fungi. Réaumur studied the larvæ of the Truffle Helomyza. The head of this fly is ferruginous, its thorax is of a brownish grey, its shoulders of a brownish yellow, its wings brownish, the abdomen yellow and brown, and the feet red. The larvæ of these insects commit depredations for which gourmands will never forgive them, destroying, as they do, their truffles. When one presses between one's fingers a truffle that is in a too advanced state, one feels certain soft parts, which yield under pressure. On opening the truffle, the larvæ of the insect of which we are speaking will be found inside. These larvæ are white and very transparent. Their mouth is armed with two black hooks, by means of which they dig into the truffle in the same way as other larvæ dig into meat. The excretions of these little parasites cause the truffle to become decomposed and rotten. In a few days the larvæ become full-grown. They then leave their abode and go into the ground, there to change into pupæ.

The _Ortalidæ_ form a tribe which is remarkable for the upright carriage of the wings, which are generally speckled, by the vibratory movement of these organs, and especially for the cradle chosen by them for their progeny in fruits and grains. Nature seems to have assigned to each species its own particular vegetable.

We will only mention here the Cherry-tree _Ortalis_, whose larva lives on the pulp of that fruit. This fly is about a line and a half long. It is of rather a metallic black colour, its head light yellow, the edges of its eyes white, and the tarsi red. The wings have four broad black stripes.

The Olive Dacus (_Dacus oleæ_, Fig. 65) is a little fly, about half the size of the house fly, of ashy grey colour on the back, its head orange-yellow, its eyes green, and its forehead yellow, marked with two large black spots. The thorax is adorned with four lightish yellow spots, and its hind part, as well as its antennæ and wings, are of the same colour. The wings are transparent, reflecting green, gold, pink, and blue, according as the rays of light fall upon them, and are remarkable for having a small black spot at their extremity. The abdomen is of a fawn colour or orange-yellow, spotted with black on each side. This fly performs sudden and jerking movements; it keeps its wings extended, and rather jumps than flies. It is a destructive insect, a perfect scourge, which causes every two or three years a loss of five or six millions of francs to French agriculture.

M. Guérin-Méneville has made some valuable observations on the Olive Dacus, and at the request of the Imperial Society of Agriculture of Paris, has indicated the way to preserve the olive from these ruinous larvæ, which generally destroy two crops out of three. We will borrow the following details from this learned entomologist: "At the time when the olives are formed the Dacus proceeds to place an egg under the skin of each of the fruits. By means of a little horny instrument, with which the female is provided, and which contains a small lancet, she pierces the skin of the olive; she moves her wings and lays her egg. She afterwards cleans and rests herself, by passing her feet over her head, wings, and other parts of her body. She then flies away, and seeks another olive, to deposit in it another egg; she repeats this operation until she has placed on as many olives the three or four hundred eggs which she bears."

Fig. 66, taken from the Memoir published by M. Guérin-Méneville, in the "Revue Nouvelle" of the 15th July, 1847, shows the Dacus laying its eggs on the olive, and the larvæ that are already hatched in another of the same fruit. The larvæ which succeed these eggs (Fig. 67) are whitish, soft, and without limbs. They pass fifteen or sixteen days in boring a gallery in the pulp of the olive, at first vertically, until they reach the stone, then on one side, and along the side of the stone. When they have reached the term of their development, they approach the surface, enlarging the first channel and leaving between it and the exterior air only a thin pellicle, in the middle of which may be perceived the first small opening by which the mother had introduced her egg in the commencement.

Fig. 68, copied from a drawing in the Memoirs of M. Guérin-Méneville, shows the gallery bored round the olive by the larva of the Dacus. The larva thus prepares an easy issue for the perfect insect. Its skin then contracts, its body diminishes in length, and is transformed into an oval cocoon, which soon gets brown, and is the chrysalis of the insect. At the side of the head it shows a curved line, a thin suture which marks a sort of cap or door, which, at the time of its hatching, the insect will be easily able to force open with its head. The fly is hatched twelve days after its metamorphosis from the larva to the pupa. It has thus taken the Dacus twenty-seven to twenty-eight days to arrive at this state, from the time the egg was laid; besides which, this species, in the warm climates of Provence and Italy, can reproduce itself several times from the beginning of July, the period at which the first flies begin to lay, till the end of autumn.

In order to save a considerable portion of the olive crop of these countries, M. Guérin-Méneville has advised hastening the harvest sufficiently for all the olives to be pressed at a time when the larvæ of the last generation, which would be preserved in the olives that are left, or in the earth, according to the climate, are still in the fruit. If a first operation were not sufficient to destroy them all, it should be repeated the following year. The sacrifice entailed by this practice would be amply compensated by a succession of good crops and the certainty of a sure and permanent profit. In fact, by an early gathering at least half a crop of oil is still obtained; whereas, by waiting for the usual period of gathering the olives, sufficient time is left for the larvæ of the Dacus to devour their parenchyma, which deprives them of the little oil that they might have yielded if their destruction had been accomplished earlier. This early gathering has the advantage of causing the destruction of a great number of larvæ, which will be so much towards diminishing the means of reproduction of the fly.