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n our paper Mr. Wilson and I thought it well to figure the upper surface of the halter as seen under a high magnification. The drawing showed the hinge on which the halter quivers--and certain basal papillae, as Weinland calls them. There is little doubt that the main function of the halteres is that of balancing and orientating the insect. They may, however, have a secondary function; in some flies they are known to vibrate with extreme rapidity. It is just possible that in these rapid vibrations the papillae of the concave surface rubbing against those of the convex basal plate may produce a note. As long ago as 1764 von Gleichen-Russworm observed that when the halteres of the common house-fly are removed the volume of the buzzing diminished. This, however, in all probability is due to the diminished activity of the wings. On the other hand, Professor J. Stanley Gardiner informs us that he has noticed that mosquitos still continue to give forth a faint note even when their wings are quite at rest, and this note may possibly be caused by the halteres.

THE MOSQUITO

PART IV

Gnats are unnoted wheresoe'er they fly, But eagles gazed upon with every eye.

The eggs of the mosquito are deposited in fresh water, and at first they are white, but they very rapidly darken until they assume a polished black appearance. Each egg is 0?72 mm. in length, and its greatest breadth, which is somewhere about its middle, is 0?16 mm. The egg is boat-shaped, and one end, as is usual in boats, is slightly deeper and fuller than the other. The under surface is fluted, and is marked by a minute network. The upper surface has a coarser reticulation which divides the surface into nearly equal hexagonal areas. The rim of the 'boat' is thickened, and these thickenings are regularly ribbed; they extend over above the median third of the egg, and recall the rounded float which runs along the edge of a life-boat: and indeed they serve the same purpose, for they are composed of air-cells, and their function is to keep the boat-shaped eggs right side upward. Soon after the egg has been laid it is of a greyish-black colour, but after a certain amount of attrition an outer membrane splits off--the membrane which has given the egg its reticulated appearance. This membrane scales off in fragments, and is of a grey colour. The egg beneath it is glistening black--as shiny and as black as patent leather.

One curious fact that Professor Nuttall and I noticed in the life-history of the egg is that when it is drawn by capillary forces a little way out of the water on to the leaf of a water-plant or some other half-submerged object, the blunt end always points downwards. Now the blunt end is the head end, and thus, should hatching take place whilst the egg is suspended half in the water and half in the air, the larva will emerge into its proper element and not into the atmosphere.

On the second or third day after oviposition , the young larva leaves the egg and begins to swim in the water. The egg hatches by the detachment of a cap-like portion of the anterior end of the egg-shell. There is no visible ring indicating the limits of this operculum, but the cap is usually more or less of the same size. Opinions differ as to how far desiccation interferes with development of the larva in the egg-shell. They do not seem to be able to stand more than forty-eight hours of drought. There is no evidence that they can survive throughout the winter period. Everything that we know indicates that the egg must pass this period within the mother's body, and that they only attain maturity in early spring, when the weather becomes warmer.

The larva of the mosquito is one of the most fascinating objects one can watch under the microscope. It is very complex, and consists of the usual arthropod regions of the head, the thorax, and the abdomen.

Without going into the question of how many typical somites make up the head, we must state that the thorax has the typical number of three, much fused together, and the abdomen nine. The first seven of these are very much alike; the eighth, however, bears the large stigmata or orifices of the breathing system, and the ninth a number of beautifully arranged hairs, by means of which the larva to a great extent steers itself. The head resembles two-thirds of a sphere, and is covered with a complete and clearly defined brown, chitinous case. The eyes are lateral, and on each side we have both a simple and a compound eye. In front of each eye is a little protuberance, which carries the antenna, and between these two eminences a band of pigment runs across the head, from which six symmetrically placed immovable feathered hairs project, wreathing the head, as it were, with a halo. There are many other hairs on the head, whose number and shape are of great systematic importance. The anterior edge of the head carries on each side of its under surface a conspicuous brush, very like a shaving-brush, the constituent hairs of which are arranged in a spiral, and it is these brushes which sweep the food into the mouth of the young and voracious larva. The base of this brush is so arranged that when depressed and bent towards the mouth the two brushes approximate, but each brush can move independently and often does so: one may be depressed towards the mouth, whilst the other remains erect.

The larva passes its life hanging on to the under surface of the surface-film of the water, its dorsal surface being uppermost. In fact, as Sidney Smith pointed out about the sloth, 'it passes its life in a state of suspense, like a young curate distantly related to a bishop.' But, since these larvae feed on any kind of organic d?bris that floats up to the top and is there arrested by the surface-film, it is obviously important that the brushes which sweep together these organic particles and carry them to the mouth should be next the surface, and to effect this the head must rotate through an angle of 180?; and the head does in fact turn upside-down on the neck so sharply and accurately that, as it comes into position, you almost think, as you are watching it, that you hear a click, just as you do when you rotate the diaphragm of a microscope.

The mouth parts now begin to vibrate upwards and forwards, and the brushes are bent downwards, backwards, and inwards. Round the mouth is a small space, the walls of which are completed by the mandibles, and into this space the brushes are suddenly bent back, at the same time the mandibles and maxillae move forward to meet them. This movement may take place as many as 180 times a minute, and it produces a current converging in concentric curves towards the above-mentioned chamber. The water filters out between the sides, and any particle of food is retained by the hairs or by the mouth appendages; from time to time the mandibles are brought together, and their stiff bristles are run through the brushes as one's fingers run through a beard; at other times the brushes disappear far into the mouth, and then are slowly withdrawn, passing through the comb-like bristles on the mandibles. The brushes are frequently swallowed, and are withdrawn in little jerks, so that the maxillae have every opportunity of combing any nutritive particles out of them. The whole operation is a most fascinating one to watch.

As far as one can judge, the currents set in motion by the action of all these forces extend in an area equal to twice the length of the larva, or even more. The currents are in the plane just below the surface-film, and any organic matter lighter than water is swept towards the mouth. In fact the larva sweeps the lower side of the surface-film of the pond or puddle just as a careful housemaid might sweep spiders and flies off a ceiling with a hand-brush.

If you have a beard.

Grassi found the intestine of the larva to contain protozoa, unicellular algae, and other organic detritus. In course of time some object too big for the larva to swallow is brought to its mouth by the currents, but after a very short struggle this is rejected. The minuter particles accumulate in the chamber for a certain time, and then are swallowed by a gulp-like motion and thus pass into the oesophagus.

THE MOSQUITO

PART V

Amongst aquatic larvae, the most beautiful and delicate are those of numerous species of gnat.--

The eighth abdominal segment bears the stigmata or the openings of the respiratory apparatus, and the ninth segment has abandoned the flattened and square cross-section of its predecessors, and is cylindrical and tapering. The posterior end of the body is cut off sharply. Round the posterior opening of the alimentary canal are four white, soft papillae, which are well supplied with tracheae and are capable of contracting and expanding. Above these are four very prominent hairs, two median and two lateral, and ventrally to the ninth abdominal segment is a fan-shaped arrangement of hairs springing from two pieces of very complicated structures. These hairs seem to act to some extent as a rudder, and they probably serve as an accessory organ of locomotion. Possibly they have also a sensory or tactile function, and act, as so many posterior filaments do in insects, as antennae 'from behind.'

Not infrequently the larva ceases to lie parallel to the surface of the water, its palmate hairs are put out of action, and then its body hangs down into the water, but it still maintains its respiratory connexion with the outer air through these breathing-pores. From time to time the hairs mentioned above are brushed over by the mouth parts and cleaned of any d?bris.

The larvae, when they leave the surface-film sink by their own weight; but they not infrequently swim actively downwards, their swimming action being very like that of an eel. When returning to the surface they are entirely dependent upon their powers of swimming, being slightly heavier than water. When the tail reaches the surface-film the larvae are at once arrested, and immediately cease their swimming-movements. They invariably move tail forwards, and the hairs which we have mentioned above at the posterior end of the body undoubtedly act as 'buffers' or 'fenders.' As a rule, when they are above, they are actively engaged in feeding; but at the bottom they lay inert, as though feigning to be dead. Kept in a glass beaker they are apt to lie with their respiratory apparatus attached to the concave film, which capillary attraction draws up on the surface of the glass. Their heads then point towards the surface of the beaker. If forcibly kept below--say, by submerging them under a watch-glass--they are frequently enabled to breathe by attaching the openings of their respiratory apparatus to an air-bubble.

The general colour of the larva is a mottled brown, darkening where the chitin thickens. The older larvae are to some extent green, possibly due to their food; but this green colour is not by any means confined to the alimentary tract. After moulting, the issuing larva is a uniform light lavender colour, which, however, very soon darkens.

When first hatched the larvae measure somewhere about 0?7 mm. to 0?95 mm., but when ready to pupate they have attained the length of 7 mm. The rate of development is greatly influenced by the temperature, and a few cold days will markedly retard the larval growth. In warm sunny weather, larvae will pupate between the second and third week, but larvae taken in August do not attain their full growth until November. The young larvae undoubtedly die in considerable numbers, and the act of pupating is also attended with certain and varying dangers. Out of 834 larvae and pupae caught in Cambridgeshire, 636 were small larvae, measuring less than 4 mm.; 181 were large larvae, measuring up to 7 mm. But only 17 pupae were taken. There are other facts which show that the larvae under natural conditions succumb in very considerable numbers.

The pupa is something like a tadpole, with its tail bent under its body and flapping up and down, instead of from side to side. The whole pupa is enclosed in a thin semitransparent membrane, through which the organs of the adult can readily be seen. As it grows older its colour darkens. Until about the time when it will give rise to the fly, the pupa floats quietly at the surface, breathing through its respiratory trumpets. When disturbed it shows considerable activity, and it is by no means always easy to capture by means of a pipette. At the least sign of danger it darts below with a series of intermittent strokes and rests at the bottom of the water. Its own buoyancy brings it back to the surface, as, unlike the larva, it is lighter than water. Not only has it a certain amount of air in its tracheae, but there is a reservoir of air at the posterior end of the thorax which acts as a very efficient float. When retreating below the surface the respiratory trumpets usually carry down with them two minute air-bubbles.

The sex of the pupa can be determined by the lobes at the posterior end of the tail: A and B representing the male, and C and D the corresponding parts of the female. The duration of the pupal life is generally three to four days, but conditions of temperature and the state of the natural surroundings exert considerable influence upon the rate of development. Howard has pointed out that the addition of creosote or creosote-oil to the water in which the larvae are living hastens the metamorphosis into pupae, and the pupa stage is passed through in as short a time as fifteen hours instead of the normal forty-eight hours of the warm waters of the Southern States in America. It has also been observed that showery weather hastens the rate of development.

The exit of the fly is naturally a very critical period in its life-history, and in many cases it is fatal. The freeing process takes between five and ten minutes. When undisturbed the emergent fly rests for a time until its wings and limbs are sufficiently hardened to enable it to fly, or at least to walk about. Sometimes the mosquito takes its first flight straight from the pupa-case; at other times it rests awhile before taking to the air. The young imago is pale in colour, the thorax being brown and the abdomen transparent, with a greenish tinge. At first the abdomen is much longer than it is later, for, almost immediately after the mosquito's exit from the pupa-case its abdomen begins to contract, and from its hinder end four or five drops of a glistening, greenish-white fluid are exuded.

The newly born imagines generally take to flight between five and ten minutes after they have emerged, and they at once begin to darken in colour, and in two hours assume the normal dusky colour of the adult. If anything hinders the insect from properly extending its limbs immediately on issuing from the pupa-case, the parts harden and remain distorted throughout life.

Anyone who has spent a day or two in Lille or Bruges, or other towns in Picardy and in Southern Belgium, will understand why, as my Uncle Toby has it, 'Our army swore terribly in Flanders.' The incessant and tireless biting of mosquitos would make any army swear, even though they were ignorant--as my Uncle Toby's army certainly was ignorant--that the gnats, as they called them, conveyed tertian and quartan ague. In Europe the trouble is a summer or early autumn trouble; but our troops are fighting in many tropical and sub-tropical countries, where the mosquitos--like the poor--are always with them.

We have seen that the larva and the pupa hang on to the surface-film of the water by means of certain suspensory hairs, and by the openings of their breathing-apparatus. Anything which prevents the breathing-tubes reaching the air ensures the death of the larva and pupa, and then there is no issuing adult--hence the use of paraffin on the pools or breeding-places. It, or any other oily fluid, spreads as a thin layer over the surface of the pools and puddles and clogs the respiratory-pores and the larvae or pupae die of suffocation.

In Ismailia the disease has been reduced to an amazing extent, and remarkable results have followed the use of these preventive measures at Port Swettenham in the Federated Malay States. Within two months of the opening of the port in 1902, 41 out of 49 of the Government quarters were infected, and 118 out of 196 Government servants were ill. Now, after filling up all pools and cleaning the jungle, no single officer has suffered from malaria since July 1904, and the number of cases amongst the children fell from 34?8 to 0?77 per cent. The only melancholy feature about this wonderful alleviation of suffering, due to the untiring efforts of the district surgeon, Dr. Malcolm Watson, is that his fees for attending malarial cases dropped to zero.

Thus, even ten years ago, a considerable degree of success had attended the efforts of the sanitary authorities--largely at the instigation of Sir Ronald Ross--all over the world, to diminish the mosquito-plague. It is, of course, equally important to try to destroy the parasite in man by means of quinine. This is, however, a matter of great difficulty. In Africa and in the East nearly all native children are infected with malaria, though they suffer little, and gradually acquire a high degree of immunity. Still, they are always a source of infection; and soldiers stationed in malarious districts should always place their dwellings to the windward of the native settlements.

It has always been a source of surprise to me that the great resources and the very evident enthusiasm of the anti-vivisection societies have not been turned in this direction. In the malarial parasite, we have a most potent vivisector of the entrails of one of the most charming and graceful of creatures, whose poetry of movement is hardly approached and never equalled by the ladies of the front row of the ballet. A little help, a very little help, would free these fascinating flies from a form of trouble far worse than that the human alternative host suffers. Yet, as far as I know, these societies and the societies for the prevention of cruelty to animals have declined to help in any way, and have knowingly allowed thousands of millions of animals to perish annually by a most painful death, and have never stretched out a helping hand to the fairy-like and fascinating mosquito.

THE YELLOW-FEVER MOSQUITO

... et nova febrium Terris incubuit cohors.

Like other branches of human activity disease has its romantic and its unromantic side. Nobody can regard mumps or measles as romantic. On the other hand, yellow fever calls up all the romance of slave-trading, pirates and the Spanish Main, buccaneers, maroonings and other grisly horrors, whose sole redeeming feature was a touch of romance. Lovers of pirate stories--and who are not?--will always remember their graphic description of Yellow Jack in the West Indies.

One of the peculiarities of the disease is that it frequently disappears from a given locality for long periods of time. For instance, it was absent in Barbados after the first outbreak until 1690, and when it recurred it was at first not recognised as being the same disease which devastated the islands forty-three years before. In the eighteenth century there was another break of fifty-four years, and similar breaks can be recorded in most of the West Indian islands.

Besides the West Indies, it is at present endemic in Brazil and on the west coast of Africa, and is common in Mexico. Whether the disease arose primarily in Africa and is part of the toll the American continent has had to pay for the slave-trade, or whether it was brought to the west coast of Africa from the other side of the Atlantic, is not certain. It apparently appeared as a regular disease in Brazil in the year 1849, and from that time onwards, with the exception of one year, has been a permanent trouble at Rio. From time to time the disease has been carried to neighbouring parts of America, especially to the Gulf, Central America, and the northern coast of South America. It has been introduced more than once into Monte Video and Buenos Ayres, and has even penetrated up the Parana as far as Asun?ion. Every few years it extends into the Southern States and has even reached Philadelphia and Boston. With the exception of an outbreak in Leghorn in 1804, European epidemics have been confined to Portugal, Spain, and the Balearic Islands.

It will have been noticed that most of these outbreaks occur on the coast and then pass up the rivers. It is thus most probable that the disease is one which is brought mainly by ships. It is obviously a disease which must be guarded against by our troops fighting near the coast in West Africa, as well as such troops as are left in the West Indies. But, above all, it must be guarded against in relation to our shipping fleet and our Navy, operating off the South American coasts. The danger, now the Panama Canal is open, of introducing the disease from America to Asia is a danger that should carefully be considered.

Yellow fever is a disease which requires a winter temperature of at least 68? F., for it is a mosquito-borne disease, and the yellow-fever mosquito flourishes best at about this temperature. It can be introduced into a new locality by the arrival of an infected mosquito, or by the arrival of an infected human being. In the former case the disease breaks out within a few days; in the latter at least ten or twelve days elapse before new cases arise, for, as we shall see later, the organism, whatever it is, that causes the fever is not capable of passing from the mosquito until it has been in its body for ten or twelve days.

They are very apt to bite one in the neck, creeping along the darker parts of the clothing until an unprotected region of the body is reached. Unless one has very thick socks they frequently bite the ankle, and they are as tireless in their pesterings as ever Mrs. Pardiggle was--no sooner are they driven away than they return to the attack. The bite is painful, and in many people raises a considerable swelling.

In the absence of man these mosquitos will suck blood from other animals, and in confinement they are generally fed on rats or canaries, and they will even suck up a drop of blood presented on a piece of cotton-wool.

If the female mosquito has been fertilised before the sucking of blood she will commence egg-laying two or three days later, and two or three days later again the larva will emerge. The larval stage lasts from nine to twelve days, and the pupa stage three to four, so that the whole metamorphosis takes from sixteen to twenty-two days. Hence, during warm weather, many generations succeed each other, but one must have a temperature of at least 20? to 27? C. Below that temperature the processes tend to slow down, and under a temperature near freezing-point the regular development is definitely interrupted. But the interruption is only a suspense, and living activities are resumed should the temperature rise again.

THE BISCUIT-'WEEVIL'

'Let us be merry,' said Mr. Pecksniff. Here he took a captain's biscuit. 'It is a poor heart that never rejoices; your hearts are not poor. No!'--

The larva of this beetle is in truth a book-worm. Its interest for us in the present series is, however, the disastrous infestation of ships' biscuits, which frequently is so severe that the sailors 'hard-tack' is rendered uneatable. Heating, of course, kills it; but the biscuits are still uneatable. The dead larvae are as unpalatable as the living. The contrivance of biscuit-tins since Marryat's time has done much to lessen the evils. Tradition has it that a great firm and a great fortune had their foundations laid, during the first half of the last century, by the accidental contiguity of a baker's shop and that of a tinsmith.

THE FIG-MOTH

All' amico mondagli il fico.

But to return to the figs. So serious was the trouble felt to be in the American fig-market that, in 1910, the authorities at Washington sent Mr. E. G. Smyth of the Bureau of Entomology to investigate the insect in Asia Minor, where the figs come from, and from his report the following account is taken:--

The manner of the fig-harvest is as follows: During August the figs are ripening on the trees, and are gradually dropping off to be collected from the ground and laid on strips of reeds, called 'serghi,' a yard broad; and here for two to five days they dry in the sun. When dried, they are packed in goats'-hair bags or woven willow baskets, and carried by horse or by camel to the fig-depots in the neighbouring villages. Here they are collected from the whole district, mixed together, and re-sacked for transmission by railroad to the coast. At Smyrna they are graded and prepared for the market: the better kind being either 'layered' or 'pulled,' whilst the inferior fruits are strung on strings or exported in the form of a mashed cake to make the 'strawberry' jam of the Western breakfast-table.

Mr. Smyth's object was first to find out at what stage the figs become infected by the moth, and then if possible to suggest preventive or remedial measures. He minutely investigated every stage in the preservation of figs, from the ripe fruit on the tree to the preserved figs in the hold on the steamer bound for New York, and the conclusion he came to is this: With very rare exceptions the eggs are never laid on the fruit whilst on the tree. The first and by far the most important infection is when the figs are gathered and exposed on the reed 'serghi.' Then about seven in the evening the moths begin to appear, and steadily increase in number as the evening wears on. The actual deposition of the ova cannot be observed, for the moths get down amongst the reeds and lay their eggs on the under surface of the fruit--usually in some crack or abrasion--so that the newly hatched larva can more easily make an entrance into the fig. From some 'counts' made at Tchifte Kaive it appears that after an exposure of one night 29 per cent. of figs were infested, after two nights 38?5 per cent., and after three nights 44?5 per cent.

With regard to preventive measures, there seems in many parts of Asia Minor to be two crops of figs--one in May and June and one later. The former produces a large, watery fig, unfit for sale. It is left to rot on the ground, but it serves as food for the larvae which will produce the myriad swarms of moths in the early autumn. Obviously these worthless figs should be destroyed as completely as possible. Equally obvious are the suggestions that the figs should be covered at night with some cheap covering whilst on the 'serghi,' and screened from the moth whilst in the depots, and their sojourn there should be as short as possible. Measures for destroying the larvae in the fig usually take the form of heat--either hot air, hot water, or steam. Each is effective, and each has certain advantages and disadvantages; still, the more progressive merchants of Smyrna were, before the War, experimenting trying to find the best means of destroying the larvae, and in time a uniform system will probably emerge.

THE STABLE-FLY

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