More Minor Horrors

Part V

In the young larva of Anopheles the head is broader and deeper than the thorax, but in the older larvae the segments that succeed the head have at least twice its diameter. It is a characteristic of true flies, or Diptera, that the thorax should not exhibit that separation into three divisions which is so usual in the less specialised insects—such as the cockroach and this is peculiarly true of the larva of the mosquito—at any rate, so far as its external structure goes. The abdomen of the larva consists of nine free segments; the third, fourth, fifth, sixth, and seventh of these bear palmate hairs on the dorsal or upper surface, something like hands with fourteen ‘fingers’ spread out. These hairs adhere to the under layer of the surface-film of the water, and help to maintain the animal in a horizontal position just below that film. When the larva relaxes its hold and sinks into the water, it not infrequently carries with it air-bubbles enclosed by these fourteen ‘fingers.’

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.’

We have referred above to the respiratory openings, and, indeed, these are the key to the whole situation. Close these openings— as they can be closed by floating petrol or other oil on the surface of the water—and ‘the trick is done.’ The larvae and the pupae can no longer breathe, and there is thus no imago to “carry on.” In Culex (the gnat), these respiratory orifices are borne on a long tube directing dorsalwards—a tube which is larger and longer than a segment of the body, and whose presence gives the larva the appearance of a Y with slightly unequal limbs. These breathing-openings are of the greatest complexity, but the outstanding fact is that these stigmata pierce through the watery film and put the respiratory system of the larva into communication with the atmosphere of the whole cosmos. If anything frightens the larva, certain side-pieces and flaps fold suddenly backwards and over the stigmata, the connexion through the surface-film is broken, and the little larva, like a German submarine when it sights an English battleship, darts below, frequently carrying with it the drop of air attached to the rim of the respiratory recess which surrounds the openings of the two stigmata.

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 débris.

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.

A strong wind passing over a pool where Anopheles eggs, larvae, or pupae are floating, will gradually pile them all up on the side towards which it is blowing. The Anopheles larvae undoubtedly are braver than those of the Culex—that is to say, a disturbance which will send all the Culex larvae scurrying to the bottom will leave the Anopheles larvae unmoved.

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 (if the autumn be cold) 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.

When the larva is about to turn into a pupa it comes to rest, and now the thoracic regions are more swollen than ever. Soon a dorsal slit appears along the larval cuticle and the pupa slowly, but gradually, emerges through this slit and leaves the larval chitinous cuticle behind it. On first emerging, the pupa measures about 6·5 mm., the head and thorax making up one-third of this. During the last larval stage many of the pupal organs have been re-forming and are more or less visible through the cuticle. The mouth parts and limbs of the third stage—the future imago—show no relation to those of the larva. They are there enclosed in their respective sheaths, but these are quite independent of the larval ‘appendages.’ The respiratory trumpets, which, as in the larva, pierce the surface-film, are ready to act as breathing-organs. Whereas the larvae breathe through two stigmata at the posterior end of the abdomen, the pupae breathe through two respiratory trumpets issuing from the anterior dorsal surface, and it is these trumpets, together with certain palmate hairs, which support the pupae in the right position and put the respiratory organs at this stage into communication with the outer atmosphere. During the pupa stage Anopheles, like the pupa of other insects, takes no food.

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 (Fig. 24) 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.

When the adult mosquito is about to emerge, a certain amount of air is secreted under the chitinous casing of the pupa. A fine streak containing air appears along the back, extending between the respiratory trumpets and the base of the head. This central streak gradually passes backwards until it reaches the seventh abdominal segment, and then suddenly the pupa extends its abdomen so that it floats parallel to the surface of the water instead of being under the rest of the pupa’s body. The chitinous integument now splits along the median dorsal line, and through the slit thus made the thorax of the adult mosquito now protrudes. By gradually pressing its abdomen against the pupa-case, the body of the perfect insect is slowly but gradually raised above the surface of the water. The head is pulled backwards and upwards, and millimetre by millimetre the mouth parts, the palps, and antennae are withdrawn, and at first remain bent backwards beneath the body of the insect. Gradually the bases of the wings and the abdomen emerge, and soon the wings are freed and immediately flatten out and begin to harden. The legs and the tip of the abdomen alone now remain to be dealt with. At this stage the insect projects far beyond the anterior end of the pupa encasement, and somewhat resembles an exaggerated figurehead on a ship. The pupa-case is still filled with air, and acts as a float to support the emerging insect. At last the front legs are being freed, the second and third pair of legs soon follow, and now the insect is standing on the surface of the water raised on its tarsal joints, the tip of the abdomen being the last part to free itself from the pupa-case.

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.

That the plague can now be checked is shown by the making of the Panama Canal; and that this check is due to British science is shown by the work on the life-history of the malarial organism, first investigated by Sir Ronald Ross, and later, as regards the human parasites, by certain Italian savants. It is also due to the public health services of one or two British medical officers of health in the East. Their methods have been applied and improved by those responsible for the elusive channel which now at times separates North from South America.

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.

Knowing the cause, we can now guard against malaria; mosquito-nets and wire-protected windows and doors are a sufficient check on the access of Anopheles to man. If the mosquito and man could only be kept permanently apart, we might hope for the disappearance of the parasite from our fauna. In relieving man from this world-wide pest, all genuine lovers of animals will rejoice that we are also relieving the far more serious lesions of one of the most delicate and beautiful insects that we know.

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.