Tropical Diseases/Chapter 9
CHAPTER IX
MALARIA: THE MOSQUITO
THE part played by the female mosquito, or gnat, in the life-history of the germ of malaria, as well as in that of yellow fever, of filariasis, of dengue, and perhaps other diseases of warm climates, renders some knowledge of the natural history of this insect a necessity for the tropical pathologist.
Although the Culicidæ or mosquitoes had received, now and again, some attention from naturalists ever since the time of Linnæus, it is but recently that the extent, complexity, and importance of this group of insects have been apprehended. Considerations of space preclude me from giving more than the merest outline of the subject; those who desire to follow it up would do well to provide themselves with one, or more, of five recent works namely, "The Mosquitoes of North and Central America and the West Indies," by Howard, Dyar, and Knab; Alcock's " Entomology for Medical Officers," Theobald's " Monograph of the Culicidæ," Blanchard's " Les Moustiques,"—— and Giles's " Handbook of the Gnats or Mosquitoes" in which will be found details of all that was known about these insects up to the date of their publication.
The Culicidæ, of which there are at least 500 known and, probably, over 200 additional and as yet undescribed species, belong to the order of Diptera, or two- winged insects, in which the posterior of the two pairs of wings of the typical insect are rudimentary, being represented by the halteres or balancers (Fig. 23). Like most other members of the order, they lay eggs, from which are hatched worm-like larvœ; these, after growth and several changes of skin, cease to eat, and become converted into nymphs 
Fig. 23.—Female mosquito.
The geographical range of the mosquito extends from the frigid zones to the equator. Given stagnant or slow-flowing water and a summer temperature, there this family of insects will be represented by one or many species. The distribution of particular species and the abundance of mosquitoes in any given place are determined, in addition to temperature and hydrographic conditions, by complicated circumstances not yet wholly understood.
The adult insect feeds on vegetable juices; the males, with few exceptions, exclusively so. In
Fig. 24.—Various forms of mosquito eggs.
addition to a vegetable diet, the females of most species, when opportunity offers, suck the blood of mammals, birds, occasionally of fishes, reptiles, and even that of other insects and their larvæ. The male mosquito, not being a blood-sucker, takes no part in the diffusion of disease; it is the female only that is a germ carrier.
Soon after impregnation the female lays her eggs (Figs. 24, 25, 26) from time to time—singly, in groups, or in boat-shaped masses, according to species—either on the surface of still water, on which they float, or in propinquity to water. The process of hatching out depends in great measure, as indeed do all the developmental processes connected with the mosquito, on temperature, being retarded or even suspended by cold and accelerated by warmth. In some species the eggs remain dormant throughout the winter or through a long term of dry weather, but in ordinary circumstances the larvæ hatch out in from two to three days, and at once proceed to feed voraciously on the organic materials suspended in the water. Being air-breathers, a great part of their time is passed at the surface of the water, where they lie in such a position which varies with species that the respiratory opening placed near the tail can function readily (Figs. 27, 28). After the several moultings the larva, now very much increased in size, passes to the nymph or pupa phase (Fig. 29), during which the little animal ceases to
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| Fig. 25.—Eggs of Anopheles maculipennis. a, Under side; b, upper side. |
Fig. 26.—Culex pipiens— egg-boat. (After Sambon.) |
feed, and for the most part floats just awash at the surface of the water. In from one to two days the pupa-case bursts and the insect, emerging, stands on the empty case till its wings have dried, when it flies away. From first to last, from egg to imago, the process of development takes about a month in the temperature of the Italian summer; but a much shorter time (seven to ten days) may suffice in a tropical climate. As each female mosquito may lay eggs many times in a season, and many hundreds of eggs each time, and as the young female can produce eggs within a week or ten days after her emergence from the pupa-case, it follows that one pair of insects can give rise to a large progeny in the course of a summer.
During cold weather the development of the larva is temporarily suspended, and the surviving adults, at all events the females, hibernate in dark and sheltered places, to become active again on the return of warm weather. In this way the species is carried over the cold weather of winter, though there are
Fig. 27.—Anopheles maculipennis, Meig, larva, showing breathing position at surface of water. (After Howard, Bull. United States Dept. Agr.)
some species that hibernate in the larval stage and others in the egg. The duration of the life of the adult insect has not been definitely ascertained; it
Fig. 28.—Culex fatigans larva in breathing position at surface of water. (After Howard, Bull. United States Dept. Agr.)
is known that some species, if supplied with water and suitable food, can live for several months.
It is suggested that the mosquito tends to return to the particular pool in which she herself was hatched out to deposit her own eggs, and that she rarely strays from the vicinity more than a few yards, quite exceptionally beyond half a mile. Occasionally she may be blown for some distance by gentle winds; and it is believed that in certain circumstances, probably connected with food supply and over-stocking, she will travel singly or in vast swarms for long distances. Such migrations, however, are quite unusual. Of course, mosquitoes may be, and often are, transported great distances in ships, railway carriages, and similar vehicles, and in this way man aids in their diffusion; but for the most part the mosquito is a feeble and timid flyer, disliking to
Fig. 29.
Pupa of Anopheles maculipennis. Pupa of Culex fatigans.
leave her accustomed haunt, and seldom rising high above the ground. So soon as even a moderate breeze springs up she seeks shelter in bush, or house, or cranny. Some species are domestic; others live exclusively in jungle or forest; some, after passing the day in the open, visit human habitations, or the haunts of birds and beasts, during the night. The great majority of species are nocturnal in habit, although many of these can be coaxed into activity by the reproduction of night-like conditions of shade and atmospheric stillness.
The accompanying figures (Figs. 23-39) give a good idea of the leading features of the anatomy of the mosquito and of the names applied to the various parts and organs, and will help the student to understand descriptions of genera and species.
Fig. 30.—Heads of Culicini.
a, Male; b, female.
The antennæ of the male insect (Figs. 30, a, and 31, c) are usually adorned with a profusion of long, silky hairs, in marked contrast to the scanty, down,
Fig. 31.—Heads of Anophelini.
c. Male; d, female.
The proboscis consists of a number of piercing elements enclosed in a sheath—the labium, which, at its free end, is tipped with two minute labella. In feeding, the mosquito raises her hind legs and presses the tip of the proboscis against the skin. This causes the labella (Fig. 32, h) to splay out and so serve as a support to the piercing elements namely, the labrum, hypopharynx, mandibles, and maxillæ (Figs. 32, 33)—which are now thrust into the skin. The
Fig. 32.—Mouth parts of female mosquito.
a, a, Antennæ; b, b, palpi; c, labrum-epipharynx; d, d, mandibles; e, hypopharynx; f, f, maxillæ; g, labium; h, labella.
labium does not penetrate; as the stabbing elements sink into the skin it bends backwards about its middle, the labella still pressing against the skin and clasping the stilettes. The secretion of the salivary glands (Figs. 11, 14) passes along the salivary duct and thence down a minute canal which traverses the hypopharynx to its tip, and so into the subcutaneous tissues of the bitten animal. It is supposed that the function of this secretion is, by irritating, to determine a flow of blood to the part bitten, and also to prevent coagulation of the blood. To many people this secretion is a powerful irritant, although repeated inoculation tends to produce tolerance, as in the case of many other organic poisons.
A buccal tube is formed by the apposition of the upper surface of the hypopharynx to the under surface of the labrum (Fig. 33). Along the tube so formed the blood is aspirated by the expansion of the gizzard-like organ (Fig. 11, b), and then driven by the contraction of the same into the stomach (Fig. 11), or middle intestine, as it is called. A mosquito will fill herself in a minute or thereabouts. She then withdraws her proboscis and flies heavily away to some sheltered spot to digest the meal. Apparently the first step in digestion is the concentration of the blood she has imbibed; this is effected by excretion
Fig. 33.—Section of mosquito. (Adapted from Nuttatt and Shipley.)
c, Labrum-epipharynx; d, mandible; e, hypopharynx; f, maxillæ; g, labium; i, sailvary duct; j, muscles; k, trachea.
of the watery portion of the the liquor sanguinis. Often while this process of dehydration is proceeding, even while she is sucking, droplets of clear fluid may be seen ejected at her anus. The concentrated blood becomes in this way a viscid tarry mass, which is gradually, in the course of three or four days, partly absorbed, and partly voided as gamboge-coloured fæces. The mosquito is now ready for another meal.
The rich pabulum supplied by blood seems to favour ovulation.
Diagnosis.—Many kinds of insects possess blood-sucking propensities. As a rule there is little difficulty in distinguishing most of these from the mosquito. There are certain Diptera, however, which closely resemble the latter in appearance as well as in habit. These the student should learn to distinguish.
The principal of the mosquito-like blood-suckers are the Midges (Cheironomidæ) and the Sandflies (Phlebotomus). The following are the diagnostic points:—
Mosquitoes have a long suctorial proboscis, and the veins of their wings are fringed with scales.
Midges are very slender and minute, have a short suctorial apparatus, and wings without scales.
Sandflies are small, slender, and very shaggy, have a comparatively short suctorial apparatus, comparatively long legs; narrow, pointed, hairy wings, and long hairy antennæ.
Fig. 34.—Wing of Culex concolor (male), to illustrate terminology.
The medical man should be able not only to recognize mosquitoes but also to determine genera and species. Obviously, it would be impossible to give here minute descriptions of the large number of species that have been described; I must, therefore, refer the reader to the monographs on the subject already mentioned. On the next page is given a synoptical table which will enable the student to identify the main groups or sub-families to which any given mosquito belongs. It has been prepared after a careful consideration of the various classifications proposed. Most authorities separate the Culicidæ into two sub-families, namely (1)
| Proboscis | Straight. |
Metanotum | Nude. |
Scutellum | Trilobate: abdomen completely invested with close overlapping scales | Culicint. | |||||
| Simple, not trilobate: abdomen never completely and uniformly covered with scales | Anophelini. | ||||||||||
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With a tuft of bristles: a pair of very large bristles projecting horizontally forwards from crown of head |
Sabethini. | |||||||||
Tapering and recurved. |
Flat scales on all parts of body: large, iridescent mosquitoes | Megarhini. | |||||||||
The Megarhini are jungle mosquitoes. From the structure of their proboscis it is very improbable that they can bite. Their larvæ, which occur chiefly in the water that collects in holes in trees, bamboo-stumps, etc., are predacious, and undoubtedly destroy the larvæ of other mosquitoes having the same breeding habits. The Sabethini are also jungle mosquitoes breeding almost exclusively in the water that collects in the axils and bracts of leaves and in "pitchers" and other such modified parts of plants.
Most workers at the group now regard the Anophelini as naturally definable in a single genus, Anopheles, which may be characterized as follows:——
GENUS ANOPHELES (Meizen)
Head only moderately broad, usually covered with upstanding forked scales, though scales of other kinds are also present in a few species. Maxillary palps long and spathulate in the male; as long as, or not much shorter than, the proboscis in the female. Free edge of scutellum simply convex, not trilobate; metanotum bare. Abdomen either sparsely hairy or with localized patches of scales; sometimes with a considerable expanse of scales which, however, never form a uniform, complete, and compact investment. Wings commonly dappled or profusely speckled, but occasionally quite spotless. Legs remarkably elongate. In repose the body is usually inclined at an angle with the resting surface.
The eggs are boat-shaped, and with rare exceptions have their investing membrane inflated laterally to form a pair of floats.
The larva has the head at least as long as broad, and has four bristles or feathered hairs projecting from the free edge of the clypeus. The long lateral hairs of the thorax and abdomen are strongly feathered. Some or all of the abdominal segments as far as the 7th carry dorso-laterally a pair of characteristic rosettes or cockades of scales. The breathing openings are situated in a hollow on the dorsum of the 8th segment, the hollow being bounded laterally and posteriorly by a system of folds or valves.
The larvæ occur not only in all kinds of stagnant water, but also in pools in the beds of rivers and mountain streams, or even in the current if there is also floating weed and debris for their protection, in domestic water-vessels, and occasionally in the water that collects in holes in trees; some species thrive in
Fig. 35.—Graphic key to generic distinctions based on scale characters.
Fig. 36.—Graphic key to generic distinctions based on scale characters.
Fig. 37.—Continuation of Fig. 36.
Megarhinus, head, scutellum, and lateral view of head scales; e, Culex, head, scutellum, and lateral view of head scales.
brackish or salt water, and each species has its own particular kind of habitat.
For those who still think it desirable to sub-divide the genus Anopheles, the following synopsis of subgenera may be serviceable:—
| Front edge of wing either not spotted or having not more than two distinct spots. |
Abdomen without scales; palpi slender; wings not or little spotted; wing scales narrow | Anopheles. | |||||
| Abdomen with some localized scales, if only a tnft on the 7th abdominal sternum; palpi shaggy; wings always dappled; wing-scales broad |
Myzorhynchus. | ||||||
| Front edge of wing broken into numerous alternate light and dark spots or streaks. |
Mesothorax invested with hairs | Myzomyia. | |||||
| Mesothorax invested with narrow upstanding scales | Pyretophorus. | ||||||
| Mesothorax invested with broad, appressed, white or yellow scales |
No outstanding tufts of scales on abdomen | Nyssorhynchus. | |||||
| Tufts of scales projecting from abdomen laterally or ventrally | "Cellia." | ||||||
Fig. 38.—Anopheles maculipennis (female).
Fig. 39.—Anopheles funestus (female).
Species that are known to foster the Malaria Parasites
Note.—In the case of several species it is known that they are not always susceptible to experimental infection throughout their range, and that at a given time and place all individuals are not susceptible.
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| Fig. 40.—Resting position of Culex pipiens. |
Fig. 41.—Resting position of Anopheles sinensis. |
Fig. 42.—Resting position of Anopheles maculipennis. |
Europe.—A. maculipennis; A. sinensis (=pseudopictus, Grassi); A. superpictus, Grassi; A. turkhudi, Liston (=hispaniola, Theob.).
Africa.—A. maculipennis, A. umbrosus (=strachani), A. mauritianus, A. culicifacies (=Pyrelophorus sergenti, Theob.), A. funestus, A. turkhudi, A. costalis, A. maculipalpis, A. pharoensis.
Asia.—A. umbrosus, A. barbirostris, A. sinensis (=jesoensis) A. culicifacies, A. funestus var. listoni (=formosaensis and cohaesa), A. minimus, A. turkhudi, A. ludlowi, [1]A. rossi (?), A. leucosphyrus (?), A. stephensi, A. fuliginosus, A. maculatus, A. maculipalpis, A. theobaldi, A. willmori.
North America.—A. crucians, A. punctipennis, A. quadrimaculatus.
South America.—A. punctipennis, A. pseudopunctipennis, A. pseudomaculipes, A. intermedius, A. cruzi, A. tarsimaculatus, A. albimanus, A. arygrotarsis.
Australia.—A. bancrofti (?), A. annulipes.
Whether other Anophelinse are efficient has not been determined. So far no other Culicidæ have been found to foster the malaria parasites of man, and experiments with Culex albopunctatus, C. penicillaris, C. vexans, C. pulchritarsis, C. pipiens, Tœniorhynchus richiardii, Theobaldinella annulata, and T. nemorosa have proved negative. Considering that Culex pipiens transmits several Hæmoprotozoa of birds, closely allied to those of men, it would be rash to assert positively that the Anophelinæ are the only mosquitoes which transmit the malaria parasite.
- ↑ This species has recently been shown by Christophers to be the transmitter of a peculiarly localized malaria in the Andaman Islands. It breeds only in brackish water, and is not found farther away from such water than half a mile.