Climate and Health in Hot Countries and the Outlines of Tropical Climatology / A Popular Treatise on Personal Hygiene in the Hotter Parts of the World, and on the Climates That Will Be Met Within Them.

by George Michael James Giles

(5) If, however, a human being infected with malaria be bitten by a mosquito of the appropriate sort, the parasites, sucked into the stomach of the mosquito along with its meal of blood, undergo further development into distinct male and female animalcules, whose union gives birth to myriads of germs which, although incapable of further multiplication within the organism of the insect, find their way into its salivary glands, which are the organs in which the irritating poison of the mosquito is elaborated, and so are necessarily inoculated into the tissues of any human being whom the mosquito may bite, with the result that a new victim becomes infected, and the chain of events commences anew.

(6) The malarial parasites can exist in the mosquito only within certain limits of temperature, and hence the disease is not found in countries where the maximum summer temperature is less than 76° F., or in places so hot that the temperature rises above 86° F. for any length of time, and the comparative healthiness of many hot continental climates is due to the fortunate circumstance that a period of excessive dry heat follows immediately on the cold season.

In considering the measures of prevention detailed below, it must be remembered that their importance is enhanced by the fact that not only malaria, but also the blood-worm disease already alluded to, or filariasis, and yellow fever, have also been shown to require the intervention of mosquitoes for their transmission from man to man, and that although not carried by mosquitoes, the germs of sleeping sickness are also probably conveyed by a winged insect, so that many of the measures detailed below will have a certain value also in the prevention of the last-mentioned disease.

Thus for the maintenance of malarial fever the co-existence of three animal organisms are essential, viz., of man, the mosquito, and the malarial protozoon; and it is obvious that even the temporary banishment of either of the three from any given locality will necessarily put an end to all possibility of the occurrence of fever; for man can be infected only by the mosquito, and the mosquito by man, and the presence of both of the others is necessary for the maintenance of the species for the parasite.

For the proper contrivance, then, of measures of prevention it is essential that we should be well acquainted with the life-history of our two partners in this curious cycle of development, and as that of the parasite has already been sufficiently described for our purposes, it remains only to describe the leading parts of the life-history of the mosquito. Mosquitoes, or, as they are called in England, gnats, are small two-winged insects whose appearance is quite familiar to most people, though midges so closely resembles them, in general appearance, that they are commonly confused with them, but mosquitoes may be easily distinguished by their possessing a long, trunk-like proboscis which is wanting in the midge, as well as by the fact that, if examined with a strong magnifier, it will be seen that their wings and the greater part of their surface are covered with minute, downy scales exactly like those of butterflies, while midges are quite devoid of any such covering. The males carry a pair of beautiful plume-like feelers, while those of the females, though quite as long, show only a few inconspicuous hairs.

Over five hundred species of mosquito have been described; and, as might be expected, their habits vary to some extent; but, speaking generally, they are mainly twilight and nocturnal insects which remain hidden in sheltered places during the day, and come forth to feed and disport themselves in the open at dusk. Owing to the peculiar structure of their mouth-parts, which consist of a long, delicate tube, supported in the midst of a group of lancets, they are incapable of taking solid food, and subsist almost entirely on the juices of living plants and animals, which are sucked up by means of the tube which is introduced into a puncture made by the lancets. With one or two doubtful exceptions, the males of all species live entirely upon the juices of plants, but the females of perhaps the majority of species are not content with so simple a diet, and attack animals of all sorts, puncturing the skin and filling themselves with blood till they are scarce able to fly. No animal is safe from their attacks, and improbable though it may appear, it is recorded, on the authority of a skilled naturalist, that young fish are killed in large numbers by the crowds of mosquitoes which pounce on them as they show their heads and backs at the surface of the water.

Fig. 6.—Various forms of Mosquito Eggs. (1) Egg-boat of Culex, seen from above; (2) the same, side view (after Sambon); (3) separate Culex eggs; (4) eggs of Panoplites (after Daniels); (5) eggs of Stegomyia; (6) the same more highly magnified (after Theobald); (7) groups of Anopheles eggs as they float on the water (after Sambon); (8) egg of Anopheles maculipennis, showing lateral floats, seen from above, × 30 diams.; (9) the same, viewed laterally (after Nuttall).

With the exception of the fleas and ticks, no insects are so admirably adapted for the conveyance of disease from one animal to another by inoculation; and owing to their enormous numbers, and their capability of flight, their powers of mischief must be far greater than those of either of these other pests. When biting an animal, the mosquito injects into its tissues a fluid specially secreted within its body, which being of an irritating character causes a congestion of blood round the puncture, whereby the insect is secured a full and ample feed of blood. The early life of the mosquito is passed in water, on the surface of which the eggs are laid by the female flying insects. These eggs are of various forms, but are all provided with some arrangement to secure their floating on the surface of the water. In many species they are glued together by their sides so as to form rafts consisting of some hundreds of eggs, while in others, and notably in the special sorts that carry malaria, they are laid separately and float on their sides. In a few rare cases they may be laid on dry surfaces, in situations that will be flooded during the rains. This is notably the case occasionally with the form that carries yellow fever; but for practical purposes it may be considered that the eggs can only be laid on water, and in any case, when the young insects are hatched out, it must needs be into water, as at that stage of their existence they are purely aquatic organisms, and live but a short time if removed from the water. In spite of this they are air-breathers, and in the anatomy of their arrangements for obtaining air, present a curious resemblance to the type of submarine boat that gets its air by means of a small tube reaching just above the surface of the water. The young mosquitoes, or larvæ, are small, wormlike animals, which may often be seen wriggling about vigorously in water, and are especially common, in warm weather, in water standing in small stagnant pools, broken crockery, old tins, and so forth.

When full grown they are about a quarter of an inch in length, and vary in colour from bright green, through brown to black. Two principal forms may easily be distinguished by the difference between the structure of their breathing organs. The first and commonest kind, of which the common English gnat (Culex pipiens, L.) is a good example, has a long breathing tube projecting from the back close to the tail, so that it looks much as if the hinder part of the body were forked.