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

On the back of each of the rings of the abdomen, or hindmost division of the body, of the Anopheles larvæ may be made out a pair of structures formed like minute palm leaves, the function of which appears to be that of keeping the little creature flush with the surface of the water.

One of these larvæ, magnified about twenty times, with these structures and certain peculiar hairs on the front of the head, which are of value in distinguishing one species from another, more highly magnified, is [figured] on the preceding page. By the aid of these illustrations the reader should have no difficulty in recognising these organisms when he meets with them. When disturbed, they dart backwards and seek refuge among the débris at the bottom, but cannot remain there long, and soon resume their resting position on the surface.

The larvæ of many species are said to be carnivorous, and even cannibalistic, though I cannot say I have ever observed this personally, and in any case their main provision is found in the minute vegetable organisms which are abundant in the sites where they are common; the Culicine larvæ confining themselves mainly to those that are found completely immersed, while the young Anopheles browses on those floating on the surface, keeping its head screwed round, a full half-turn, so as to bring the mouth uppermost. The abrupt way in which this action is performed and reversed is extremely quaint, so that, as a writer remarks, one is rather surprised at its not being accompanied by an audible click.

The duration of the period of larval existence depends on the temperature of the water and on the abundance or scarcity of food. When first hatched out from the egg, the larvæ are barely visible to the naked eye, whereas when full grown they contain within themselves the entire material of the adult flying insect. Once full-grown, the weather being favourable, they change into “nymphs” or pupæ, not unlike small tadpoles in form, the head and thorax being enclosed in an almost spherical envelope, to which is appended a sort of tail, formed by the abdomen, which is usually kept folded under the body, as shown in the [figure] below. In this stage the animal breathes through a pair of horns springing from the back of the thorax. The mouth is completely closed, so that the pupa is incapable of feeding, but is in no sense quiescent, as it is capable of lively movements and tries to elude capture almost as briskly as the larva. After a sufficient time has elapsed for the completion of the extraordinary anatomical changes that convert the larva into the adult insect, the pupa-case bursts along the back, and the perfect gnat gradually disentangles itself from its temporary home, and flies away, very shortly to reinitiate the cycle of events by laying a fresh batch of eggs. As has already been remarked, the time required to complete these changes varies greatly in different species and under varying environments, of which climate is the most important factor, but under moderately favourable conditions the time required is a fortnight or three weeks, the greater part of which is passed as a larva. In countries where water freezes during winter for any length of time, all larvæ that have not completed their metamorphosis by the end of autumn, must necessarily perish; and the continuity of the species is maintained entirely by the survival of pregnant females that hide themselves in warm corners and pass the winter in a truly hybernating or dormant condition. Further south, both males and females hybernate, and the dormancy is so much less pronounced that the insects are often tempted to issue from their hiding places on exceptionally warm days. In such climates, which include Italy and most of the sub-tropical zone, the species finds an additional string for its bow in the survival of larvæ, which, though they cannot be said to hybernate in the strict sense of the term, being always lively and alert in their movements, are yet incapable of growing, and appear to remain at whatever size they may have previously attained until the return of warmer weather. Quite recently Dr. Bancroft, of Queensland, has discovered that in some species these wintering larvæ can give birth to small broods of young larvæ, so that in such cases the bow has no less than three strings. Further south again, in truly tropical climates, breeding goes on all the year round.

Fig. 9.—Larva and Nymph of Anopheles, to show habitual resting position. The head of the larva is shown looking up, as when feeding.

No adult mosquito can stand the direct rays of the sun, and hence all of them have to seek the shelter of houses, trees, or some such protection, during the heat of the day; moreover, with very few exceptions, they are nocturnal or twilight insects, so that there is comparatively little danger of being bitten during the hours of daylight. For the majority of species too, extreme atmospheric heat has much the same effect as great cold, so that during periods of intensely hot, dry weather the number of species to be met with in a state of activity is very small, and fortunately those that convey malaria are not amongst the number; but on the other hand, the one or two sorts that possess this tolerance are so enormously prolific that their numbers far exceed those of all other species combined.

Another point of importance is that, for practical purposes, mosquitoes cannot fly far, and hence never wander far from the puddle on which they were born. Being tolerably long-lived insects, it is of course possible for them to slowly spread amongst trees or bushes for considerable distances, a few straying further and further away every night from the place of their birth, but the number that can travel in this way for any considerable distance, is so small as to be scarcely worth consideration; and any considerable expanse of bare, open country is practically impassable to them.

Mosquitoes are most active during the periods of twilight, and especially in the evening, at which time they quit the shelter in which they have dozed away the day, and come out into the open to seek their food, remaining outside for the most part throughout the night, though the females of those species that attack man and animals, it is needless to say, will find their way back into houses in search of their favourite food. As soon, however, as the sun has got well above the horizon they may be observed trooping back into the house, and if a window be closed it is very amusing to watch the numbers that will alight on the glass at this time of the day, and their efforts to find a way through the obstruction. The special importance of keeping all means of entry closed at this particular hour can therefore easily be understood, though in ordinary tropical practice, it is the very time at which every door and window is habitually thrown open.

Mosquitoes are found all over the world, it being a great mistake to imagine that they are confined to tropical climates. Some score of species are to be found in England, and though in high latitudes they cease to be dangerous as carriers of disease, there are perhaps no localities where they are so numerous and troublesome as certain parts of the North American Continent, and in Scandinavia, close up to the Frigid Zone.