LIFE HISTORY AND SEASONAL PREVALENCE 137 of October, though neither they nor the two persistent species of Culex, above noted, exhibit any tendency to bite, so that there is no longer any need for the use of Mosquito nets, whether on the score of comfort or safety, for in India, just as is the case in England, we find that the question as to whether or not Mosquitoes will attack human beings is entirely one of air temperature. Celli says that the adult Anopheles will bite as soon as it is out of the pupa case, and after they have digested their meal, bite every two days or so, and ovulate several times. As far as I can judge by the rather desultory experiences of correspondents and myself, I fancy this conveys rather too high an idea of the gastronomic and reproductive capabilities of individual insects : that they generally take some four days to renew their appetite after a full feed, and that the laying of a couple of batches of eggs is about the limit of the reproductive capabilities of most of them ; but doubtless they vary in this, and their intolerance of captivity makes such matters difficult of determination. However this may be, there can be no doubt of the enormous capabilities of multiplication possessed by the entire family, and this strongly tends to neutralise any check that can be imposed upon it by natural enemies.
Like the larvse, the adult insects may be roughly classified as domestic, field, and jungle species ; and they generally agree in these respects with the corresponding larvae, though Celli emphatically states than An. maculipennis is domestic, while An. bifurcatus is a field insect ; whereas Ficalbi regards the habits of their larvae as the reverse. It is impossible, however, to make distinctions of a rigid character in such matters, and the looser our conceptions of their signification, the better, though they are very useful for general purposes if employed with such reservations. Although some few species are active throughout the day, e.g., An. bifurcatus, in Italy, taken as a family, gnats are essentially nocturnal, and especially crepuscular insects. Speaking generally, Mosquitoes start the day's work just after sundown, and are very busy among the plants and animal victims that furnish their food, till it falls dark,
138 GNATS OR MOSQUITOES — CHAPTER VII by which time, though many continue in evidence throughout the night, most of them have satisfied their needs, so that there is a sort of lull in their activity till dawn, when those that have failed to secure a breakfast reappear, in vicious earnest to secure a supper. Once, however, is the tropical sun above the horizon, and they may be seen taking immediate measures to escape from the power of his rays.
The window-door of my last little Indian work-room was closed by means of Mosquito netting stretched over the frame, so that it was possible to use the microscope with it open, without being tormented with flies ; and I was often much diverted by watching the persistent efforts, not only of Anopheles, but of all the other domestic species about, to get out to feed in the evening, and to return to shelter in the morning. Shelter, as we have seen, they must have during daylight, and speaking generally, an observer who confined his observations to those hours might report that no such insects existed in the tropics, so sluggish are they during the heat of the day and so cunningly do they hide themselves in dark corners. During the rains, however, the Stegomyiie, which are common at that season, may be seen flying about such places as orchards, and will occasionally even venture upon short flights across patches of sunshine. Within the bungalow, too, they will often bite viciously, even in the middle of the day ; but as they are rather field than domestic in their habits, they give less trouble in this way than would otherwise be the case.
When settled on some surface, the position of these insects is very characteristic. As a rule, they support themselves entirely on the four anterior legs, the hind pair not being brought into requisition, but kept held up well above them, so as to serve as extra feelers. During the active hours of their existence they are kept slowly waving about in the air, as if feeling for something ; but when quiescent, though the hind legs are still rarely placed to ground, they are kept quite still, so much so that I found little difficulty in photographing the living insects, placed in the glass box described in the first chapter, with even an entire minute's
PLATE VI. .A> ^ ^m^. Photograiihs of living Anoi>helcs and Ciilc.v. Face ]).
LIFE HISTORY AND SEASONAL PREVALENCE 139 exposure. The Cullces sit humped up, with the abdomen drooping, or at most horizontal, and the proboscis held out in front of them, so that even when they are looking at the surface on which they are standing, it is at least kept off the ground. The palpi are kept arched backwards and upwards, and usually slightly separated from the proboscis, and the antennae usually sloped well behind its line. In Anopheles, on the other hand, there is a strong tendency to keep all the appendages of the head in the same plane, and in both sexes, the four basal joints of the palpi are kept closely held against the proboscis. In the female the whole organ is so held, but the clubbed terminal joints of those of the males are kept spread out at an angle of about 45 degrees. On this account the trunk of this genus of gnat looks almost as thick as the rest of the body. In addition to this, instead of bunching themselves up as the Culices do, they keep the whole body and cephalic appendages nearly in one straight line, obliquely, or it may be, almost vertically to the surface on which they are resting, with the abdomen held up, and the proboscis pointing at the surface on which they rest, as if they contemplated boring themselves into it. This attitude is so characteristic that it affords a ready means of recognising the genus, when found settled. In the case of A71. sinensis, An. Bossii, An. Jamesii, An. argyrotarsis, An. albipes, and also, to judge even from Sambon's own figures, in those of An. superpictus and An. maculipennis, there can be no doubt that the habitual resting attitude is such as has been described ; but I do not for a moment suppose that it can be taken as an absolute rule for all species, either of Anopheles or Culex ; and just before I left India I met with a new species of Anopheles which entirely deviated from the common rule in this respect, as every one of some dozen living specimens of both sexes was found resting in a position exactly such as is regarded as characteristic of Culex. The resemblance is further aided by the fact that in the fresh state the abdomen is conspicuously banded in a manner most exceptional in the genus, so that when I arrived in the rest-house in which I found them, late one night, I was completely taken in by the counterfeit, mistaking them for
140 GNATS OR MOSQUITOES — CHAPTER VII C. imj^ellens, to which they present a close resemblance. On this account I have named the species A71. culicifacies sp. n. On the other hand, I suspect that C. mimeticiis, Noe simulates Anopheles, not only in costume, but in its habitual position when at rest, though I have seen too few living specimens to be sure. It must be remembered that these remarks refer to habitual attitudes only, and that there is nothing absolute in the application of any natural history rule of the kind, so that there is nothing noteworthy, as some writers appear to have imagined, in finding an individual Culex with its tail cocked in the air, or an Anopheles with the abdomen drooping ; for being living animals, they may be occasionally found in any possible position, however characteristic certain particular poses may be of their usual ways.
When the season favourable for active existence and breeding is over, the majority, at any rate of species, proceed to find some suitable shelter, in which they may lay up, protected from cold and injury, till the return of spring, or, at the least, the surviving impregnated females do so. In Northern Europe and similar climates this appears to be the sole provision for the maintenance of the species through the winter, and the males rarely or never hybernate ; but in the south, as we have seen, this means is supplemented by the hybernating of the larvae as well ; while in really tropical regions there is no interval of inactivity, at any rate from cold.
There can be no doubt, however, that not only cold, but other climatic conditions unfavourable to the free multiplication of a species, such as intense heat with drought, cause Mosquitoes to retire into a similar condition of inactivity, as the truth of the suggestion advanced in the first edition of this book to that effect has been amply verified by my observations during the past year in India. There can, in fact, be no other way of accounting for the absolute disappearance from public ken, for long periods, of certain species, such as the Stegomyice, which are active only for certain limited seasons. The larvae are certainly not concerned in the matter, as none of the kind are to be found,
LIFE HISTORi' AND SEASONAL PREVALENCE 141 and water of a character suitable to their needs is nowhere available, so that it is certain, that though nowhere in evidence, they must undoubtedly exist somewhere in hiding ; and though I cannot say I have actually traced them to their lairs, the unlimited accommodation for secreting themselves afforded by the structural imperfections of an Indian house make this in no way astonishing, and the same is the case in the cold season with Anopheles, which appear to have vanished from the earth, so well are their hiding places chosen.
During the cold weather, of the twenty or thirty members of the family that form the gnat family of the North-west Provinces, the imagines of but two, with their larvae, and the latter stage of a couple or so more, are alone to be found ; and one of the former, I believe, also disappears into seclusion when the fierce heat of the dry season commences, leaving C. fatigans as the only adult Mosquito in evidence. Now, as the insects are in no sense migratory, it is an obvious conclusion that a sufficient number of adults, either impregnated females only, or representatives of both sexes, to maintain the breed, must be lying by somewhere. In countries such as England, where buildings are carefully finished and their ceilings fairly accessible, it is generally possible to obtain living gnats at any period of the year, provided one knows the kind of situations wherein to search for them ; but in India, the ample space between the ceiling cloth and the thatched roof, and the innumerable gaping fissures in the rough mud plaster, that is all that covers the carelessly laid bricks leave so many crannies and corners, that it is by no means surprising that one may fail to unearth the insects from their hiding places without pretty well pulling down the house, an operation by no means unattended with risk, as one is apt to disturb other skulkers of a larger and more immediately dangerous character. This habit is spoken of as hybernation, and in Northern Europe, where the males apparently all die before winter has fairly set in, only impregnated females survive the winter, and it is through their agency alone that the continuity of the species is maintained. In more moderate climates, where
142 GNATS OR MOSQUITOES— CHAPTER VII water rarely freezes during the winter, this expedient may he assisted by the survival of larvae also, as we have seen is the case with certainly some species of Anopheles, and in that genus also, at least in India, it is not the females alone, but the surviving members of both sexes that seclude themselves in this way, for towards the end of the season of activity, males are to be met with in houses, in far larger numbers than their spouses, and are obviously sinking into the same lethargic state. Mosquitoes, when hybernating, creep into any hiding place that will afford warmth and darkness, and prefer to utilise for the purpose narrow crevices, into which one would hardly expect so delicate a winged insect to creep.
When in this state they are extremely sluggish, and may often be found stationary in the same place for long periods, without having, to all appearance, moved for days together; and provided one can find them, it is naturally not difficult to catch them ; but they nevertheless retain sufficient strength and alertness to escape from careless or clumsy attempts to capture them, though, with such exceptions, they neither feed nor perform any of the other active functions of life. All Mosquitoes are greatly affected by cold, and refuse to move or feed on a cold morning, even on their more habitual vegetable food, while the most troublesome species never attack animals, except in warm weather, so that although a couple of species are fairly common in houses throughout the cold weather in Oudh, Mosquito curtains can be safely laid aside.
It is only during the times of the year that they are actively breeding that Mosquitoes attack animals, and the habit is probably indulged in to supply the large amount of nourishment required to supply material for the relatively enormous bulk of eggs laid by them. Many species are, it seems likely, unable to mature their eggs without having first obtained a feed of blood. In Southern India, and doubtless in similar climates elsewhere, it is never cold enough to drive Mosquitoes into hybernation, and in places within the equatorial belt of uniform climatic conditions, the heat is rarely so extreme, or drought so prolonged, as to
LIFE HISTORY AND SEASONAL PREVALENCE 143 force them into retirement on the opposite account ; so that in such places, all correspondents tell me that they are present, in greater or less numbers, throughout the year ; though even with them, the degree of prevalence is a good deal influenced by such climatic variations as occur. Thus there exists for many species, what may be called a southern limit of hybernation, to the north of which it is compelled to retire into seclusion during the winter ; whereas, further south, they are able to remain in activity to a greater or less extent throughout the year. In the case of our local species in India of Anopheles, this line lies somewhere about the 26th parallel of latitude, for while they were certainly still hybernating in Agra, in the middle of February, I found them numerous and fairly active some hundred miles further south, at Hoshangabad, in the Central Provinces. It should, however, be needless to remark that it is impossible to lay down any rigid limits in a case of this sort, and that it must needs vary from year to year with temporary variations of the character of the seasons.
Our knowledge of the seasonal prevalence of individual species is very limited, and in any case, can only be stated for areas of small extent, but it may be broadly stated that in each locality it may be fairly predicated from an inspection of the annual weather reports, as while some are tolerant of the greatest heat, others are confined to seasons of abundant rainfall. Were we even much better informed than we are, it is therefore obvious that no general account could be afforded within any moderate limits of space, so I confine myself, as an example of what may be observed in some one region, to the following sketch of a year's experience on this point in the North-west Provinces of India, and Oudh. Presuming a visitor to arrive in January, he would find about houses and in gardens, but two species, C. fatigans, Wied, and C. impellens, Walker, and even these, neither troublesome, nor numerous.
In the garden tanks he would find the larvae of these species and also those of Anopheles Bossii, Mihi, and sinensis, Wied, but, owing to almost all natural collections of stagnant water of moderate size being already dried up.