114 GNATS OR MOSQUITOES — CHAPTER VII Food then, alike for larval and adult insect must be obtainable in close proximity. The pupa does not eat. In the adult stage, the usual food of probably all species of both sexes is the juices of plants, and although the females of many attack and suck the blood of vertebrates, food of this sort does not seem to be in any way essential to their welfare, as most species will lay their eggs equally well on a purely vegetable diet. Moreover, even in the case of notoriously troublesome species, it is often difficult to induce them to bite animals, although they will still feed on their more usual vegetable food. A high atmospheric temperature appears to be the main determining condition of these outbursts of sanguinary instinct, and this is probably the reason why, in spite of gnats being quite common, we are so rarely troubled by their attacks in England, for it is noticeable that whenever there occurs a spell of exceptionally hot weather, we find in the press notices of an invasion of England by Mosquitoes, which, however, always turn out to be common indigenous species when submitted to a competent entomologist. The same thing may be observed in Northern India, where it will be found that Mosquitoes cease to be troublesome, some time before they have finally hidden themselves for their cold weather rest. Speaking generally, only the females bite, and it is noticeable that different persons differ greatly in their reaction to the stings of these insects. When Mosquitoes are unable or unwilling to obtain blood they suck the juices of plants. They are frequently found on flowers, and especially in England on the catkins of the willow. On hot days and in places exposed to sunshine they remain at rest until the evening, but especially in wooded localities they often are active in the middle of the day, and may often be observed perched on leaves and making a sort of balancing movement of the body, by alternately bending and extending their legs in the same way as do many of the TipulidcB. In India too, the most usual recognisable constituent of the contents of the stomach, of all species I have examined, in both sexes is pollen. Now some pollen grains look not unlike the printed representations of zygotes, and I have
LIFE HISTORY AND SEASONAL PEEVALENCE 115 met with an instance when they were mistaken for them. No fairly expert microscopist is of course likely to be misled in this way ; but it will be at least a harmless precaution, for anyone who has to depend for guidance on figures, to make himself familiar with a few of the commoner pollens before starting zygote hunting.
In addition to the juices of living plants and animals, many species will also feed on such articles of human food as are suitable to the structure of their mouth parts and they show a marked preference for sweet things. Hence mosquitoes may often be seen clustered on the moist sticky sugar exposed for sale in the shops of an Indian bazaar, and a few will generally be found feasting on the preserved fruit and sweets laid out for desert on an Anglo-Indian diningtable, or on puddings put aside in the larder. Not indeed that they confine themselves to such articles ; they are not adverse to milk, and will also attack raw meat, or indeed, any article of food out of which anything can be got by suction. On one occasion I found some Mosquitoes unmistakably feeding on filth, but believe such a habit is adopted only as a last resource, for at the time I observed this, all plants were dried up and juiceless. As is well known, many butterflies which also normally feed on plant juices, exhibit the same incongruous taste. Although one of our commonest Indian species is reputed to do so, I have never actually seen a male mosquito bite, and beheve that when they do so it is only an exceptional indulgence. There is, however, no doubt that some few species do so occasionally.
On this point Mr. Austen remarks : " While it is certain that in a natural state only an infinitesimally small proportion of all the Mosquitoes that come into existence can possibly taste the blood of a warm-blooded animal, it is reasonable to suppose that primitively all Gulicidcs fed upon the juices of plants. Indeed it has been stated that at the present day some species are still exclusively vegetarian in both sexes ; that in others, while the males are vegetarian, the females suck animal blood — in some species only exceptionally, in others habitually ; and, finally, that there
116 GNATS OR MOSQUITOES— CHAPTER VII are species in which both sexes suck animal blood (this is said to be the case in an Egyptian species and in two Itahan ones; it has also been observed in two species found in Madagascar, and has been noticed at Bannu (N. India)."
When in the humour to attack, it is not only man, but also other mammals as well as birds that are laid under contribution ; and it is propable that they do not even conjfine their attention to warm-blooded animals, for if, as is most likely the case, they are the habitual intermediate hosts for the majority of the class of protozoal blood-parasites, of which that of human malaria is the best known example, it seems likely that tortoises and frogs are the qhosen victims of certain species, as haematozoa closely aUied to Laverans parasite are well known to occur in animals of these classes.
Bovine malaria has been shown by Smith and Kilborne to be carried by a tick {Ripicephalus annulatvs), but owing to the amphibious habits of the former animals it is unlikely that the alternative host of their malaria can be any wingless arthropod.
' The staple food of the larva? of all the well-known genera is undoubtedly of a vegetable character, such as monocellular and filamentous algae, and in dissecting larvae I have never met with anything else in the contents of their stomachs. There can be no doubt, however, that they feed also on animal matter, and may be not only predatory, but actually cannibaHstic. The larvae of Coretlira are said to be especially remarkable for procHvities of this sort, but I have no personal experience of this genus, and otherwise there seems but little difference in the habits of food of the different genera. The Indian species of Anopheles are no exception to this. Quoting from a communication to the Bombay Natural History Society, advance proofs of which have been kindly sent me by the author, Mr. E. H. Aitken, one of our best field naturalists, I extract the following graphic description of their habits : — " The favourite vegetable food is a soft fucus, very like cotton wool dyed green, which is found in clear running
LIFE HISTORY AND SEASONAL PREVALENCE 117 water ; or that thick spongy growth which clothes the sides of fountains in which there are no fishes, and gets detached and floats like a thick, dark green scum. The principal animal food is the cast-off skins and pupa cases of water larvae, including those of its own kind, and the remains of dead mosquitoes and other small insects. These often collect in patches on the surface of a pond, and one dip under such a patch will secure hundreds of larvae after you have searched the rest of the pond in vam. Now the larva of Anopheles, unlike that of Gulex, floats flat on the surface of the water, and it is much more unwilling to go down than Gulex. If green food is to be had not more than two or three inches deep, it will go down and feed, but it comes up again very soon, and would evidently rather not go down at all. As it floats you will see two little organs on the front of its head incessantly stirring the water. These are the " whorl-organs." They are crowned with little brushes of bristles, and their function is to keep up an eddy, by which every little floating particle which passes by is sucked in towards the insect's mouth. With a lens you can see this quite plainly and observe it seizing the little particles with its jaws, sometimes eating them and sometimes throwing them away with an angry toss of its head. I do not think that even living objects, if small enough, are refused, and, in fact, I am almost sure that the larger larvae sometimes eat the little ones. They are all very ill-natured and bite savagely at each other when they get the chance. This way of feeding explains why Anopheles likes a certain amount of motion in the water, for it brings food, and why it must starve in deep water unless there chances to be a great deal of food, animal or vegetable, floating on the surface. The larvae of Gulex dive much more freely and are more promiscuous in their diet, and, since they float with their heads down, they do not care much for floating matter. Hence they dislike any motion. The more stagnant the water is and the more dirty, the better it pleases them. Lastly, Anopheles larvae must have sunlight, though they will hate it more cordially than other mosquitoes when they come to mosquitos' estate."
118 GNATS OR MOSQUITOES — CHAPTER VII The necessity of sunlight is mentioned by Meinert, and is also noted in a letter I had received from Dr. St. George Gray, of St. Lucia, in the West Indies, and furnishes an explanation of the failure of all our efforts to keep these larvae in health in captivity. In speaking of the habits of Gulices, Mr. Aitken refers to the specially domestic species, for those of many sylvan gnats are less tolerant of dirty water than any Anopheles of my acquaintance, and those of Mr. Theobald's new genus Stegomyia, are the most dainty larvae I know of in this respect, which probably accounts for the fact that they are all essentially insects of the rains. Species, such as C. fatigans, which live in reall}^ foul water, subsist largely on the vibriones with which it swarms. Waterless tracts, or such as are desert owing to the character of the soil, are necessarily free from Mosquitoes, except through the indirect agency of man, but in by far the majority of situations, food for both their stages of existence is obtainable, and gnats of some sort are included in the fauna ; though the species harboured will vary with the sort of accommodation offered.
In searching for larvae, it is little use attempting to do so by a mere inspection, as owing to their universally protective colouring, it is easy to overlook them, even when present in large numbers, if one merely stoop down and peer into the puddle. The examination should be made by dipping up some of the water from the surface of the puddle in an ordinary tumbler, and examining it by transmitted light. If care be taken not to disturb the mud at the bottom, it will usually be easy to do this at once, but should the water get stirred up, the specimen must be put aside till it settles.
The habits of the larvae, not only of the different genera, but also of individual species of the same genus differ greatly, and much misconception has arisen from hasty attempts to generalise ; for, as a matter of fact, it is impossible to say that you will find the larvae of Anopheles in certain situations, or in water of a given degree of purity or otherwise, as those of some species frequent only fairly pure collections, while others are hardly less particular than many, even domestic species of Culices.
LIFE HISTORY AND SEASONAL PREVALENCE 1 Lynch Arribalzaga (L. A., p. 23) classified Mosquitoes, as regards their habits, into domestic, field, marsh and wood Mosquitoes; and Ficalbi (F. Y. S., p. 62) into " foveal," which I take it, may be translated as domestic, paludal, and sub-paludal species.
The word paludal, however, appears to be a misnomer based on an unconscious remembrance of bygone notions of the etiology of malaria, for all marshes of dimensions worthy of the name, shelter too many of the greatest enemies of the larvae to be regarded as the typical habitat of any species ; and for practical purposes, it is more convenient to divide them into domestic and field Mosquitoes ; though the true forest gnats, such as the genera Megarhina, Sabethes, Mdes, Mdoinyia, and Ceranotoenia mostly, which inhabit the depths of tropical jungles, far from the haunts of man, form a class by themselves. Psorophora and Mucidus are field rather than jungle gnats, and Stegonujia and Armigeres, are rather field than domestic, and are never, so far as I know, found in dirty water. I know of no really jungle species of Anopheles, all species being either field or domestic, but Culices are found in all possible situations.
It would be a mistake to attempt to attach too rigid a signification to terms of this sort, and I would particularly caution against premature attempts at generalisation, but broadly speaking, the domestic gnats are those which infest habitations, while their larvae are mostly found in small artificial collections of water, such as garden tanks, broken crockery filled by rain, waste water puddles and so on ; while the field species, though often found in and about habitations, pass their larval stage in fairly clean, natural pools, or at any rate in such as afford conditions similar to those found in the open.
Some typically domestic gnats, such as C. fatigans, will deposit their eggs in the foulest of water, and I have found its larvae in apparently flourishing condition, in the small cemented tanks, for the temporary storage of sullage water in Indian towns, known as nahhddn, the contents of which are filth, far more concentrated than the sewage of any
120 GNATS OR MOSQUITOES — CHAPTER VII European town. On the other band, unless my suspicion that however close their resemblance, they are really distinct forms be correct, the same larvae will be found during the rains, alone or in company with Stegomyia larvae, in almost drinkable water. Culex pipiens is of course another typically domestic gnat, and C. annulatus, spathipalpis, nefiiorosus, viridiventer and pulchriventer may be cited as examples of field species, though the last is perhaps, strictly speaking, a forest form. The habits of the different species of Anopheles larvae also vary greatly An. Bossii is certainly essentially a domestic species, and so too, I think, is An. Sinensis with us, but An. Jamesii, which appears to be the species that swarms in the Madras paddy fields, is probably a field species, and so too, it may be, is An. culicifacies , sp. n., which is probably the species referred to by Mr. Aitken, in his above-quoted note.
During the past year I have observed the larvae of An. Bossii in a variety of situations, but all essentially "domestic." In the spring months, in upper India, none are in evidence, and I first found them towards the end of April, breeding in a pool beside one of the piers of the old bridge across the Goomti, which flowed just beneath my house in Lucknow. The pool was but a few yards long by not more than six feet wide, and though it did contain a certain amount of green filamentous vegetation, was extremely foul. Still it is the nearest approach to the "Anopheles pool" of the West African Malaria Commission that I have met with inhabited by the larvae. Typical pools of the sort I have indeed come across by the dozen, but in no case have I met with them in such pools. The horizontal posture of the larvae was, however, very noticeable, and when placed in a tray under lens, I could also verify their peculiar trick of screwing round their heads so as to look upward. During the dry, hot weather, however, their appearance is with us exceptional, though in the damp eastern and southern parts of India they may be found at any time of the year ; and it is not until the rains .-are fairly established that one can expect to find them in any numbers.
LIFE HISTORY AND SEASONAL PREVALENCE 121 In July and August last, my duties involved an extensive tour through the province during the rainy season. In the early part of the period it was evident that though Anopheles larvae were common, they had not long been so, as even in places where they were plentiful, I could find no adults. It was not indeed till the end of the month that I began to find them at all commonly in bungalows. The situations, however, in which I found the larvae entirely upset all the notions I had gathered from recent writings on the subject. I began, of course, by looking for the typical Anopheles pool of Ross, but such as I found never held any of the expected larvae, and the first place I met with them was in the garden of the Meerut Club, in the small irrigation tanks described in the next chapter Here they were present in enormous numbers, sometimes alone, but more frequently in company, and apparently on excellent terms with the larvae of C. fatigans. It was, however, noticeable that while the Culex larvae for the most part remained in the middle of the tanks, those of Anopheles generally kept themselves floating with their tails touching its side walls, and so might easily be overlooked. In my subsequent wanderings, I met with Anopheles larvae in a variety of situations, but always these small irrigation tanks were the " surest find," and further I never met with them at any distance from human habitations, so that I am inclined to suspect that females are unable to mature and deposit their eggs until they have had a feed of blood. I have also met with Anopheles larvae in muddy pools of some size in brick-fields, in the overflow from stand-posts in large cities supplied with a regular filtered water supply, and even in a very shallow depression in the concrete surface of the platform of a bustling railway junction, also fed by a stand-post. In Hong Kong, Dr. J. C. Thomson writes, in The Government Gazette of November 24th, 1900. "The usual habitat of the larvae of the Anopheles Mosquito is the natural water courses, and their favourite locations little breaks in the rocky surface by the side of the stream, where the merest trickle from the stream itself prevents entire stagnation, and where there is no thi-ough-wash." This, it will