Panama: on mosquitoes

No less than eleven species have been collected (see Table 12.1), yet it is improbable that more than three are of economic importance from carrying malaria.

It is not unlikely that tarsimaculata is a variety of albimanus, since it differs from it only in the arrangement of the white bands in the palpi. It is also probable that A. gorgasi is an abnormality of tarsimaculata; only one specimen has been found, and it is somewhat damaged.49

“The most important and the most easily recognised species among our American Anopheles is Anopheles albimanus, Weidemann,” writes Mr Knab.50 And this being so, it will give some idea of the difficulty there has been in determining accurately what mosquitoes exist in a place, if I repeat from Dr Knab’s paper the various names and combination of names which have been applied to this easily recognized insect (see Table 12.2). It has had an even more confusing nomenclature than A. maculatus (or willmori) in Asia. As will be seen later on, this mosquito exists in British Guiana, and it is the great malaria carrier in tropical America and the West Indies.

On the Canal zone A. albimanus is not only the most important Anopheles, but it is the commonest. Although it cannot breed in swiftly running water, it can and does breed in the grass of sluggishly running water, in isolated pools, in stream beds, and in backwaters, especially where there is a growth of algae. Although it is considered necessary to oil the sides of the lake at Pedro Miguel, it may be said that the mosquito does not like the larger collections of water; marked sewage contamination, too, is un congenial, indeed is inimical to the larvae, but they have been found where pollution is slight.

“With the exception of foul or swift water, they may occur in almost any collection of water, however small or seemingly unsuited to mosquito propagation. Hoof-prints, wheel-ruts, the smallest puddle or thinnest film of water seeping upon the ground from a wet hillside, particularly if the ubiquitous algae are present, are points of danger, and must be included in the control work. … While not domestic in the same sense as Stegomyia calopus, Anopheles albimanus is closely associated with man, and finds its most congenial surroundings about his habitations, and in the conditions he creates in the course of agricultural, engineering, and other works.”51

The outstanding impressions I received of this mosquito were, first, that it is essentially a puddle-breeder, preferring the smallest puddles it can get, although to be found in other places where the other conditions were favourable; and secondly, that the existence of algae, or the presence of dead grass, dead wood, and other dead vegetable matter created, above all, a condition favourable to it, and that where the conditions were favourable the insect might be found in enormous numbers.

Near to dwellings and to man’s work the conditions are inimical to vegetation, and so favourable to the mosquito. But happily, as proved to be the case in British Guiana, agricultural operations need not necessarily be favourable to the insect or to malaria; indeed, if properly conducted, they may be the most effectual way of eliminating the disease. In the Canal zone everything was in favour of the mosquito, and no agricultural operations presented themselves to the study of the Sanitary Department; so this great truth had not been realized; whereas in the Federated Malay States the opening up of land had been like a great experiment specially designed to show the importance of agriculture in the prevention of malaria. In discussing this with Dr Darling one day, I was told by him that from time to time he had had an opportunity of examining villages outside the Canal zone, and he kindly gave me the following memorandum:

Board of Health Laboratory, Ancon, 31^st May, 1913

Memorandum for Dr Malcolm Watson: Blood Examinations for Malaria in Children at Native Villages in the Republic of Panama.

La Chorrera, 23^rd August, 1910

Eighteen persons examined; three positive for malaria. Anopheles albimanus abundant in the village.

Chame

Sixteen children examined; six positive. The only children having enlarged spleens in this series were two that were also infected with parasites. Anopheles albimanus breeding in the village.

Guarare, 25^th October, 1910

Ten children examined; blood negative. No Anopheles52 detected in the village, and no larvae found in few pools outside of the village.

La Ville de Los Santos

Eleven children examined; all negative for parasites. At Los Santos thirty-four boys were examined for splenic index, and two had enlarged spleens.

(Signed) S. T. Darling.

At one of the places where there was little malaria, I have forgotten which, Dr Darling had been greatly impressed by the prosperity and the activity of the inhabitants. In the general appearance of the village, and in their business enterprise, the inhabitants presented a strong contrast to the ordinary Panamanian. Beyond these, Dr Darling had not had any opportunity of making observations outside of his ordinary work, and the relationship of agriculture to malaria had not been specially studied.

Of these it is not necessary to say very much, for they have little or nothing to do with malaria. The following is taken from Mr Jenning’s paper:53

A. argyritarsis is less common than A. albimanus, and although widely distributed, its numbers are never very great, and it is not very frequently found in buildings.

It is the only species of isthmian Anopheles which breeds readily in artificial containers. In ground-water it prefers the smaller collections. … It seems quite dependent upon the presence of algae.

A. pseudopunctipennis occurs abundantly from ocean to ocean, but is somewhat more discriminating than A. albimanus in choice of breeding places. It prefers as a rule water of greater purity and rapidity of current. The larval food, like that of A. albimanus, is by preference the soft green algae, though it does not scorn, lacking better, many places departing widely from the chosen type. At times its abundance is enormous, though usually far fewer of this species will find their way into buildings than is the case with A. albimanus, and its flight is less vigorous. Darling’s experiments (see Chapter 14) show it is less easily infected with malaria than A. albimanus, and probably it is not a carrier in nature.

A. malefactor is widely distributed, and abundant. It is a large handsome species, a vigorous biter, active in entering houses. Yet apparently it does not transmit malaria. Its favourite breeding place seems to be, from what I saw and was told, large and deep shady pools.

A. eiseni is a54

strictly sylvan species, and breeds in depressions in the rocky beds of mountain streams, where protection from the rapidly flowing current is afforded, also in tree-holes and bamboo stumps. It is fairly abundant in favourable localities; yet I have never observed adults of the species at large by day, even in the dim light of the dense forest, nor at night when camping in the vicinity of active breeding places. I have no record of its entrance into buildings, and have never taken the larvae even a few yards beyond the edge of the forest. The relation of A. eiseni to malaria is not known, as it has been impossible to collect and breed sufficient material with which to work.

A. cruzi is55

remarkable for its habit of breeding only in water held in the leaf-axils of various species of epiphytic bromeliads. … Like the last, this species is not observed free in the adult state, even when in camp near its breedingplaces.

In Panama, as in the Federated Malay States, we find each species of Anopheles with a preference for a special kind of breeding place, and each with habits so definite that anyone devoting some attention to the subject can easily learn them; so that when he goes over a piece of land and sees a stream or pool, he has a very good idea what its mosquito inhabitant is. It is true that on rare occasions larvae may be found in unusual breeding places. Mr Bath, divisional inspector, told me that only on two occasions in all his experience of the zone had he found Anopheles albimanus breeding elsewhere than in water on the ground: once larvae were found in water in the bottom of an abandoned French railway car or truck; on the other occasion they were in water in a clean kerosene tin. I think the explanation is very simple. With all other pools oiled, the mosquito was forced to lay her eggs on the only clean surface she could find. Accidents happen in the best regulated families; more than one human being has been born in a train.

Of great interest are many of the observations made on adult mosquitoes on the zone.

In Havana it had been observed that Aedes (Stegomyia) calopus was a domestic mosquito, and practically never was found more than 200 yards from houses; and the scheme of mosquito destruction put into effect there, “though entirely directed against yellow fever, was almost equally successful against malaria.”56 The idea that the flight of Anopheles was also limited to this small distance was brought to the zone from Havana; and Colonel Gorgas, writing in July 1908, says:

The inspector is required to keep the tropical undergrowth cut off within 200 yards of villages and 100 yards of isolated houses. Within this area the grass must be kept less than a foot high. I consider this scarcely less important than drainage. Bush and grass shelter the adult mosquito, and they will reach a habitation from distant breeding places by short flight if they have the continuous protection of bush, whereas the Anopheles will not cross a cleared area of 100 yards.

As time went on and the anti-mosquito work became so nearly perfect that mosquitoes were rarely seen by the ordinary inhabitant of the zone, and in a week only two or three hundred would be captured by the trained mosquito catchers in a large station, a condition was produced that presented unparalleled opportunities for observations on the flight of mosquitoes; and these, I need hardly say, were eagerly seized upon by the Sanitary Department. It is obvious that, if in ordinary times a place is practically free from mosquitoes, and the few that enter the area are captured and counted, any increase of the insect immigrants will be noted at once; and so it was in Panama. Immigration on a large scale has occurred on different occasions, and search having been made for the breeding places, the flight of the mosquitoes was found to be much greater than had generally been supposed.

The first great flight, or immigration of Anopheles, occurred in 1911, and was caused by a hydraulic fill on the east side of the Canal to the north of Gatun. The photograph in Figure 10.6 shows the final condition of the fill, when the swamp had become solid dry land free from sun-cracks. At one stage, and in one part of this fill, extensive sun-cracks had developed in the mud, and in the water in the cracks enormous numbers of mosquitoes were produced.

The second great flight occurred at Gatun in 1913, again as the result of a hydraulic fill. On this occasion the larvae came, not from water in cracks in sun-dried mud, but from the portion of the swamp where the salt water of the “fill” was killing out freshwater jungle. In the sun-cracks there was probably some special food for the mosquitoes, of which, however, I do not think mention was made; and there would certainly be great security from such enemies as fish. The attraction in 1913 was, of course, the dead wood; and with abundant food and a certain amount of cover, the mosquitoes seemed to take little harm from the fish, which swarmed in the swamp. As the subject is of great importance, and is well told in the reports of the Sanitary Department for the months of January and February 1913, I cannot do better than reproduce these in full.

From the Report for the month of January 1913

Some very interesting data have been collected in reference to the increase of adult mosquitoes at Gatun, which was mentioned in the December report.

The weather conditions on the Atlantic slope and at Gatun have been somewhat different from those of recent years, and toward the end of the month several showers occurred at a time when continuous dry weather is usually expected.

As the number of Anopheles increased until the number of adults in and about residences in Gatun was very much larger than had ever occurred previously, it appeared that the origin of the sudden influx must have been caused by some new condition that did not exist in other dry seasons. It was evident that such condition must be due to topographical or other changes made since the previous dry season. The prominent changes made during the past year were four in number, viz.:

1. (1)The rise of water level in Gatun Lake to elevation Plus 55, followed by a collection of vegetable debris along the shore, and the rapid collection and growth of aquatic vegetation along the shore. Green algae did not occur there.
2. (2)A hydraulic fill north-east of the town was completed and brackish surface water only partially drained off, leaving but little dead vegetation near the surface.
3. (3)The natural drainage of an area north of bridge No. 9, between the old and new Panama railroad locations, had been affected by the silting of ditches in adjacent territory.
4. (4)A large fresh-water swamp, west of the French Canal, was being filled by hydraulic dredge, the water used for carrying the mud from present Canal channel being seawater.

All of these areas were beyond the limits of where Anopheles control had been found to be essential in previous years.

In order to determine the source of the unusual Anopheles influx, all previously controlled areas were very carefully inspected and found to be in a satisfactory condition.

The four areas already mentioned were then examined in the order given above.

An examination of water in debris along the lake shore showed a few scattered Culex larvae and very few Anopheles larvae. The numerous small fish present were keeping down the number of Culex larvae, but it is thought possible that in time conditions will not be so favourable for the good control now accomplished by the fish.

An examination in the high vegetation on dry land along shore showed relatively few adult Anopheles in daytime and quite a few Culicines. The latter did not follow the observers out into the full sunlight.

Examination made by boat at night along the outer line of aquatic growth, within 50 feet or less from shore, showed an absence of adult Anopheles. This same state of affairs occurred near the shore at a point less than 300 feet from a row of labourers’ cars, where over one hundred adult Anopheles were collected early each morning from each car. Moreover, no adult Anopheles were found in shade of brush near the cars in the daytime. The lake was, therefore, temporarily discarded as the possible source of supply.

The second area, the hydraulic fill north-east of the town, proved to be sterile as a production area, although some of the native huts not far away contained large numbers of ungorged female adult Anopheles in the daytime. This area extends from about 6000 to 10,000 feet from the Gatun railroad depot near the lake. It was temporarily discarded as having been a previous important source of the adult Anopheles, sufficient to account for the quantity present at the time the observations were made.

Third, the area north of bridge No. 9, extending from about 6000 to 10,000 feet north of the concrete railroad depot, which had been wet, dried up completely while the investigation was being made, and as at the time the Anopheles influx was still increasing rapidly it evidently should not be considered.

The remaining area, west of the French Canal, was next taken up. The inspection thereof was started at its western boundary, about 5000 feet from the railroad depot. The water there was fresh swamp water with no taste of salt. Anopheles adults were quite numerous. Plenty of water was present, and some places looked favourable, but no mosquito larvae were noted. The north-east end of the swamp was visited next, and water there was fresh. Adult Anopheles were very numerous, but no larvae present in water at that point. Adult Anopheles albimanus was present on nearly all tree trunks near the ground, and in all other suitable resting places.

It was decided to start from this point about 4500 feet from railroad depot, and to wade through the swamp in a general north-westerly direction. At about 200 yards from shore, in water about 18 inches deep and thickly overgrown with bunch grass, we noted young Anopheles larvae. The water near by was just perceptibly brackish. It was noted that the Anopheles larvae increased as the water became more salty. In the wet area that was decidedly brackish, Anopheles larvae and pupae in all stages of development were noted. It is safe to state that they were more numerous than in any of the places in the Canal zone that have been encountered since 1904.

During the eight years of sanitary work on the zone only one case was recorded where Anopheles have bitten a person standing in the full rays of the sun. That occurred at 8 a.m. At the breeding place above mentioned this rule did not hold good, the A. albimanus and A. tarsimaculatus bit quite freely there in the full sunlight at all hours of the day. It is of interest to note that the larvae were so numerous as to be quite close together. More than twenty-four were noted on one plant leaf 6 inches long. The places where the larvae and pupae were most numerous contained from 30 to 75 percent of sea-water. Culex larvae were relatively scarce.

In the fringe of brush about 300 feet wide, on dry land between the edge of the swamp and the French Canal, Anopheles adults were very numerous in the daytime, and at night it was impossible for observers to remain there without being bitten quite frequently during a period of one minute.

The eastern edge of the swamp is approximately parallel with the present Panama railroad, located about 3000 feet west of it. The larger part of the settlement at Gatun is east of the Panama railroad, and some portions more than 6000 feet from the infected swamp and about east thereof. The prevailing winds blow from north to south, and hardly ever from west to east or east to west. There were many more millions of Anopheles coming from the swamp than were necessary to account for the influx at the houses east of the railroad track and more than a mile from the swamp.

It was necessary to determine at once whether this area was the source of the influx at the settlement. A rowboat containing several men known to be attractive to Anopheles albimanus was rowed very slowly up and down the French Canal each night for several nights. The Anopheles that came aboard were few in number, and not sufficient to account for the large influx at the settlement a mile distant.

It was assumed that such a large number of adults as would be produced every twenty-four hours at the production area must of necessity spread out or travel a considerable distance in order to get blood sufficient to satisfy them. It was thought within the limits of possibility that they might fly high and not be noted by persons in a boat. It was also assumed that the period of the long flight might be of limited duration. Other factors bearing on the question and previous results noted by specially trained and competent sanitary inspectors of the Canal zone in connection with the study of habits of malaria-conveying species of Anopheles were given due consideration. It was considered necessary to make several more thorough observations lasting over twenty-four hour periods before drawing any conclusions.

On 20^th January, at 4.30 p.m., observers were posted on the opposite side of the French Canal from the propagation area and faced the latter, to observe any marked flight that might occur. Previously to 6.20 p.m. practically no mosquitoes were noted. At that time birds of the kind that feed on flying insects in the air, appeared to be very active, and were apparently feeding on insects in the air at an elevation of thirty or more feet above the water surface. Later, the birds operated at a lower elevation, and about 6.30 p.m. were feeding at about 6 feet or less above the water surface. It was at this time that the observers on the bank and in the boat noticed the first appearance of Anopheles. The flight was from west to east, and quite marked. As it became darker the quantity of flying Anopheles increased, and by looking past a dark object against the clear sky hundreds of Anopheles could be seen passing by. These observations were continued for four consecutive nights, and the time of the start of the flight period remained about the same. After dark the flight was markedly reduced, and practically stopped completely before 9 p.m. Also, observers on the east, short of the French Canal, were attacked continuously during the period of flight, but failed to find a single Anopheles at 9 p.m., although they were very numerous on the west shore near the propagation area.

It would appear, then, that the Anopheles albimanus and A. tarsimaculata in the area mentioned ovideposit in water that is decidedly brackish, and that the adults fly eastward for long distances between six and eight o’clock. There is relatively little travel after that hour. Adult Anopheles were stained with dye and liberated at the swamp. Subsequently some of them were collected on the opposite side of the river, at the locks and in houses 4700 feet from the liberating station. It should be stated that the Anopheles’ flight was decidedly marked and was easily noted by half a dozen witnesses, when their attention had been drawn to it. Even so, not one person in areas thickly infested did note the flight until shown the way to observe it.

It is very interesting to note that there is apparently no large or marked return flight to the swamp from east to west. It may be that such occurs, but that it is of a different nature from the direct flight and not yet understood. The most surprising part of the observations made was that the flight did not extend very far beyond the inhabited area where the employees live. It was expected that, with thousands of Anopheles adults travelling from the swamp to the settlement each night, some would go well beyond the settlement in the apparently direct line of flight. Such extension did not occur.

As the number of Anopheles occurring in houses and barracks increased rapidly, more Anopheles traps were installed, and eight labourers were employed exclusively for destroying mosquitoes in the barracks. Previous to the influx, there were relatively few cases of malaria or Anopheles at Gatun.

The combinations of such conditions with the work of the mosquito catchers and use of traps has prevented to a large extent the expected increase of the sick-rate at Gatun, as shown by the following figures: [see Table 12.3]57

Apparently, up to date, millions of adult Anopheles have recently occurred near the settlements at Gatun, but due to the control methods used there has been no perceptible increase of malaria fever.

The sequel is told in the February Report:

In the report for January, data were given concerning the presence of adult Anopheles at Gatun. Two large ditches were made to drain the production area referred to, and it is now under control. The number of adult Anopheles in the settled area has decreased. Toward the end of the present month the prevailing species near the settlement was Anopheles tarsimaculata. Very few were noted in the office buildings in the daytime, even though a considerable number were resting in cracks in the ground under such buildings, and would bite quite freely there. It should be explained that the buildings are raised several feet or more above the ground surface. No Anopheles were found resting in shaded places on the under side of the floor system. Several stained Anopheles were found near the breeding area more than two weeks after they had been stained at that locality. It is of interest to note that some of the marked specimens were retaken more than 6000 feet from the point where they were liberated.

The work of destroying adult Anopheles in quarters at Gatun had been continued daily, with gratifying results. The table given below shows the increase of adult Anopheles at Gatun, and the corresponding malaria sick-rate. It indicates the value of such auxiliary control work: [See Table 12.3]

Extensive hydraulic fills are now being made on low lands between Diablo Hill, Balboa, and Panama. Observations show that the Anopheles tarsimaculatus and A. albimanus increase when salt water is introduced into the propagation area. That the number of larvae of these species is quite large even when the seawater content exceeds 60 percent, if other conditions are favourable. The areas being treated will have to be carefully watched and controlled temporarily to prevent Anopheles production. Ultimately the filled area will be non-productive, and will save the expense of anti-malaria work in future years.

Not least in interest is it to note that the malaria rate of Gatun did not increase. Whether this was entirely due to the mosquito catching or not, I am not prepared to say. It must not be forgotten that the population of Gatun had for many years been very free from malaria, and that the number of malarious people who could infect the Anopheles must have been smaller than in a place where a large number of Anopheles were continuously present throughout the year. The mosquitoes were the result of a condition so artificial that it could not remain permanently to produce these enormous numbers; and I think it would be unwise to argue that because mosquitoes have been known to fly over a mile in large numbers for a short period, measures to prevent malaria should be extended to a similar distance. I gathered that such occasional and temporary flights are not considered on the zone to be of any special danger, even if the mosquito catching were not carried out.

This occurred, as I have said, when the reservoir was being filled, and when abundant food in the form of dead timber was present in the water. The reservoir is situated about 4000 feet from the houses, and directly south from them. The prevailing wind being from north to south, the mosquitoes thus traveled against the wind for nearly a mile.

At the beginning of the wet season in 1912, Corozal was invaded by an unusual number of Anopheles. They came from a mangrove swamp cut off from the sea by a bank, in which, as a result of the rainfall, the salinity of the water was less than usual. Whether or not this caused the death of some of the vegetation, I did not ascertain. Certainly it did not produce a complete destruction of the whole vegetation similar to that in the hydraulic fill at Gatun in 1913, of which there is a photograph in Figure 10.5, and it is unlikely that there were sun-cracks. Whatever was the cause, the effect was a great flight of Anopheles. Three weeks later the malaria rate of the station rose sharply from i percent to 2 percent and a month later to nearly 3 percent per week. The great influx lasted about two months, when it came to something about treble the normal rate, and remained so from the middle of July to the middle of November, when there was again a sharp increase.

Figure 12.1: Chart of the malaria rate and the mosquito catch at Corozal

At Corozal the violent fluctuations of the Anopheles catch and the malaria rate are much less directly connected than one would imagine, and raise the problem of how much of the malaria which occurs on the zone in the wet season is the result of the labourers getting chilled and suffering from relapses of old infections; how much is from new infections produced by the increased number of Anopheles, and how much is due to relapses among the many new arrivals who come from the West Indian Islands, many of which are highly malarious.

I do not propose to discuss this, for it is practically certain now that the annual rise is mainly from relapses among people who have imported their malaria. Year by year it has been getting less, until in 1913, for the first time, there was no increase during the wet season; and it is significant that in his report for 1912 the Quartermaster says 941 men were recruited from the small islands adjacent to Barbados, and adds, “This undoubtedly marks the last shipment of unskilled labour to the Canal.” It is more than a coincidence that in the year after the stoppage of recruiting the annual malaria wave should disappear; and this taken with the fact that the population of the zone had reached the highest point yet attained, shows conclusively the origin of most of the relapses.

Strong winds drive mosquitoes to cover; but where the air current is slight, mosquitoes have been observed on repeated occasions to fly or swarm head to the wind.58 Great migrations of mosquitoes from salt marshes have been carefully investigated by Dr John B. Smith, of New Jersey, who has no hesitation “in stating that these salt marsh species may migrate inland for forty miles.” The salt marsh mosquitoes, however, immigrate, possibly aided by the wind, but as more recent evidence shows, more probably flying into the wind.59

On the Canal zone it is apparent that whatever it is that attracts mosquitoes, they can and do travel against air currents of at least 5 miles an hour; and that more mosquitoes are found to windward of a swamp than to leeward. Mr Le Prince discusses this in a recent paper.60 On the zone the prevailing wind is from north to south, and “it was noted that Anophelines were not as numerous at the labourers’ camp at Ancon as at other stations, although there was a large swampy area to the north of it that contained Anopheline larvae.” At Corozal, 3 miles north of Ancon, there was a swamp immediately to the north of the camp, and another to the south, “a little over a mile distant. … During the dry season of 1911 there were no Anopheles production areas within one mile to the south of Corozal, and anti-malarial operations were concentrated at the swamp to the north. In spite of the work done, many thousand Anophelines appeared each week at Corozal, and from one to two thousand were caught in one building. As the result of the work on the windward side was a failure, an extra brigade of labourers and oilers was put to work in the swamp more than a mile to the south of Corozal (to the leeward). As soon as that area was under control, the Anophelines at Corozal practically disappeared.”

As an experiment oiling was then stopped in the swamp to windward at a distance of 2500 feet, but although “millions of Anophelines’ larvae and pupae soon appeared, the number of adult Anophelines that reached the camp at Corozal decreased. It seemed quite evident that there was very little flight in the direction of the wind towards Corozal. During November 1911, when the wind blew from south to north, the Anopheline catch at Corozal at once increased.” Observations confirming these were made at Miraflores, the station on the north of Corozal; so there seems little doubt that, like a bird in the wind and a fish in a stream, a mosquito is most comfortable when heading the current, and that it prefers to travel against the current when moderate in force. The practical point is, of course, that if we could depend on the wind blowing always in one direction, we should make the centre of the sanitary mosquito-free area not coincident with the centre of the settlement, but to leeward of it.

Many years ago, someone suggested that mosquitoes might be trapped in boxes painted black inside; the theory being that in the morning the mosquitoes in a house would retire to rest in a dark box. The lid of the box was then to be closed suddenly; a few drops of chloroform were next to be dropped into it through a small hole, and when stupefied, mosquitoes were to be taken out and burned. Of course this is not sanitation; nor would it produce enough excitement in the dullest spot to warrant a gamble on the catch.

But in Panama, Mr Bath, one of the two divisional inspectors, invented a trap of much Interest. It is made in three pieces, which can be taken apart, and is on the principle of a fish trap. The bottom piece of the trap is prism-shaped; the base is open; the two sides lead up to the apex, where there is a narrow slit, through which the mosquito readily enters, and from which it rarely escapes. Passing through another bottom with a similar slit, the mosquito is securely inside the trap. The trap can be fastened to any part of a house, with the opening either out or in, and it gives some idea of the number of mosquitoes which try to enter or leave the house. It may be left for a month, if the catches are small; but it is unwise to leave it longer, for spiders may close it up with their webs.

During the great flight of mosquitoes at Miraflores, one trap caught 1018 mosquitoes between 6 a.m. and 7 a.m. In sixty days an average of seven traps caught 37,268 Anopheles, and in addition many Culex. In forty-four traps the catch averaged from 90 to no mosquitoes for the individual traps, and 96 for the whole number. On another occasion eight traps were exposed for six nights, and it was discovered that the maximum flight was from 9 p.m. to midnight. The actual figures are given in Table 12.4. The wind was blowing from four to seven miles an hour. These figures are of interest, because they are not in accordance with what was observed in the Gatun flights in 1913.

A point of practical importance has been demonstrated by these traps; it is that, to screen a house without giving special care to the part next to the eaves, is to court failure; for the largest catches have been got when the trap was just under the eaves. This is probably because all the mosquitoes which seek to enter the house, gradually work their way up the screening to the eaves, and there move along, constantly trying to get in. A trap in such a position would, therefore, get a large proportion of the mosquitoes which had tried to get into any portion of that side of the house. I have often heard people argue that as mosquitoes do not fly far from the ground, it is unnecessary to screen between the wall plate and the ceiling. In one hospital in the Federated Malay States this space was actually left open, although the rest of the hospital was screened. Needless to say the hospital became a mosquito trap, and, since then, mosquito-proof buildings have been consistently condemned by some people in the Federated Malay States as worse than useless. The largest catches were always got under the eaves on the lee side of a house, showing again that mosquitoes travel against the wind, and are not blown away by it.

Mr Bath has also found that mosquitoes rarely enter an unoccupied house. Unoccupied huts have been observed for some time; no mosquitoes have been found in them. Then men have been put to sleep in them for a few nights, when mosquitoes immediately appeared and could be caught. Lights do not attract mosquitoes, possibly smell does. The next largest catches are got at holes in the floor.

Mr Bath has patented his trap.

Figure 12.2: Mr Bath’s mosquito trap

To test the direction of the flight of mosquitoes a simple apparatus was devised; it consists of two pieces of wood at right angles to each other, mounted on a pole 42 ft. from the ground, much like a signpost at crossroads. The surface of the cross-pieces is coated with a mixture something like bird lime, and any insect which alights on it cannot escape. The side on which the largest number of insects is found faces the direction from which the mosquitoes are coming.