The effect of climatic conditions upon populations of insects

325 Proceedings

of an

Ordinary Meeting of the Society held at M a n s o n H o u s e ,

28, Portland Place, L o n d o n , W.1, at 8.15, on T h u r s d a y , 8th D e c e m b e r , 1932. D r . G. CARMICHAEL LOW, F.R.C.P., President, in the Chair.

PAPER. THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS. BY

PATRICK A. BUXTON, M.R.C.S., L.R.C.P., D.T.M. & H.

Director, Department of Entomology, London School of Hygiene and Tropical Medicine. PAGE

THE PHENOMENA

..

325

T H E ANALYTICAL METHOD

326

T~MPERATURE

327 333

..

HUMIDITY . . . . OTHER CLIMATIC FACTORS FIE,LD WORK .. APPLICATIONS

346 347 352

SUMMARY .. REFERENCES. . . .

353 354

. .

..

T ~ E PHENOMENA. J u d g e d b y the standard of man, whose life extends to several decades and whose reproductive capacity is low, insect populations increase and decrease with surprising rapidity. I n the course of a few months or even weeks, one m a y witness a thousandfold alteration in n u m b e r s . I t is well known that the n u m b e r s of insects are altered b y m a n y different causes. O f t e n enough the p h e n o m e n o n is c l i m a t i c : at other times it is due to increase or decrease in enemies or parasites, ¢ and here again the alteration in n u m b e r s of hosts m a y be indirectly d e p e n d e n t on climate even if it is not possible at present to trace the ¢It is a curious fact that many biting insects appear not to be attacked by parasitic insects. It seems that no parasitic insects are known from any Culicidm (the larv~ of which fall a prey to a host of predators) ; the sucking llce and biting lice (Anoplura and Mallophaga) are also free from parasitic insects. In spite of the fact that many have been bred, we only know of one parasitic insect which attacks fleas (Siphonaptera). (See WATERSTON). My friend, Dr. W. R. THOMPSON,has been good enough to confirm these statements after referring to the extensive index o f parasitic insects which is maintained at the Parasite Laboratory of the Imperial Institute of Entomology.

89,6

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

relationship. In many cases, also, insect populations are suddenly increased by immigration. The most familiar example of migrant insects are the locusts, but many sorts of dragon-flies, butterflies and other insects are known to migrate in large numbers over great distances. The rapid growth and decrease of a population of insects is of great importance to all applied entomologists. To us, as medical people, the subject matters because the geographical spread of human diseases and the seasonal occurrence of certain epidemics appear to be directly due to alteration in the numbers of insects which are the essential vectors of these diseases. Our ultimate objective is to know the numbers of particular sorts of insects which are capable of infecting us with the organisms which they carry. But in these days of ignorance we should be content to study the gross numbers. It is familiar that seasonal alterations in the numbers of a particular sort of insect can frequently be related to increases and decreases in the disease with which they appear to be positively correlated. As examples one may quote the relation between rat fleas (especially Xenopsylla cheopis) and plague in rats and men in many tropical countries, and that between lice and relapsing fever in Northern India (CRACG). There is also the fact, familiar to many though not yet sufficiently studied, that excess of winter rain in Mediterranean countries is followed by marked increases in the number of Anopheles, and by outbreaks of malaria. Nor should we forget that the geographical distribution of an arthropod is often limited by climatic factors. It is the purpose of this paper to consider the available evidence about the effect of climate upon insects of medical importance. T H E ANALYTICAL METHOD.

The subject may be approached in two ways : it may be studied in the field and the laboratory. The field method has met with most favour in the past. It has been felt that it is important to study the insects under natural conditions, and it has been said that field work brings the investigator closer to the practical problem. On the other hand, it may be urged that field observations can hardly ever be precisely interpreted, because so many factors are varying at once, and because insects certainly react to fa~ctors which we cannot perceive and never measure. My own belief is that field observations are a necessary preliminary, but that their function is to set problems which should be solved in the laboratory. The laboratory method will eventually carry us further than we can go by working in the field, because experiments can be devised in which single factors vary. One therefore obtains a result which is precise and repeatable; from this established base further advances in knowledge can be made. Our ultimate objective is the understanding of the laws of physiology and also of physics, which will give an interpretation of what is seen in the field. But that interpretation, which is synthetic, must be based on work which is essentially analytical.

PATRICK A. BUXTON.

827

In the present paper I shall confine the discussion as far as possible to what is known about insects of medical importance. But the problems are general problems of insect physiology, and no apology is needed, or given, for introducing references to important work on insects which themselves have no relation to medicine. TEMPERATURE.

The temperature of the air is of particular importance because, generally, the body temperature of the insect approximates to it. Changes in air temperature have therefore an immediate effect on digestion, excretion, movement, reproduction, and indeed every vital activity. The effect of temperature upon insects can be dealt with briefly because a large collection of facts has been brought together by UVAROV. It is therefore sufficient to give a few illustrations of the effect of temperature upon the growth of insect populations. It is perhaps because temperature was first investigated many years ago that much of the work now appears to be inaccurate and uncritical. It is not always clear that temperature was measured among the experimental insects. Furthermore, a number of authors have exposed insects to a range of different temperatures neither measuring nor controlling humidity, though one may assume that the insects which they exposed to high temperatures were also under the influence of a high saturation deficiency. There are also particular difficulties, which we can hardly overcome at present, which arise when insects are allowed to feed at a number of different temperatures ; in such experiments it is almost impossible to separate the effect of temperature upon the insects from its effect upon the food. Much of the work on the relations of insects to temperature is fragmentary. For one species we know the temperature which is fatal to the egg on a five minute exposure; for another the temperature at which the adult will not suck blood or lay eggs. It appears that no thorough investigation has been made of the effect of temperature on any single insect of medical importance. Upper Limit.--In general it is found that, if insects are exposed to high temperatures for one hour, they die between 40 ° to 45 ° C. (104 ° to 113 ° F.). After an exposure of twenty-four hours, the lethal temperature falls generally to 35 ° to 40 ° C. (95 ° to 104 ° F.).* Though a few species can survive much higher temperatures, these figures hold good for the great majority of those insects which have been investigated. The figures are surprisingly low, particularly when one remembers the fact that, in many subtropical countries, the shade temperature goes far beyond these points for several hours daily during three or four of the hot months of the year. It is perhaps for this reason that certain insects become rare in these countries at particular seasons. The * T h e humidity of the air modifies the lethal temperature for s o m e insects, b u t not all. See page 333. B

~28

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS,

curve published by LEDINGHAM, showing the numbers of flies (mostly Musca) caught in a trap at Amara, Mesopotamia, during two consecutive years, illustrates this point. The flies were extremely abundant in April and in November, but during the hot weather and also during the winter, the numbers fell almost to zero. The figures must be accepted with reserve, because they only give the number of flies caught in the trap. It is possible that a large population of adult flies existed during the winter, but that they were rendered inactive by the cold so that they did not seek food or enter the trap. The fact that the upper lethal temperature of insects is relatively low is made use of, especially in Canada and the U.S.A., to control domestic insects and those which infest flour mills. In those countries the provision of heating is generally generous, because the winter is so cold. If the heating is used in summer, and the building is completely shut, it is easy to attain lethal temperatures in the rooms. The only difficulty is to secure sufficient rise of temperature in the actual spots where the pests exist. Inasmuch as the upper lethal temperature of most insects is about 40 ° to 45 ° C. (104 ° to 113 ° F.), it should be possible to make more use than is done at present of solar radiant heat. Many insects in hot countries live at temperatures which are not far from lethal. For instance, CHORLEY has observed that, if the pupae of Glossina morsitans are buried beneath an inch of soil, they are killed if the soil's surface is exposed to the sun. It appears that 48 ° C. (118-4 ° F.) is fatal to these pupm on half an hour's exposure. CHORLEYsuggests that death by heat may account for a considerable seasonal mortality which is observed in the pupae. There are other facts which suggest that partial control of insects might be effected in a similar way. To take another example : it is known that larva~ of A~des argenteus die after a five minute exposure to 44 ° C. (111"2 ° F.). Pupa~ fail to hatch if they are exposed for five minutes to 46 ° C. (114"8 ° F.), and eggs are killed at 49 ° C. (120.2 ° F.) though they survive 48 ° C. (118.4 ° F.) for five minutes (MACFIE, 1920). I can find no readings of the temperature in rain water gutters, but it is at least possible that in a black gutter exposed to the sun all stages of this insect would be killed ; this will not achieve major control, but it might be developed into a useful subsidiary measure. We have some reason for thinking that the temperature of water limits the natural distribution of Anopheles bifurcatus. It is a matter of observation that in Northern Europe the larwe are frequently found on the shaded side of a pond or river. In the Mediterranean they are generally common in winter and difficult to find in summer. In Palestine they breed throughout the year in cool subterranean water, for instance caves and cisterns, and in winter they are occasionally found in swamps and water exposed to the sun. One might suppose, therefore that temperature is a limiting factor, and this is borne out by the observations of WRIGHT that exposure of the larwe for one hour to 35 ° C. (95 ° F.) is fatal though they survive 32 ° C. (89-6 ° F.). It will be interesting to know what is the upper temperature limit of aquatic stages of A. gambit*

PATRICK A. BUXTON.

829

(costalis), which often breeds in shallow p o n d s fully exposed to the sun, and whether other African species have a low limit. Lower Limit.--We have just seen that, at least in hot countries, many insects exist at temperatures which approach closely to the fatal. In contrast to this it is generally true that insects can survive temperatures many degrees below what they encounter in n a t u r e ; some can also survive prolonged exposure to cold. For instance, the egg of A~des argenteus is not killed by eleven days at freezing Point (DAvis) ; according to HASE, if eggs of Cimex lectularius are kept at 2 ° C. (35"6 ° F.), there is little or no abnormal mortality if the exposure lasts ten days, and a few eggs survive thirty-five days' exposure to this temperature. Even Glossina palpalis, whose normal range of temperature is believed to be so narrow, possesses unexpected powers of surviving exposure to cold ; ROUBAUD (1909) put a pupa on melting ice for twenty minutes without killing it ; but according to MACFIE (1912), adults of this species do not recover completely after a few minutes at 2.5 ° C. (36-5 ° F.), though exposure to 7 ° to 10 ° C. (44.6 ° to 50 ° F.) for an hour is not fatal. F r o m these facts it seems probable that exposure to the degree of cold which normally occurs in an insect's natural habitat does not generally cause its death. T h e fact that large numbers of insects are dormant but living during cold seasons is important. Critical Points.--It seems that the upper or lower temperature limits which are inconsistent with the life of a population of insects come into action at one or two critical points in the cycle. If, for instance, a hypothetical population of mosquitoes can continue to exist at 18 ° C. (64.4 ° F.) but not at 16 ° C. (60.8 ° F.), it will not be found that all activities cease at the lower temperature. It is nearly always the case that a single essential action cannot be performed ; for instance, the larva fails to emerge from the egg, or the adult from the pupa, or else the adults are unable to pair or to feed. So far as insects of medical importance are concerned, it must be confessed that we cannot yet indicate all the critical points for a single species. But as examples one may quote the fact that Anopheles quadrimaculatus will oviposit at 12.8 ° C. (55 ° F.) but not at 12.2 ° C. (53..96 ° F.) (MAYNE, 1926). According to NUTTALL, Pediculus humanus does not oviposit at 21 ° C. (69.8 ° F.) : at 22 ° to 23 ° C. (71.6 ° to 73.4 ° F.), eggs are laid though the number per female per day is under one, the normal n u m b e r being about ten. But if eggs are kept at 22 ° C., they fail to hatch. It should, moreover, be remembered that communities of insects consisting of hosts and parasites generally have different critical points, so that a slight rise of temperature may affect the host m u c h more than the parasite, or vice versa. F r o m these facts one concludes that the studies on the effect of temperature on insects should be carried out critically stage by stage. Velocity of Development.--Between the upper and lower temperature limits, the rate at which an insect grows or passes through the stages of its life history depends on temperature. The relation is approximately expressed

880

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

by a simple law. The curve expressing the rate of development of some stage of an insect in its relation to temperature is approximately a hyperbola. This phenomenon is known to occur very widely throughout insects ; indeed it is a general biological law. Attention is called to the matter here because of its obvious value in relation to the study of insect outbreaks. A curve such as I

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60 ~n

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Pea,rs.

1927.

g .lO0 5

10

15

20

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FIO. 1.--Mean duration of larval stages of Musca domestica in days (curve AB, and scale at top) at a number of different temperatures. Line CD gives relation between temperature and 100 (scale at bottom). aays

have described is shown in Fig. 1, the original facts for the length of life of the larval stages of Musca domestica being derived from PratIRS. It is clear that the curve (AB) which expresses the relation of time to temperature is approximately a hyperbola. One may test this by graphing the relation of t -1" ~ to temperature ; it can be seen that this gives a straight line (CD), at any rate over the greater part of the temperature range. This shows that the first curve is in fact a hyperbola, and it is a matter of practical convenience because it makes interpolation so easy. i f the straight line in Fig. 1 be examined, it will be seen that at 35 ° C. (95 ° F.) the duration of larval life is a little longer than would be expected, and at 8 ° C. (46.4 ° F.) a little shorter. These divergencies will be found to occur generally, indeed they are frequently much greater than those shown in the figure. See, for instance, similar collections of fact for the length of larval and pupal stages of Lucilia, Calliphora and Sarcophaga (PEAIRS). These figures are based on very large numbers of individuals exposed to temperatures

PATRICK A. BUXTON.

881

between freezing point and 35 ° C. (95 ° F.). A number of authors have endeavoured to discover mathematical formulae which best fit the curves for particular insects, especially at high and at low temperatures where the curves depart from the hyperbola. In several cases the precision of mathematical argument exceeds the precision of the technique with which the original facts were collected. It is only if insects are maintained at constant temperatures that the rate of development follows the law which has just been mentioned; but under natural conditions they are subjected to fluctuating temperatures, the effect of which has been studied on numerous occasions. In general, the method pursued has been to establish the relation between temperature and velocity at several constant temperatures. Patches of insects are then subjected to temperatures which fluctuate in a regular manner, the mean temperature being recorded. The velocity under these fluctuating conditions is generally found to be greater than when the insects are kept at the same mean temperature without fluctuation. This has been shown for a large number of species, including many important pests of agriculture, and we may assume that it is of general but not universal occurrence. The phenomenon has not, as far as I am aware, been explained. If food is ample and other factors are favourable, the majority of insects will go through their life history at a velocity which is determined by temperature, and will continue to do so for an unlimited number of generations. We have at the moment live stocks of a large number of insects in the London School of Hygiene and Tropical Medicine. We have had strains of Rhodnius prolixus, A~des argenteus and Ornithodorus moubata for many years ; Triatoma rubrofasciata and ttavida, Cimex lectularius and Lucilia sericata for three to five years ; Ceratophyllusfasciatus, Xenopsylla cheopis and astia for nearly three years. One cannot say that these stocks have been under detailed scrutiny all the time, but I feel confident that under favourable conditions breeding is continuous. It is probably true to say that the majority of tropical insects, which live in an environment where the temperature is never low, are able to breed in this continuous way. Many insects of temperate countries are able to do the same, and several of them appear in the list given above. Even Anopheles maculipennis, which goes so regularly through two annual generations and no more in Northern Europe, will breed continuously if it is kept at a high temperature. Our laboratory stock is now in its eighth generation, and there has been no sign of hibernating. Ludlia sericata normally passes the winter in Britain as a full-fed larva (DAVIES), but in the laboratory it is easy to maintain a stock which breeds continuously. In our laboratory, and in others, cultures have been kept for several years without any break. On the other hand, if full fed larva~ are allowed to enter the hibernating condition, there is evidence that their development is arrested and that they cannot be rapidly brought to maturity till hibernation has lasted for several weeks or perhaps months. It must be this fact which led ROUBAUD (1922) to state that, in this species, a winter rest is obligatory, and that when winter

88~

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

approaches the larwe pass into a state of hibernation which continues irrespective of temperature. But it is probably true that in m a n y insects the number of generations per a n n u m is definitely limited to one or two, and that a prolonged winter rest in some stage must be gone through irrespective of the temperature at which the insects are kept. The subject requires very thorough investigation by a cautious and critical worker. Mortality in Relation to Temperature.--It has been shown that a population of insects has absolute limits of temperature outside which it cannot exist; the effects of temperature generally make themselves felt at certain critical points in the life history ; within the limits which are favourable to the insect, the velocity of development follows a simple physical law, or at least approximates thereto. But the effect of temperature upon insect populations is not completely defined by these statements, because a considerable mortality is generally observed if insects are kept in conditions which approach the upper or lower limits of temperature. T h e figures in Table I derived from PRAMS provide TABLE I. PERCENTAGEEMERGENCEOF LARVm(L) ANDPUPAE(P) oF Ludlia ccesar, Cadliphora vomitoria AND Musca domestica.

Calliphora.

Lucilia.

°C.

°F.

35 30 25 20 15 10 8 6.5 5

95 86 77 68 59 50 46.4 43.7 41

I

L. 46 71 79

I

P" 57 65 81

I

Musca

L°

P.

L.

P.

39 64 83

54 63 82

75 86 94

65 88 87

89

89

92

99.

97

96

84 54 36 5.2 0

83 67 7

82 59 33

87 43 1

78 72 6

83 71 20

0

0

0

an excellent example. It will be seen that, in each case, the mortality is lowest both among larvae and also pupae at 20 ° C. (68 ° F.) than at higher or lower temperatures. It is not easy to understand w h y about 90 per cent. of the individuals should go through at this temperature, with a progressively greater and greater mortality above and below it. If, for instance, the conditions at 35 ° C. (95 ° F.) permitted 46 per cent. of larv~ of Lucilia to pupate, it is difficult to understand why 90 or even 100 per cent. should not have done so. One may perhaps guess that this would not occur if perfect experimental conditions could be attained : if all factors were standardised and nothing varied except temperature, a more uniform result would be expected. But none the less it is

PATRICK A. BUXTON.

883

a matter of general observation both in the field and the laboratory that considerable mortality occurs at temperatures which are many degrees removed from the approximate limits. HUMIDITY.

It has been known for many years that the numbers of fleas, tsetse-flies, and other insects are in part dependent on atmospheric humidity. A more complete view of the matter is now possible because we can control and measure humidity in relation to entomological problems. Considerable technical advances in hygrometry have been made in relation to the storage and maturation of tobacco, etc. ; much work has also been done on problems of hygrometry in cold stores. Some of the methods which seem particularly appropriate to entomological problems have been discussed elsewhere (BUXTON, 1931b). Thus advances in one specialised field have led to unexpected results in other directions. Precise work upon the effect of atmospheric humidity upon insects is so recent that it is not possible to present a complete and logical account of it, but we can already say that humidity differs greatly in its effect on different insects, and that it may affect many physiological processes other than loss of water from the body. I propose to direct attention to a number of points which appear to be important; some of them help us to understand the effect of atmospheric humidity on insects, though their relevance in an account of insect populations is not at the moment evident. I have recently published a rather full summary of what we know about relations between insects and atmospheric humidity (BuXTON, 1932b) ; here I shall confine attention as much as possible to insects of medical importance. Spenders and Savers. We may approach our subject by observing that in relation to loss of water there are two main types of insect, connected by a series of intermediates. The insects which belong to the first type are the most numerous ; they exhibit some of the following general characteristics. They live in moist environments and eat food which contains a high proportion of water ; the proportion of water in their bodies is also high. They lose large quantities of water in their excreta and possibly through their cuticle and their respiratory system. They have no need to practise economy of water, and as they are continually taking it in with their food it is necessary for them to pass it freely. Insects of this type are not tolerant of saturated air, in spite of the fact that they normally live under conditions not far from saturation, if they are kept in relatively dry air, they lose water freely and die. The insects of the second type are capable of existing in dry environments and of living on food which appears to contain no water, many of them are resistant of starvation. They support life in deserts and also in stores of dried fruit, cereals, hairs, feathers, etc. We believe that these insects practise several different types of water economy. Indeed, it is evident that it would be useless for such an insect to reduce the loss of water from its surface if it did not also reduce the

884:

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

loss in excretion. It is convenient to have English words with which to distinguish between these two types of organism, and I propose to call them the " spenders " and " savers." An insect which is of great interest to our Society and which is clearly a " spender " i s the tsetse fly (Glossina). It has been known for at least a quarter of a century that it depends on a high atmospheric humidity. The work of MACFIE (1912), STUHLMANN (1907) and ROUBAUD(1909), not to mention other investigators, showed a long time ago that dry air was unfavourable to the continued life of these insects. Making use of the methods which were then available, each of these three authors attempted the laboratory analysis of the problem. But in spite of the fact that the control of the tsetse fly depends almost entirely on altering the climatic conditions in its haunts, very little laboratory study of its relation to dry and moist air has since been undertaken. It is my belief that a little work in the laboratory might explain and systematise the observations that have been made in the field. It should be possible for the insect physiologist to delimit the conditions of temperature and humidity which are favourable and unfavourable to the life of tsetse flies and to their digestion and reproduction. In this way, the field worker's objective would be defined with much greater precision than it is at present. I feel that we have been content too long to associate particular species of Glossina with the " BerliniaBrachystegia association" and " pioneer xeroseral communities," and to state that the fly is (or is not) to be found in " open mbugas," " kurimis," " mchaka," " miombo " and other types of environment. Enough of this Bantu-Latin ecology; let us express ourselves in physical units. It is perhaps rash to suggest a list of other insects which will be shown t o be spenders of water, but one might with confidence name the adults of Lucilia, Calliphora and Musca, the larva of X. cheopis and perhaps of other fleas. Africa can also provide a good example of a " saver." The tick which transmits African relapsing fever, Ornithodorus moubata, is widely distributed in the drier parts of tropical Africa. But it is not found in the wetter parts of tropical Africa, and if one compares the map of its distribution given by BECQU~RT with a rainfall map of the continent, one can see that it does not exist in that part of West and Central Africa in which the rainfall exceeds 60 inches per annum ; in Madagascar the tick is only found in the dry, northwesterly part. On the other hand, its power of living for months in dry sand, without food or drink, is well known. There is also experimental evidence (CuNLIFFR, 1921 and 1922) showing that, if ticks of the genus Ornithodorus are reared in moist air, the mortality is greater than in dry air ; at the same time the rate of development of those in moist air is slightly retarded. Other examples of water savers of interest to medical science are the adult X. cheopis and perhaps adult fleas in general ; also the bugs Cimex and Rhodnius. The contrast between a saver and a spender may be illustrated by the adult and the larval flea. It can be shown that the adult X. cheopis loses so

PATRICK A. BUXTON.

335

little water on an exposure of one or twenty-four hours that its thermal death point is not affected by humidity (MF.LLANBY, 1932a). LEESON showed nearly the same thing, exposing unfed adults of this species at 37 ° C. (98.6 ° F.). The mean duration of life at six different humidities ranging from 0 per cent. to 80 per cent. saturation only varied between 1.3 and 1.7 days; even at 90 per cent. saturation, the duration was only 1.9 days ; the differences in duration between successive humidities, as shown in his Table I, were not statistically significant. Clearly, therefore, the span of life of the unfed adult depends mainly on temperature and very little on humidity. But the larva is utterly different. On an exposure of one hour, it dies at the same temperature irrespective of humidity, but during an exposure of twenty-four hours its loss of water is so great that it can just survive 22 ° C. (71.6 ° F.) in dry air, 27 ° C. (80.6 ° F.) at a relative humidity of 30 per cent. and 36 ° C. (96.8 ° F.), at 90 per cent. (Fig. 2, page 336). It may well be asked what is the essential difference between the adult and the larva of this flea. It is evident that the adult is a saver of water. The loss of water from its surface, from its tracheal system and from its alimentary canal must be extremely little; one wonders whether the larva loses water from all these situations or from one of them only. The question may be answered by observing living larvae and adults by transmitted light (WIccL~SWORTn, 1932). It can be seen that the contents of the upper part of the hind gut of the larva are quite fluid and that Malpighian excretion is liquid. Lower down the hind gut there is a dilated chamber lined with large epithelial cells, and here and also in the rectum the contents of the alimentary canal become more concentrated, but the f~eces are passed as a sticky semi-solid material. Similar observations on the adult indicate that it is less affected by dry air partly, at least, because it is less wasteful of water in its excreta. In the same paper, WICGLESWORTa has published much evidence to show that the rectum absorbs water in insects of many orders; in some this function is apparently performed by the rectal epithelium as a whole, in others, by collections of special cells--the so-called " rectal glands." In some insects absorption is extremely efficient, in others less so, and the creatures' ability to live in hot dry air depends in part on this. The same author (WIGGLESWORTH, 1931a) has shown that the water which is necessary for nitrogenous excretion in R h o d n i u s is used many times. The upper part of the Malpighian tube excretes a clear solution of potassium or sodium acid urate ; water and base are absorbed in the lower part of the tube, and uric acid is precipitated, the water and base being circulated again. We have at present very little information about the loss of water from the general surface of insects, and about the possibility that they may control the loss from the tracheal system by closing the spiracles. What little evidence exists has been summarised elsewhere (BuxTON, 1932b, p. 283). It is clearly of great importance that investigations upon the permeability of chitin to water should be pushed forward. Chitin and the substances which occur with it form the barrier between the insect and its environment, and the properties of that barrier

8'86'

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

are all the more important because insects are so small that their surface is great in proportion to their weight or volume (page 338). Investigations at Sublethal Temperatures.--It has already been pointed out that the temperature which is fatal to an insect is frequently between 40 ° and 45 ° C. (104 ° to 113 ° F.), and it is clearly important to discover whether this is affected by atmospheric humidity. Much has been learnt by exposing insects to the highest temperature at which they would live, at the same time controlling the humidity of the atmosphere. It is generally found that the upper limits which are consistent with life are extremely sharply defined ; less than 1 ° C. separates the conditions which are lethal to all individuals from those i

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FIG. 2.--Highest temperature survived by certain insects, when exposed for one hour, oz" twenty-four hours, humidity being controlled. Original data from B ~ T T m for Galliphora erythrocephala adult, one hour o n l y ; and MdSL/.~UCSY(1932a) for Pediculus humanus adult, Tenebrio molitor larva, Xazopsylla cheopis adult and larva.

PATRICK A. BUXTON.

887

under which all of them survive. Fig. 2 shows that different sorts of insects react very differently to such experimental treatment, and several classes appear to be distinguishable.

Class 1 .--There is the simple case in which death is due to heat and occurs at the same temperature irrespective of humidity. The larva of Rh. prolixus falls in this group, also the adult of X. cheopis and of C. lectularius. All these insects die at a particular temperature irrespective of humidity, both on an exposure of one hour and of twenty-four hours. The larva of X. cheopis and adult of P. humanus and L. sericata fall into the same group if the experiment lasts one hour. Class 2.--Certain insects lose so much water in dry air that the temperature which is fatal to them is lower in dry air than in moist. The larva of X. cheopis and the adult of Pediculus and Lucilia show this type of reaction on a twenty-four hour experiment. It is particularly clearly shown by the larva of X. cheopis. Class 3.--Certain insects are capable of surviving a higher temperature in dry air than in moist, at any rate for an exposure of one hour. This can be shown for mealworms, provided large individuals are chosen. Class 4.--Several insects, of which the only one of any medical interest is the adult Calliphora, fall into an intermediate group. On an exposure of one hour, Calliphora can survive 39 ° C. (102-2 ° F.) in saturated air and 40 ° C. (104 ° F.) in nearly dry air. But at a relative humidity of 70 per cent. the insect can survive 41 ° C. (105.8 ° F.). " Survival " signifies that at least eleven out of twelve experimental insects were alive twenty-four hours after the experiment was carried out. (The data quoted above can be found in the following papers : BEATTIE, 1928 ; ]~ODENHEIMER,1927 ; BUXTON,1931a ; ~IELLANBY,1932a ; NECHELES, 1924. The facts for Cimex have not been published ; adults survive 43.5 ° C. (110"3 ° F.) for one hour and 39 ° C. (102"2 ° F.) for twenty-four hours at humidities from 0 to 90 per cent.). If the facts set out above are carefully considered, it will be seen that the four classes are not to be rigidly distinguished from one another. One may assume that the insects in the first class are savers, but many insects only fall in this class if they are exposed for one hour ; if the exposure is prolonged they lose a material amount of water in dry air in which the fatal temperature is lower than it is in moist air. Reference to Fig. 2 shows that the adult Pediculus is an example of such an insect. On a one hour exposure it falls in Class 1, surviving 46-4 ° C. (115.5 ° F . ) ; but it survives 38 ° C. (100"4 ° F . ) i n moist air and 33 ° C. (91"4° F.) in dry air on a twenty-four hour exposure, falling therefore into Class 2. Even such an insect as C. lectularius, which can only just survive 39 ° C. (102.2 ° F.) at all humidities on a twenty-four hour exposure, is in fact losing water. For if bugs are kept at 37 ° C. (98.6 ° F.) for three days, it is found that six out of ten die in dry air, whereas only one or two out of ten die at higher humidities. A similar effect of dryness on length of

888

THE EFFECT OF CLIMATIC CONDITIONS U P O N P O P U L A T I O N S OF INSECTS.

life is observable at lower temperatures, provided the exposure be longer (MELLANBY, 1932c). The insects which are placed in Class 3 are able to survive a higher external temperature in dry air than in wet. One must assume that they evaporate water when they are placed in dry air, and cool their bodies in this way. It is quite easy to show that such cooling takes place ; as MELLANBY (1932a) has pointed out, it can easily be demonstrated by inserting a small mercury thermometer into the vagina of a cockroach. If the insect is exposed to moist air at 45 ° C. (113 ° F.), its body temperature quickly rises to that figure, but if it is exposed to dry air at the same temperature it can maintain its body temperature at about 39 ° C. (102"2 ° F.) for a considerable period. A similar phenomenon is known to be widely spread among the insects ; it has been recorded in hawk moths, locusts, grasshoppers and certain beetles. MELLANBYmade the interesting observation that it is only large mealworms, weighing about 100 mg., which are able to withstand a higher temperature in dry air than in moist on an hour's exposure. Smaller individuals weighing under 30 rag. have not this power. This led him to consider the physical process involved, and the effect of size upon gain of heat and loss of water from the body. It will be remembered that the volume or weight of a cylindrical object varies with the square of the radius, whereas the area of its surface varies with the radius. If, therefore, we compare two insects of similar proportions but different size, we shall find a great difference in the ratio of their volume, which is a measure of the amount of water in them, to the area of surface through which they receive heat by conduction. The figures given in Table II will make this clear. T~

II.

S H O W I N G THE RELATIONS OF SURFACE TO V O L U M E I N

Insect. A B

C

Dinxensions ram.

10×1 10×3 100 × 10

Surface sq. ram. ratio. 33 108"5 3300

1 3.3

100

THrum

IDEALISED INSECTS.

Volume cu. ram. I ratio. 7.8 71 7850

m•

1 9 1000

Area of Surface corresponding tO 1 C.C.

sq. ram.

ratio.

41.2

1 "0

15.3 4-1

0"37 0.10

I have taken three insects--A, B and C--which are assumed to be cylinders with flat ends, of the dimensions given in the table. From the dimensions the area of the surface and the volume of each of the three insects have been calculated. Insects A and B are of the same length but different thickness ; it will be noticed that the increase in respect of surface is as 1 to 3.3, but the increase in volume as 1 to 9. Knowing the volume of an individual insect and the area of its surface, one may calculate the area which corresponds

PATRICK A. BUXTON.

889

to 1 c.c. of in6ect. The figures are given in the last two columns of the Table, and it will be seen that if A and B are compared, there is a reduction of surface per unit volume from 1.0 to 0-37. If A and C are compared, the differences are much greater, the increase in surface being one hundred times and in volume 1,000 times. The reader will note that the dimensions given for A and C are by no means extreme; many insects increase in a much greater proportion between hatching from the egg and the end of their growing period. The conclusion that the larger the insect, the less surface it has proportionately, is important; it means that at high temperatures the larger insect will receive proportionately less heat by conduction. If it is to balance the receipt of heat by expending water which is evaporated, the larger insect is more favourably situated than the smaller. MELLANBY(1932a) has illustrated this by two concrete instances. A cockroach weighs 1 gram, and has a surface of 8 sq. cm. If we assume certain values which have not yet been measured, we find that, if the insect's body is at 40 ° C. (104 ° F.) and the air is at 45 ° C. (113 ° F.), it will absorb 40 calories per hour. To balance this it will evaporate 80 mg. of water or 8 per cent. of its weight. These are experimental figures showing that loss of weight at this rate is possible. But a louse weighing 3 mg. with a surface of 18 sq. mm. would receive 1 colorie under the same experimental conditions; to balance this it would have to evaporate 2 mg. of water or about 70 per cent. of its weight per hour. This is clearly impossible, though one must remember that even a small insect may be able to regulate its temperature a little if the external temperature is not so high as the figures quoted, particularly if the insect is continually feeding--as an aphis or a caterpillar is. We may state in general terms that a large insect placed in hot dry air may (or may not) regulate its body temperature by evaporation. But a small insect cannot do this, and if it is to exist under these climatic conditions it must do so by being a saver and by having a high thermal death point. The extent to which some small insects can resist loss of water is remarkable. The unfed first stage larva of Rhodnius weighs only 0.5 mg. and is extremely slender, so that its ratio of surface to volume must be very high; but even on an exposure of twenty-four hours, it dies at the same temperature in moist and in dry air (BuxTON, 1931a). Harvesting ants, Aphenogaster barbara, have been observed in Palestine running on the soil at mid-day when the temperature of the surface was between 50 ° and 55 ° C. (122 ° to 131 ° F.) (BuxTON, 1924). But it is doubtless true, as KENNEDY says, that the great majority of really small insects inhabit very moist niches. Law of Saturation Deficiency,--It has been shown that the metabolism of the fasting mealworm does not of itself cause loss of weight. The same is true of the bed-bug (MELLANBY,1932 b and c) From this it follows that if these insects are starved and weighed at intervals, their loss of weight is a direct measure of loss of water. Work on the fasting mealworm has also shown that its loss of weight is

840

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

proportional to the saturation deficiency* of the atmosphere, at any rate between 23 ° and 37 ° C. (73.4 ° to 98.6 ° F.), except in very dry and very moist air (BuxTON, 1930 ; MELLANaY, 1932b). But it is clear that this law, in its simple form, cannot be applied to insects in general. F o r instance, the loss of water from Cimex is m a n y times greater in moist air (or less in dry air) than it would be if it were directly proportional to the saturation deficiency. I t is certain that Cimex possesses some mechanism for controlling loss of water into dry air, and it seems probable that it does so b y closing or partly closing the spiracles (MELLANBV, 1932C). Investigations have also b e e n made on the loss of weight of fasting Rhodnius, at 23 ° C. (73.4 ° F.) and different controlled humidities. Individuals kept in dry air lose more water, but also oxidise less solid matter, than those kept at higher humidities. As this complication exists, loss of weight of Rhodnius does not measure loss of water and has no relation to saturation deficiency (BUXTON, 1932C). An attempt has also been made to discover whether the duration of life of fasting insects is determined by the saturation deficiency of the air. Working at a single temperature (32 ° C. 89"6 ° F.) with adult X . cheopis, the figures accumulated by BACOT and MARTIN showed that this was so, and their paper was the.first in which a p h e n o m e n o n in insect physiology was related to saturation deficiency.t T h e y worked on fleas of u n k n o w n age, taken from a cage containing a rodent. Similar work, but on unfed fleas known to be less t h a n o n e day old, has been recently published by LEESON. He has studied *The term "saturation deficiency" is not yet generally understood. It is familiar that a space will hold more water vapour at a high temperature than at a low. From physical tables one discovers that the quantities of water vapour which will saturate the same space at 15° and 30 ° C. are as 12.8 to 31-7 : (actually these figures are vapour pressures expressed as millimetres of mercury, but the figures for grams per cubic metre would be in the same proportion). From this it follows that if two atmospheres at 15° and 30 ° C. were each of them 60 per cent. saturated, the additional quantities of water required to complete saturation would be as 5.1 to 12.7. In fact, the amount of water required to (12.7~ produce saturation at 30 ° C. would be about 2½ times ks-Ti-/ that required to saturate the same volume at the lower temperature. In other words, these two samples of air, though they are at the same relative humidity, would require very different quantities of water to bring them to saturation. The quantity of water which is required to do so is what is referred to as the " saturation deficiency." It will be observed that, if the loss of water from an insect is proportional (as we now believe) to the saturation deficiency, it cannot be proportional to the relative humidity, if experiments at different temperatures are being compared. t T h e epidemiology of plague in India appears to be best measured, so far as atmospheric humidity is concerned, by saturation deficiency. This was pointed out some years ago by BROOKS; the same measure of humidity is used by ROOERSin forecasting plague. On the other hand, OTTEN has shown that the relation between plague and saturation deficiency in Java is the reverse of what it is in India. The epidemiology of plague is perhaps outside the range of the present paper. It is possible that in Java conditions are so moist in the wet season as to be unfavourable to Xenopsylla, and that conditions are more favourable in the so-called dry season (which is by no means so dry as that of Northern India).

PATRICK A. BUXTON'.

841

the duration of life of X, cheopis and two other species over a wide range of conditions of temperature and humidity, and showed conclusively that the duration of life c o u l d not be directly related to saturation deficiency at any temperature. The discrepancy between the work of BACOT and MARTIN, and that of LEESON, is very great, in spite of the fact that both investigations command the reader's confidence. One may venture to suggest an explanation, which has not yet been tested. W e may suppose that LEESON'S fleas, unfed and of uniform age, died of exhaustion of some solid reserve ; this accounts for the small effect of atmospheric humidity upon them. But BACOTand MARTIN'S fleas, which came from a rodent, and were of uncertain age, had had opportunities of feeding, and some at least of them died of exhaustion of water; for this reason their length of life was proporiional to the saturation deficiency. At any rate, it is clearly proved by LEESON that, so far as unfed fleas are concerned, humidity (however we measure it) has little effect on the duration of life, which is mainly determined by temperature. FREEBORN (1932) has discussed the effect of temperature and humidity on the life of A. maculipennis under wild conditions in California. He has also carried out laboratory investigations under controlled conditions of temperature and humidity. He gives no indication of the numbers, age or sex of his specimens, nor of the consistency of the results. His figures seem to show that the effect of humidity on length of life is little, and that the effect of temperature at about 25 ° C. (77 ° F.) is critical ; for instance, at 80 per cent. humidity and 24"5 ° C. (76 ° F.) the mean duration of life was 30 days, at 26-7 ° C. (80 ° F.) it was 3-5 days. One cannot help thinking that the experiments were carried out with insects which were in quite different states of nutrition, or that some other grave source of error was not eliminated. He observes that the duration of life is shorter at higher temperatures (at all humidities). It is, of course, familiar that all biological processes are more rapid at higher temperatures ; his hypothesis that the water-binding powers of the insect are a function of temperature has no experimental basis, and seems to have little inherent probability to support it. JONES' work on the unfed first nymph of the bed-bug (C. lectularius) shows that the length of life of individuals kept under identical conditions varies greatly, so that no very close approximation to any physical law is to be expected, particularly as he only used twenty-five insects in each experiment. But it cart be shown from his figures that duration of life is much more nearly proportional to saturation deficiency than to any other measures of humidity. There is also evidence that the loss from certain insect eggs is directly proportional to the saturation deficiency. PARKER exposed eggs of a grasshopper to ten different humidities at each of four temperatures and delimited the conditions under which 50 per cent. and also 20 per cent. of the eggs hatched. I have been able to show that his limits, far from being empirical, are in fact lines of equal saturation deficiency; and there are other collections of data

84~'

THE EFFECTOF CLIMATICCONDITIONSUPON POPULATIONSOF INSECTS.

showing that the eggs of insects are frequently subject to the same physical law (BUXTON, 1932b, p. 307). It may be well to illustrate how this knowledge can be applied. JONESexposed eggs of C. lectularius to five different temperatures (15 ° to 33 ° C. ; 59 ° to 91.4 ° F.), at 75 per cent., relative humidity in each case. It is clear, therefore, that the saturation deficiency is greater at the higher temperatures, indeed it is 3.2 mm. of mercury at 15 ° 'C. and 9.4 mm. at 33 ° C. If the eggs of Cimex lose much water, we should expect a mortality under conditions of higher saturation deficiency ; failing that we should expect a prolongation of the egg stage. (This is consistent with what is known of the m

o Egg

10 20 stage ~n days

°c 30

25

20

D

"-----~ B 5

io

15

20

1oo

F]o. 3.--Duration of egg stage of Cimex lectularius in days (curve AB), in relation to temperarare ; also relation of temperature t _1o--~ 00 (CD). days Original data from JONES. eggs of many insects.

See BUXTON, 1932b, p.312). But if we plot the figures 100 given by JONES, using temperature against ~ (Fig. 3), we obtain a straight

line, except for one aberrant observation. From this we conclude that rate of development depends on temperature, and that increasing saturation deficiency has no measurable effect on these eggs. This is confirmed by the fact that there was no mortality in any of the experiments. # To sum up we may say that the loss of water from certain species of insects and their eggs is directly proportional to the saturation deficiency over a wide range of conditions. Others possess powers of controlling the loss to some extent, especially when they are placed in dry air. Some of them possess such powers of reducing their loss of water that they are able to survive very high saturation deficiency for hours, days or weeks. But the fact that the loss of

PATRICK A. BUXTON.

~4~

water is determined in the first instance by saturation deficiency and not by any other measure of humidity is of great importance. In analysing insect populations, whether in the laboratory or in the field, we now have an appropriate scale. The Physiology of Water Saving.--Several insects of economic importance, among them bed-bugs and pests of stored products, are obviously " savers." The fact that they can resist dryness and starvation increases the difficulty of control. Studies on their reaction to climate are therefore relevant to this discussion. Another fact of some interest was discovered in the course of the work upon the mealworm. If mealworms are exposed to different conditions of humidity but the same temperature, the weight at the end of the experiment is determined by the amount of water that had evaporated from them, but the proportion of wet and dry matter in the insects is maintained in spite of great alterations in gross weight. The regulatory mechanism is chemical, the animal burning fat at a rate sufficient to replace the water it loses by evaporation; in other words, the rate of metabolism is directly determined by saturation deficiency. WIGGLESWORTH (1931b) has demonstrated that the proportion of water in the tissues of a flea remains constant ; he makes use of a different method. It will be remembered that the ultimate twigs of the tracheal system contain fluid, and he has shown that the extent of air into these twigs is almost certainly a measure of the osmotic pressure of the tissue fluids. WIGGLESWORTH observed the extent of air in the tracheoles in a fasting flea (Geratophyllusfasdatus), and noticed no alterations until after the death of the animal ; from this one may conclude that, in spite of evaporation, the osmotic forces are stabilised ; one must therefore suppose that the proportion of water in the insect's body is maintained at a constant or nearly constant amount. But other insects, which lack this mechanism, accommodate themselves to great alterations in their water content during starvation. Normal adult C. lectularius contain about 33 per cent. of solid matter. If they are kept fasting, the proportion rises to 45 per cent., when some of the insects die, though some of them will still be found alive when the proportion has risen to 50 per cent. The amount of dry matter which a fasting Cimex oxidises is determined only by time and temperature, and not by humidity (MELLANBY, 1932C). Experiments with adults of Rh. prolixus show that in many ways it reacts to humidity in the same way as the bed-bug does, but Rhodnius is more complicated than Cimex for it consumes more dry matter in moist air than in dry, other things being equal (BtlXTOl~, 1932c). In spite of the fact that all three insects are " savers, " Rhodnius and Cimex are entirely unlike the mealworm, in that they both tolerate very great increases in the proportion of dry matter in the body. Other insects are known, none of them of medical importance, which are capable of resisting very great loss of water, and of living in a dry and dormant state for months or even years. The mealworm possesses the mechanism for regulating the proportion of

844

THE EFFECT OF CLIMATIC CONDITIONS U P O N P O P U L A T I O N S OF INSECTS.

water in its body, to which reference has just been made. The only condition under which it cannot maintain its balance between solid and water is when it is kept at a humidity of 90 per cent. or over. If mealworms are exposed without food to a 90 per cent. humidity at 23 ° or 30 ° C. (73"4 ° or 86 ° F.), most of them gain in weight, and the gain is water. The ~verage gain is so great that it cannot be due to the insect retaining water formed during metabolism, and it must be due to absorption of water from the atmosphere. This power of absorbing water is a source of embarrassment to a mealworm if it is kept continually at 90 per cent. humidity. But it might be of great value to it under more ordinary climatic conditions, where the relative humidity approaches saturation by night. It is probable that other insects possess a similar power, but only one insect of medical importance is thought to do this. This is the newly emerged larva of Cimex, which appears to be able to absorb moisture from the atmosphere, but WICaLESWOaTH (1931C), in describing his observations, recognised the possibility that the larva was drinking water of condensation, and the matter requires further study. The adult bed-bug certainly has no power of taking in atmospheric water (MELLANBY, 1932c). The mechanism by which this absorption might take place is obscure, and the whole matter is very little understood. It has been discussed elsewhere, and I have given reasons for thinking that this power of gaining water from an atmosphere which is not saturated with water may be widely distributed among insects (BUxTOl~, 1932b, p. 280). If this is the case, it is a matter of the greatest interest and importance. Transmission of Micro-organisms.--The reader who has observed that the effect of humidity on insects is very far-reaching, may well ask whether it can extend to the insect's power of transmitting a micro-organism. It is at least possible that the whole internal economy of a vector may be so different in dry air and in moist, as to limit its power of transmitting a parasite under certain conditions of atmospheric humidity. The point is clearly important, for the hygienist is concerned directly with the individuals which are capable of conveying infection rather than with the gross population of the insect. It is probable that many workers in the tropics have asked whether humidity could have this effect ; the earliest reference I have been able to trace is to BENTLEY (1911). The question has been investigated by MAYNE (1928). He worked at Saharanpur, Punjab, between February and September, and dissected a large number of Anopheles of several species. Up to the middle of June, the temperature rose and the air was dry ; after that date, owing to the influence of the monsoon, the humidity rose, and in July and August was very high. The results of dissecting Anopheles are tabulated fully ; up to the middle of August, 1,306.4. culidfacies were dissected, all of them being negative ; after that date, 715 were dissected, five of them being positive. The author goes on to ask whether the facts are to be explained by supposing that the duration of life of the insect was generally so short in the dry part of the year that infection

PATRICK A. BUXTON.

845

with Plasmodium was impossible or very rare ; or whether the high evaporation during the earlier part of the investigation prevented the development of Plasmodium in the mosquito. In a later paper (MAYNE, 1930), he endeavoured to answer that question. He kept the mosquitoes in lamp chimneys in incubators and influenced the humidity by dishes of sulphuric acid. The method appears to be unlikely to give any accurate control of humidity, owing to leakage whenever the humidity outside and inside the incubator was widely different. Experience shows that in so large a space some mechanical circulation of air is requisite. Furthermore, the mosquitoes were allowed to feed on moist sponges and in some of the experiments birds were present. Moreover, the humidity was measured by a stationary wet bulb thermometer, a method which gives erroneous results, particularly when the humidity is low. I make these criticisms of technique because the investigation is clearly of great interest and importance and because the technical difficulties are not insuperable. The facts published by MAYNE show clearly that the duration of life of adult Culexfatigans and adult Anopheles of several species is much affected by humidity, the insects living a longer time in moist air than in dry. The rate of development of the infection of Plasmodium praecox in C. fatigans appears to depend on temperature alone. In one crucial experiment, some of these mosquitoes were fed on a sparrow and then divided into two lots which were kept at different humidities. N o significant difference in the development of the infection can be detected, mosquitoes killed at the same interval after infection being compared. The problem of the transmission of Filaria bancrofti by mosquitoes under different climatic conditions is similar though more complex. There seem to be good a priori grounds for supposing that low humidity imposes limits on infection with Filaria ; indeed one might say that filarial infection is a disease of the damp tropics. It is known that many efficient vectors of the worm exist, among them C. fatigans, several species of Anopheles and members of other genera. Remembering how abundant these mosquitoes are in human dwellings in nearly all warm countries, it is surprising to observe that filarial infection is absent from large parts of the world where the vectors are abundant, and that in the countries in which it occurs its distribution is not uniform. Recent work on the distribution of filarial infection in India provides an excellent illustration. The map published by ACTON and RAO shows that the infection is present in most of Bengal and Assam, along nearly the whole extent of the east coast of Peninsular India and most of the west coast as far north as the Gulf of Cambay. It is absent from Orissa and the feudatory States except the seaboard ; also from a part of the Madras Presidency which reaches down to the coast opposite Ceylon ; also from Sind and the Punjab, the recent work of KORKE extending our knowledge so far as the Punjab is concerned. If the map published by ACTON and RAO is compared with a rainfall map of India, and particularly with a map for the monsoon period from June to October, it will be seen how closely the presence of infection coincides with the area of high

846

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

rainfall. It is interesting to observe that that part of the Madras Presidency from which filarial infection appears to be absent is just that part in which the monsoon rainfall is below 10 in. ACTONand tD,o, and also KOaKE, have called attention to the fact that the disease is almost entirely limited to low lying coastal plains, and that it is often associated with rice growing. May we n o t explain these observations, and also the relation to rainfall, by supposing that atmospheric humidity is a limiting factor ? What can be the explanation of the absence of filarial infection from the regions of lower humidity ? Three possibilities exist : that under certain climatic conditions the insects cannot live long enough to become infective ; that humidity affects their physiology in such a way that, though they live long enough, the worm cannot reach its infecting stage ; that dry air prevents infection of man at the critical moment when the larvae leave the proboscis of the mosquito and pass on to the surface of the skin. We have not at present facts to enable us to say which of these explanations is correct. RAO and IYENGAa have observed that, if C. fatigans is fed on men whose blood contains about 20 microfilari~e per 20 cu. mm., the proportion of mosquitoes which becomes infected varies with the season ; also the number of worms which complete their development appears to vary. We now require experiments under controlled conditions of temperature and humidity. The results will demand the most critical scrutiny, because doubtless individual mosquitoes fed and maintained under identical conditions will develop widely different numbers of filari~e. Other Effects of Humidity.--In addition to the points to which attention has been called, it is now becoming evident that humidity affects insect life and reproduction in many ways. It affects the duration of life, not only of active feeding stages (adults and larvee), but also of eggs and pupae. Low humidity causes mortality among certain pupae (Stomoxys, MELVIN; Lucilia, WARDLE). Atmospheric humidity is presumed to have great effect on the behaviour of insects, and it is evident that some must possess the power of finding water to drink or to lay eggs in ; and it appears probable that certain insects can choose positions where evaporation is less, if the proportion of water in their bodies is too low. If further information is desired, it may be found in a paper in which I have recently summarised our knowledge of this subject (BuxTON, 1932b). It appears that humidity also has an effect upon the efficiency of certain fumigants (LINDGREN and SHEPARD). OTHER CLIMATIC ~ACTORS.

I have dealt at some length with the effects of temperature and humidity because it is clear from field observations that these factors are important, and a considerable body of experimental fact is available. With regard to other climatic factors, we have at present very little knowledge. One has the best of grounds for supposing that the quantity and quality

PATRICK A. BUXTON.

847

of the radiation which we receive from the sun at different times of year, will later be shown to have a considerable effect on insect populations. It is already known, for instance, that grasshoppers dispose themselves in such a way as to take up radiant heat, unless the intensity of radiation passes a certain height, when they dispose themselves so as to avoid it. This enables them to regulate their internal temperature and to raise it above that of the air, particularly in the early morning; this in turn affects their activities. Moreover, when one remembers the extreme importance of sunlight to all green plants, and its effect on green crops at different times of year, one must suppose that an indirect effect upon phytophagous insects will eventually be demonstrated. This may have an immediate relation to tropical hygiene, for the larv~ of Anopheles and many other mosquitoes live to a large extent upon green algae. But perhaps the most important of all the effects of radiation upon insects is in relation to their behaviour. One has only to remember their trophisms towards light or away from it, their colour choices, the fact that some at least of them perceive ultra-violet (LuTz) and the fact that many of them will only pair or only lay eggs under certain conditions of illumination. The recent work of TATE and VINCENT, showing that hibernating C. pipiens can be caused to lay eggs if they are exposed to bright light, serves to indicate how important illumination may be, and how unexpected in its effects. We have therefore many reasons for supposing that the activities and also the numbers of insects depend on the solar climate. The possibility that barometric pressure has effects on insect biology has often been discussed, without giving rise to much critical work. The effect of wind and convection currents is perhaps considerable. They doubtless increase evaporation, and may cause places to be unsuitable to insects of the spender type ; it is known that wind has this effect on certain green plants. The transporting effect of wind, and particularly of steady currents in the upper air, may lead to the wide dispersal of small insects, and to their occasional anival in numbers in places in which they are not normally found ; of this there is a considerable body of positive evidence, some of it collected by aeroplane survey. FIELD WORK. It may justly be said that the results of physiological enquiry in the laboratory are of little value in themselves because experimental conditions are artificial. If we wish to interpret our knowledge and apply it in the field, we require measurements of the climatic conditions in the spots where the insects actually live. It is essential to realise that great differences exist between the climate in these spots and that which is generally studied and recorded by the meteorologist ; moreover, if measurements are taken in spots which are perhaps only a few yards from one another, astonishing differences are sometimes revealed. It would perhaps be true to say that the important differences between the insect's environment and the conditions under which the meteorologist exposes his

848

THE EFFECT OF CLIMATIC CONDITIONS U P O N P O P U L A T I O N S OF INSECTS.

instruments are due to two main causes ; most meteorological instruments are carefully protected from solar radiation, which may produce important effects on the climate where the insects are ; evaporation of moisture from the soil or from plants very frequently raises the absolute humidity in the same positions. In general terms, one may classify the environments in which we are interested into those which are exposed and those which are sheltered. A typical exposed environment is the surface of the soil. Here, owing to high temperature by day, great climatic differences exist not only in temperature but also in the evaporative power of the air ; insects in this position are also exposed to light, etc. It appears that the exposed environment has little to do with problems of medical entomology, for few if any of our insects exist in these places. Most of the insects which are vectors of disease exist in sheltered environments, for example--mosquitoes which hide by day in bushes, bugs which take refuge Retative

humidity

40 12 +

•

15 +

cent. 80

o

•

+

,,.

•

o +

Screen.

Shed. Barn.

~ ~/

,s

/,.:o

o +

.,~~" •

/

per

60

~/

/

7 /

o. o,,/

/

/

1~°o,,

/,,÷"

/

/

3 •

FIG. 4.--Three-hourly means of temperature and humidity, derived from three thermohygrographs exposed in a meteorological screen, a shed and a barn in Jerusalem, 14th to 25th July. The three curves are lines of equal saturation deficiency

349

PATRICK A. BUXTON.

in cracks in walls, fleas which live in rat-holes or among the fur of the animal, and the louse which exists between the body and the clothes. The methods which have been standardised by the meteorologist are suitable for investigating some of these environments. Readings can easily be taken in a forest, a cellar or a cave, whirling dry and wet bulb thermometers being the most suitable for measuring temperature and humidity ; recording instruments can also be installed in these places. A considerable body of fact, collected among growing plants, in caves, etc., already exists. An example may illustrate the method, and the type of result which it gives. During the summer of 1931, I had an opportunity of making studies in Jerusalem, Palestine. Similar instruments were exposed in three positions : the first was a Stevenson's screen in a good open position, the second and third were both in a building which was solidly constructed of stone and mortar. The ground floor, or " shed," was used for cows by day and by night ; the floor was wet with manure, and ventilation was very restricted. The second floor, or " barn " had a tiled roof and no ceiling ; ventilation was very free through the tiles and also at both ends of the building. The barn was full of straw and stacks of grain, and contained no animals. In each of these three positions I exposed a thermohygrograph for twelve consecutive days during July. From the collected records of these instruments, three-hourly means of temperature and humidity have been calculated. They are shown in Fig. 4, the original data have been published elsewhere (BUXTON, 1932a ; Table I). The figures are summarised in Table III. TABLE III. READINGS OF TEMPERATURE AND HUMIDITY FROM A STANDARD METEOROLOGICAL SCREEN,

A COW SHF_J),AND THE BARNOWR IT, IN JULY, IN JERUSALEM. Temperature (°C.)

Humidity. r

Screen Shed

.. ..

Barn

..

I,

Mean

Vap.

Sat.

Mean.

Max.*

Min.*

Range.

Rel. Hum.

Press. mm.

Def.

24.0 26.2 26.7

33.0 29-5 35.0

16.5 22.5 20-5

16.5 7.0 14.5

63-4 61.6 56.6

14.0 15.7 14.8

8.4 9.8 11.5

#From records of therraohygrographs. The following points should be noticed. If the screen and barn are contrasted, it will be observed that the barn is several degrees the hotter, the mean difference being 2"7 ° C. (4.9 ° F.). The range of temperature in the two places is not very different. As to humidity, the fact that the vapour pressures are nearly identical shows that little evaporation was taking place from the contents of the barn, and the significant difference in saturation deficiency is therefore due to temperature alone. A contrast between the screen and the shed shows

850

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

that the shed is warmer than the screen and that the range of temperature in it is less than half. This is doubtless due to the fact that it is a solid, rather illventilated building. The vapour pressure of the shed is materially higher than in the screen, doubtless owing to loss of water from the cows, manure, etc. In spite of the higher mean temperature in the shed, saturation deficiency is only a little greater than outside. The difference between these two environments is material ; a community of insects could reproduce itself more rapidly in the shed than outside, and if one were studying the matter one would be misled if the meteorological data were employed. For instance, if we interpolate from the figures published by BODENHEIMER(1931), we find that at the mean temperature of the screen (24.0 ° C., 75.2 ° F.) M. domestica would go from egg to adult in sixteen days, at the temperature of the shed (26.2 ° C., 79.2 ° F.) in fourteen days. The reader will notice that the difference between the screen and the barn is simple, that is to say there is a difference in temperature but almost none in vapour pressure. But the difference between the screen and the shed is complex, being due in part to temperature and in part to evaporation, which raises the vapour pressure in the shed. It may be said that many insects of medical importance do not breed in spaces so large as sheds. What can we find out about the conditions in a rathole or in the middle of a heap of grain ? It is much less easy to study environmental conditions in small places. Temperature can be read, and even recorded, provided the worker is ingenious and has adequate material resources, but humidity presents technical difficulties and I am not familiar with any method of obtaining a current record of humidity in a remote and inaccessible spot ; it is, however, possible to take readings from time to time ; it appears that a small dewpoint apparatus is the most convenient. Inasmuch as we know that temperature in the ground varies very little within the twenty-four hours, we may assume that evaporation will also be constant, so that observations made at intervals inside a rat-hole probably give a fairly accurate indication of the climate which prevails there at all times of the day and night. Working in the shed to which reference has already been made, I carried out investigations i n two rat-holes, measuring temperature with a very short thermometer tied to ~a piece of wire, and aspirating small samples of air with an india-rubber tube and estimating their humidity by dew point.* Though the holes were very close to one another, the climate in them was quite different, as was shown by observations made on six separate occasions during the hours of daylight. The original figures have been published elsewhere (BUXTON, 1932a, Table II), and the raeans are shown in Table IV. *Messrs. Casella have recently designed a small dew point apparatus, based on suggestions made by myself and Dr. F. MARSH. T h e mirror is flat, which makes observation of the dew easier than on a rounded thimble. T h e sample of air under examination, only a few c.c., is enclosed between a flat sheet of glass and the mirror. T h e apparatus is small and robust, and I think it is more convenient for most entomological purposes than any other type with which I am familiar.

851

PATRICK A. BUXTON.

TABLE IV. SHOWING HUMIDITY IN A COW SHED, AND IN Two RAT-HOLES WHICH OPENED INTO IT, IN JULY, JERUSALEM. i

Mean Temp. °C.

Mean Vap. Press. nltn.

Shed Hole A Hole B

.... .... ....

14.4 21.9 16.7

27.4 27.0 27.0

Mean Sat. Def. Inrn.

12.9 4.7 9.9

F r o m these figures it will be noticed that, though the temperature was the same, at any rate at the times when the observations were made, there are great differences in humidity. T h e vapour pressure in Hole B was a little higher than in the shed, b u t in Hole A it was very m u c h higher. T h e effect of this was to reduce the mean saturation deficiency in Hole A to about two-fifths of what it was in the shed. T h i s difference in saturation deficiency is enough to have a great effect on the life of flea larwe. Investigations made in other rat-holes in Palestine showed that the humidity was high in nearly all of t h e m ; evaporation from the soil was active though the work was done during the rainless season (see Fig. 5). ,Saturot;on

deliciency

mm

Hg 88

4

FIG. 5.--Climate inside and outside rat-holes, in Palestine, during hours of daylight in June and July. Each line represents a single observation, the black spot indicating temperature and saturation deficiency 2 or 3 feet down the hole, the other end of the line indicating conditions in the open at the same moment. (From Ind. j%urn. Med. Res., xx, 291.)

859.

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

Another niche of interest to the hygienist is that between the clothes and the body. One may assume that the temperature is relatively stable. It is known that a man at rest loses quantities of water by evaporation through the skin (WHITrHOUSE, HANCOCK and HALDANE), and during exertion additional quantities are secreted as perspiration. But without actual measurement, one cannot discover what the saturation deficiency is. The recent work of MrLLANSY (1932d) has provided some information. He made use of a chemical hygrometer, and aspirated into it a known volume of air (about 10 e.c.) from beneath the shirt. This air was then dried with sulphuric acid and the reduction in its volume observed. He worked only with a man at rest, and made no studies of the conditions that follow exercise. The man was allowed to sit in air at a number of different temperatures. His readings of the temperature and saturation deficiency of the air of the room and beneath the shirt are summarised in Table V. TABLE V . CLIMATIC DATA IN AIR OF ROOM, AND BENEATH CLOTHES OF SEDENTARY INVESTIGATOR.

Temperature (°C.). Max.

Min.

Range.

Outside

41

0

41

Under shirt ~~

37

23

14

Saturation Deficiency. (mm. Hg.). ! Max. Min. Range 37 18

i

1.4 13

,

35.6 5"6

It is evident that the conditions beneath the clothes bear little relation to those in the room. The range of temperature was one-third of that in the room, and the range of saturation deficiency only about one-sixth. APPLICATIONS.

It is sometimes said--at least by those who have devoted little thought to the matter--that the study of the effect of climate on insects may be interesting, but that it has no relation to practical problems because climate is uncontrollable. Even if it were true that we cannot control climate, the argument would be misleading because a better knowledge of this side of insect physiology will surely enable us to forecast outbreaks of insects. At present nearly all the forecasting is based on standard meteorological data collected in places where the insects themselves do not live. Moreover, many of the smaller meteorological stations only observe their instruments once in the day, generally at 9 a.m., so that their figures give no indication of the average conditions of the spot where they were collected. Fuller knowledge of the effect of climate on insects should also enable us to map the areas to which certain troublesome insects may eventually spread. For instance, the chigger flea (Tunga penetrans) is a

PATRICK A. BUXTON.

858

native of South America which has spread widely over parts of Tropical Africa, and which has recently established itself in a small part of India. The most effective of malaria carriers, A. gambice, a native of Tropical Africa, has appeared in an area on the east of Brazil ; it seems that it may have arrived by aeroplane. It is surely important to know which parts of the world are threatened by these and other insects. One would also wish to know whether certain insects of wide distribution in the tropics, for instance X. cheopis and A~des argenteus, have yet reached the limits imposed by climatic factors. But, in fact, man can and habitually does control climate. We build houses which are warmed by radiation in summer and combustion in winter. In those houses one finds a population of foreign cockroaches, bugs, clothes moths, etc., which exist only in the house and cannot breed if they are exposed to external conditions. To take another example : the control of Glossina is largely brought about by cutting and burning undergrowth. It seems probable, though no one has proved it, that this affects the fly by exposing it to conditions of greater evaporation. Recent work by NASH contains evidence which is confirmatory, though not conclusive. It is suggested that, if we made more detailed studies of insect physiology and of the climatic conditions in the places where certain species live, we could turn the knowledge to very good effect. For instance, the spread of plague depends partly on rats and fleas, but partly on conditions which are made by man. I refer to the relation between plague and commerce in cereals ; also to the fact that the rats and their fleas live for the most part in or beneath houses and other buildings. It should be possible, by collaboration between a physicist, a builder and an entomologist to define conditions which are desirable in a tropical warehouse. If, for instance, the roof were black or constructed of glass permeable to radiant heat, and if it were built normal to the mid-day sun at a critical season, one could raise the temperature in the stored foods materially. The control of ventilation would allow moisture evaporated from the contents of the warehouse to be liberated, whenever the vapour pressure in the warehouse was greater than that outside. In this way one could increase the saturation deficiency in the warehouse, and prolong the season when flea-breeding was at a minimum. I do not suggest that this would completely eradicate plague, or that village granarie s could be built in this way. But it is clear that if a government department or a large corporation builds a grain store, it should be constructed so as to reduce the multiplication of fleas, rather than as a vivarium. SUMMARY.

It is clear that many of the periodical changes which may be observed in the abundance of insects are due to climatic factors. The factors which limit geographical spread of species are also frequently climatic. In the present state of knowledge, two climatic factors--temperature and humidity--have been investigated. With regard to their effects on insects of

854

THE EFFECT OF CLIMATIC CONDITIONS UPON POPULATIONS OF INSECTS.

medical importance, a considerable amount of information exists, some of which is fragmentary. Some of the effects, especially of humidity, are quite unexpected and could only have been discovered by investigations in the laboratory. Certain general laws are now known, but very great differences exist between different species of insects• It is possible that, as knowledge increases, it will be found that some parts of solar radiation are at least as important as temperature and humidity in controlling insect populations. T h e view is put forward that observations made in the field serve to ask questions which can be answered by precise physiological work in the laboratory. There is need for a new type of field work ; in order that the bearing of the physiological work on natural phenomena may be elucidated, large collections of critical data are required from the spots in which the insects actually live. It is thought that, if such facts were available, control of certain harmful insects might be achieved by the intelligent modification of climate.

REFERENCES. ACTON, H. W., & RAo, S. (1931). The diagnosis of lymphatic obstruction of filarial origin. Indian Med. Gaz., lxvi, (1), 11-17. BAeOT, A. (1914). The influence of temperature, submersion and burial on the survival of eggs and larvm ofCimex lectulaHus. Bull. Ent. Res., v, 111-117. BACOT, A.W., & M.amTIN, C . J . (1924). The respective influences of temperature and moisture upon the survival of the rat flea (Xenopsylla cheopis) away from its host. a~l. Hyg., xxiii, 98-105. BEATTm, M. V. F. (1928). Observations on the thermal death points of the blow-fly at different relative humidities. Bull. Ent. Res., xviii, 397-403. BECQUAERT,J. (1930). Medical and economic entomology. The African Republic of Liberia and the Belgian Congo, ii, 797. Harvard University Press, Cambridge. B~rrLEY, C.A. (1911). Report of an investigation into the causes of malaria in Bombay. Government Press, Bombay. BODm~HEIMER,F . S . (1927). Ueber die Voraussage der Generationenzahl von Insekten. III. Die Bedeutung des Klimas ffir die landwirtschaftliche Entomologie. Zeits. ang. Ent., xii, 89-122. • (1931). Erfahrungen fiber die Biologic der Hausfliege (Musca domestica L.) in Pal~istina. Ibid., xviii, 492-504. BROOKS,R. ST. J. (1917). The influence of saturation deficiency and of temperature on the course of epidemic plague. Jl. Hyg., Plague Supplement, 5,881-899• BUx'roN, P . A . (1924)• Heat, moisture and animal life in deserts. Proc. Roy. Soc. B., xcvi, 123-131. (1930). Evaporation from the meal-worm (Tenebrio : Coleoptera) and atmosph'eric humidity. Ibid., cvi, 560-577. • (1931a). The thermal death-point of Rhodnius (Rhynchota, Heteroptera) under controlled conditions of humidity, aYl.Exper. Biol., viii, 275-278. • (1931b). The measurement and control of atmospheric humidity in relation to entomological problems• Bull. Ent. Res., xxii, 431-447. 297. (1932a). The climate in which the rat-flea lives. Indianyl.Med. Res.,xx, 281• (1932b). Terrestrial insects and the humidity of the environment. Biol. Revs., vii, 275-320.

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855

Bt~rON, P . A . (1932c). T h e relation of adult Rhodnius prolixus (Reduviidae, Rhynchota) to atmospheric humidity. Parasitol., xxiv, 429-439. • (1932d). Climate in caves and similar places in Palestine• ffl..4nim. Ecol., i, 152-159. CHORLE~', J. K. (1929). T h e bionomics of Glossina morsitans in the Umniati fly belt, Southern Rhodesia, 1922-23. Bull• Ent. Res., xx, 279-301. CRAOG, F. W . (1922). Relapsing fever in the United Provinces of Agra and Oudh. Indian ~l. Med. Res., x, 78-189• CUN-LXFF~, N. (1921). Some observations on the biology and structure of Ornithodorus moubata, Murray• Parasitol., xiii, 327-344. • (1922). Some observations on the biology and structure of Ornithodorus savignyi, Andouin. Ibid., xiv, 17-26. DAvIEs, W . M. (1929). Hibernation of Lucilia sericata Mg. Nature, cxxiii, 759-760. DAvis, N. C. (1932). T h e effects of heat and cold upon A~des (Stegomyia) aegypti. ~liner. ffl. Hyg., xvi, 177-191• Fm~ORN, S . B . (1932). T h e seasonal life history of Anopheles maculipennis with reference to humidity requirements and " hibernation." Ibid., xvi, 215-223. HASE, A. (1930). Weitere Versuche zur Kenntnis der Bettwanzen, Cimex lectularius L. und Cimex rotundatus Sign. (Hex. Rhynch.). Zeits. Parasitenk., ii, 368-418. JONES, R . M . (1930). Some effects of temperature and humidity as factors in the biology of the bedbug (Cimex lectularius Linn.). Ann. Ent. Soc. ~liner., xxiii, 105-119. ~DY, C. H. (1927). Some non-nervous factors that condition the sensitivity of insects to moisture, temperature, light and odors• Ibid., xx, 87-106. KORKE, V . T . (1932). Observations on filariasis in some areas in British India. Part V I I I . Indian• ffl. Med. Res., xx, 335-339. LF-~XNGH~LM,J. C . G . (1920). Dysentery and enteric disease in Mesopotamia from the laboratory standpoint. ~l. Roy. Army Med. Coll., xxxiv, 189-203,306-320. L~SON, H. S. (1932). T h e effect of temperature and humidity upon the survival of certain unfed rat fleas. Parasitol., xxiv, 196-209. LINDGREN, D. L., & SHEPARD,H . H . (1932). T h e influence of humidity on the effectiveness of certain fumigants against the eggs and adults of Tribolium confusum Duv. ffl. Econ. Ent., xxv, 248-253. LUTZ, F . E . (1924). Apparently non-selective characters and combinations of characters. ~Inn. New York/Icad. Sci., xxix, 181-283. MACFIE, J. W. S. (1912). Experiments and observations upon Glossina palpalis. Bull• Ent. Res., iii, 61-72. • (1920)• Heat and Stegomyia fasciata ; short exposures to raised temperatures. Ann. Trop. Med. ~ Parasit., xiv, 73-82. MAYNE, B. (1926). Notes on the influence of temperature and humidity on oviposition and early life of/Inopheles. U.S. Pub. Health Repts., xli, 986-990• • (1928). T h e influence of relative humidity on the presence of parasites in the insect carrier, and the initial seasonal appearance of malaria in a selected area in India• Indian ffl. Med. Res., xv, 1073-1084. • (1930). A study of the influence of relative humidity on the life and infectibility of the mosquito• Ibid., xvii, 1119-1137. MELLANBY, K. (1932a). T h e influence of atmospheric humidity on the thermal deathpoint of a number of insects, ffl. Exper. Biol., ix, 222-231. • (1932b). T h e effect of atmospheric humidity on the metabolism of the fasting mealworm (Tenebrio molitor, L., Coleoptera). Proc. Roy. Soc. B., cxi, 376-390. (1932c). Effects of temperature and humidity on the metabolism of the fasting bed-bug (Cimex lectularius, Hemiptera). Parasitol., xxiv, 419-428. • (1932d). T h e conditions of temperature and humidity of the air between the skin and shirt of man. ffl. Hyg., xxxii, 268-274• MELVIN, R. (1931). Notes on the biology of the stable-fly, Stomoxys calcitrans Linn. ~Inn. Ent. Soc. ~liner., xxiv, 436-438. NASH, T . A . M . (1931). T h e relationship between Glossina morsitans and the evaporation rate. Bull. Ent. Res., xxii, 383-384.

856

DISCUSSION.

NECHELES, H. (1924). l~ber Wirmeregulation bei wechselwarmen Tieren. Ein Beitrag zur vergieichenden Physiologic der Witrmeregulation. Pflager's Archly f. ges. Physiol., cciv, 72-86. Nt/TTALL, G. H . F . (1917). The biology ofPediculus humanus. Parasitol., x, 80-185. OTTEN, L. (1932). The problem of the seasonal prevalence of plague. Jl. Hyg., xxxii, 396-405. PAIntER, J. R. (1930). Some effects of temperature and moisture upon Melanoplus mexicanus mexieanus Saussure and Camnula pellueida Scudder (Orthoptera). Agric. Exper. Sta. Bozeman, Montana, Bull. 223. PEAIRS, L . M . (1927). Some phases of the relation of temperature to the development of insects. West Virginia Univ..4gric. Coll. Bull. 208. RAo, S. S., & IYENGAr, M. O . T . (1930). Studies on the influence of season on the development of Filaria bancrofti in Culex fatigans. Indian Jl. Med. Res., xvii, 759-768. ROGERS, L. (1928). The yearly variations in plague in India in relation to climate : forecasting epidemics. Proc. Roy. Soe. B., ciii, 42-72. ROUBAUD,E. (1909). La Glossina palpalis R. Desv. ; sa biologic, son rrle dans l'rtiologie des trypanosomiases. Thesis, No. 1344, Paris University. (t922). Etudes sur le sommeil d'hiver prr-imaginal des Muscides. Bull. Biol. France et Belg., lvi, 455-544. STUHLMANN, F. (1907). Beitriige zur Kenntnis der Tsetsefliege (Glossina fusca und Gl. tachinoides). Arb. Kaiserl. Gesundheits., xxvi, 301-383. "FATE, P., & VINCENT, M. (1932). Influence of light on gorging of Culex pipiens L. Nature, cxxx, 366-367. UVAROV, B.P. (1931). Insects and climate. Trans. Ent. Soc. Land., lxxix, 1-247. WARDLE, R . A . (1930). Significant variables in the blowfly environment• Ann. Appl. Biol., xvii, 554-574. WATERSTON, J. (1929). On a chalcidoid parasite bred from a flea larva. Parasitol., xxi, 103-106. WHITEHOUSE,A. G. R., HANCOCK,W., 8¢;HAt.DANE,J . S . (1932). The osmotic passage of water and gases through the human skin. Proe. Roy. Soc. B, cxi, 412-429. WtOGLESWORTH,V . B . (1931a). The physiology of excretion in a blood-sucking insect, Rhodnius prolixus (Hemiptera, Reduviidae). III. The mechanism of uric acid excretion. Jl. Exper. Biol., viii, 443-451. • (1931b). The extent of air in the tracheoles of some terrestrial insects. Proc. Roy. Soc. B., cix, 354-359: . (1931c). Effect of desiccation on the bed-bug (Cimex lectularius). Nature, cxxvii, 307-308. (1932). On the function of the so-called " rectal glands " of insects. Ouart. Jl." Micr. Sci., lxxv, 131-150. WalGHT, W . R . (1927). On the effects of exposure to raised temperatures upon the larvm of certain British mosquitoes. Bull. Ent. Res., xviii, 91-94.

DISCUSSION. Dr. C, B. Williams* said that although his work was connected with agricultural and not medical entomology, the same problems were encountered. I n fact, the problem of varying n u m b e r s of insects was of even greater importance in medicine, as the farmer could always afford to lose a small proportion of his plants, and it was only when the insect pests increased to very large n u m b e r s that his p r o b l e m became acute. T h e medical man on the other hand had to protect every individual. * Chief Entomologist, Rothamsted Experiment Station.