Saturation Deficiency Research Articles

Mosquito ecology in central Alberta. II. Adult populations and activities.

Regular sampling of Aedes mosquitoes in an aspen forest in central Alberta shows that there are several peaks of abundance during the summer, each due to a different species or group of species, and that the times, amplitude, and composition of the peaks differ in two successive years. Hourly sampling over a 24-hour period shows that the mosquitoes are crepuscular, but the activity cycles of certain species show marked differences. Mosquitoes in this environment tend to remain close to the shrub layer. The factors regulating mosquito activity are discussed, and it is suggested that saturation deficiency and light intensity are the two most important environmental factors. The forest has the highest flight activity, and the willow scrubland the highest biting activity. Observations are also given on the biting activity at different levels in the forest during the afternoon, the biting activity at night and the effect of moonlight on this activity, the movement of mosquitoes into buildings, and swarms of male mosquitoes. A key to the adult female mosquitoes in the study area is included.

The influence of a deep river valley system on the dispersal ofAedesmosquitos

The influence of a deep river valley system on the distribution of dispersingAedesmosquitos was studied during 1958–59 in the vicinity of Edmonton, Alberta, Canada. Mosquito breeding within an area extending two to three miles beyond the city limits was prevented by anti-larval measures, and almost the only remaining breeding places were situated on the plains several miles to the north-west and south-east, yet considerable numbers of mosquitos appeared every spring in that part of the city included in the valley-ravine system of the North Saskatchewan River. Previous work had suggested that mosquitos dispersing across the plains were collected by the valley-ravine system and entered the city by passing down the main valley. The present paper attempts to account for this directed dispersal in terms of the behaviour ofAedesmosquitos in response to certain physical factors in the environment. The species mainly concerned wereAedes cataphyllaDyar,A. fitchii(Felt & Young),A. excrucians(Wlk.) andA. stimulans(Wlk.).The initiation of dispersal flight downwind across the plains at twilight was observed. This took place soon after emergence and was not prevented by moderate winds. It was shown experimentally that neither high wind speeds nor turbulence inhibited the flight of newly emergedAedesmosquitos, but that individuals several days old were reluctant to take flight in turbulent conditions. Since wind speeds on the plains were generally higher than those in the valleys, these reactions would tend to confine older mosquitos to the valleys while not preventing dispersal of newly emerged mosquitos across the plains.The direction of the initial dispersal flights from the breeding places was observed in mosquitos escaping from rearing cages. Take-off was characteristically into the wind, but re-orientation downwind took place when the mosquitos began to be blown backwards in flight. It was shown experimentally, by releasing mosquitos in the field when the wind was very light, that the preferred direction of flight uninfluenced by wind was towards the lowest part of the horizon, both on the plains and in the valley. This response would tend to cause mosquitos on the plain to fly into ravines and valleys, and thence down-stream within them. Dispersing mosquitos would thus tend to be held by any valleys or ravines in their path.Swarming was never observed near the pools from which the mosquitos emerged, but was mainly seen in the valley or on the edge of the valley and ravines. Swarming ofA. cataphyllatook place during the evenings after the initial dispersal. The swarms were predominantly male but mating occurred during swarming. A cohesive factor seemed to be involved in the behaviour of the swarms, and further dispersal also seemed to occur gregariously; the movement of large male swarms down the river valley was observed.By the application of optomotor theories of insect flight to previously published data on the flight speed and behaviour ofA. aegypti(L.) andA. punctor(Kby.), the relationships between wind speed and permissible heights of flight of these species upwind and downwind were calculated. These relationships were consistent with the observations on dispersal ofAedesmosquitos in the Edmonton area, and could largely explain the mechanism by means of which deep valleys attracted and held dispersing mosquitos.In contrast to the open plains, the wooded river valley provides a more favourable environment for adult mosquitos. The supply of nectar is greater, saturation deficiency and wind speeds are lower and less variable, and shaded resting sites are more abundant.Observations on mosquitos in chambers with either vertical or horizontal humidity gradients suggested that the behaviour ofAedesmosquitos is not greatly influenced by humidity.

The Effect of Temperature On the Waterproofing Mechanism of an INSECT

ABSTRACT Previous methods for determining the temperature characteristics of the permeability to water of insect cuticle are criticized on five fundamental points, and in particular it is shown that they do not measure the temperature of the cuticle which will be substantially different from that of the surrounding air. The physical principles for accurate measurement are outlined, involving the accurate control of air temperature and air circulation, the measurement of cuticle temperature and of water loss within a sealed enclosure, and the assessment of true saturation deficiency. Apparatus for meeting these requirements and making these measurements is described. Determinations have been made of the permeability of the cuticle of Periplaneta americana nymphs. When the permeability is plotted against cuticle temperature a sudden and substantial increase in permeability is seen to take place at around 30° C. The temperature at which this change occurs is higher the longer the time which has elapsed since the nymph previously moulted. A method of measuring the permeability of the cuticular grease, when spread as a layer over an isolated water droplet, is described. This layer, whose thickness is of molecular dimensions, has much greater absolute impermeability to water than the thick grease on the insect. It also exhibits a sudden increase in permeability at about 30° C., and the order of change is considerably greater than that found with intact cuticle. The findings are quantitatively compatible with the postulate that, at lower temperatures, the grease on the cockroach consists of an orientated ‘monolayer’ organized by the attraction of its polar groupings for the aqueous cuticle surface underlying it, together with a much greater thickness of unorganized grease above it. In absolute units, the grease is about five times as permeable to water as is the ‘monolayer’, but at the transition temperature the measurements agree with the supposition that the molecular organization of the ‘monolayer ‘breaks down into the disorganized state of the major part of the grease with a corresponding increase in permeability. The suggestion that films of grease on water, and the lowermost portion of the grease on the animal, have orientated molecular organization is supported by some observations on the effect of dust particles on the films and by information previously published.

Studies on Asaphes Fletcheri (Crawford), a Hyperparasite of Aphidius Testaceipes (Cresson) and Praon Aguti (Smith), Primary Parasites of Aphids1

This pteromalid hymenopteron was cultured on both the braconid species named above. Mating was preceded by courtship. This began, on the average, 68 minutes after emergence, continued for about four minutes, and was followed by copulation which required about 15 seconds. Each male was capable of mating with more than one female, but mated females were completely unattractive to males. Fertilized females produced mostly female progeny, unmated ones only males, and the progeny of the unmated ones were fewer than those of mated females. Though generally ready for oviposition as soon as they dried after emergence, if females were mated there was an interval of about 45 minutes before oviposition was begun; however, if suitable hosts were available to them, newly emerged females did not court or mate but proceeded with oviposition. Deposition of an egg consumed about six minutes. The total period of oviposition per female ranged from 2 to 13 days when they were allowed to attack 2 hours per day; the maximum number of attacks occurred on the first day, and ovipositional attacks were most numerous at 28° C. The life cycle from egg to adult required a maximum of about 27 days at 23° C. and a saturation deficiency of 3 mm., and a minimum of about 10 days at 30° and a saturation deficiency of 8 mm. Males completed this cycle more rapidly than females, but adult females lived longer than adult males. Marked discrimination was shown in selecting hosts. Only dead, sessile, parasitized aphids were chosen for attack. The early pupao of A. testaceipes, contained in such aphids, were preferred to late larvae, but with P. aguti the late larvae were more frequently attacked than the pupae. On the whole, a greater percentage of hyperparasitism was accomplished on Aphidius testaceipes than on Praon aguti; and in the field an increase in the population of Asaphes fletcheri was seen to bring about a marked scarcity of A. testaceipes, especially in one locality where both this species and P. aguti were present.

A Study of the Blackfly, Simulium ornatum Mg. (Diptera), with particular Reference to its Activity on Grazing Cattle

SummarySamples of aquatic stages, taken from a small lowland stream near Durham, England, showed that the blackfly, Simulium ornatum Mg., emerged in quantity from April to October (7 months) and that oviposition took place between May and October. Oviposition was confined to the period between sunset and dusk.Flies emerging in the early summer of 1952 were smaller than those emerging in the spring and late summer of that year.S. ornatum landed on cattle from dawn to dusk, with usually a small peak 2–4 hrs. after dawn, and a large peak between sunset and dusk on warm, sunny days. On cool, cloudy days the number of flies landing showed irregular fluctuations throughout the day.The number of flies landing on untethered cattle was not markedly dependent on air temperature or saturation deficiency. Winds of over 5 m.p.h. markedly decreased or inhibited landing activity.On average about 70 per cent, of all blackflies landing spent less than 10 min. on cattle after landing, and this interval sometimes decreased as the total number of flies landing per unit time increased.Of the flies which landed, some 8 to 25 per cent, were calculated to bite the cow, and the proportion which bit varied significantly from day to day. The number of bites sustained by cattle in the district appeared to cause no ill-effect, apart from the formation of scar tissue in the navel region where most bites were inflicted.

Studies in Diurnal Rhythms

ABSTRACT The species of woodlouse investigated are known to stand in the following order as regards the rate of water loss by transpiration : Philoscia muscorum > Oniscus asellus > Porcellio scaber > Armadillidium vulgare. The intensity of the responses of the four species to humidity at room temperature (18+ 2° C.) are compared and are shown to stand in the same order. The intensity increases as desiccation proceeds. At low temperatures (3 ± 2° C.) the intensities of the reactions to humidity are much reduced and are the same in all species; at higher temperatures (30 ± 2° C.) the humidity responses of Ph. muscorum and O. asellus are again somewhat reduced, and it is suggested that this may be correlated with a lower thermal death-point and the need to effect a reduction in body temperature by evaporation. The antennal cone sensillae are shown to be contact chemo-receptors ; no specific sense organs are concerned with the appreciation of humidity. The response is correlated with saturation deficiency rather than with relative humidity and is probably engendered by dehydration. Ph. muscorum is the most strongly photonegative of the species and there is a graduation through O. asellus and P. scaber to A. vulgare which is the least so. Ph. muscorum is also shown to be the most intensely nocturnal in habit, A. vulgare the least. It is therefore suggested that the degree of nocturnal activity is correlated with the ability to withstand water loss by transpiration. The results obtained are discussed in relation to the ecology of the species.

Similarities in Killing by Heat and by X-Radiation in the Insect Dahlbominus fuscipennis (Zett.)

The ability of adult insects to withstand extreme doses of ionizing radiation is generally known. Heidenthal (1) has reported that males of Habrobracon wasps failed to recover after approximately 250,000 r but survived for more than 2 days after a dose of 185,000 r. Sullivan and Grosch (2) employed X-radiation doses as high as 180,250 r in showing that the mean survival times of Habrobracon were increased by irradiation. This relative insensitivity of insects in comparison to most other animals lends particular interest to the study of the mechanisms of resistance involved. The object of this paper is to point out similarities in response to lethal high temperatures and lethal doses of X-rays in an adult insect. The species used in the work was the small chalcid Dahlbominus fuscipennis, an imported hymenopterous parasite of the spruce sawfly. In this species, the males are haploid and possess only six chromosomes, whereas the females are diploid. Unmated females lay eggs which by parthenogenesis produce only males; the eggs of inseminated females produce both males and females. The insects are reared in the laboratory on cocoons of the sawfly Neodiprion lecontei (Fitch) under controlled conditions of temperature and humidity (230 C and a saturation deficiency of 5 gm/m.3). Irradiations were done with a 2-Mev X-ray machine delivering 12,000 r/min at 10 cm from the target. Techniques employed in testing the temperature tolerance of Dahlbominus and results of heat exposures have been reported previously (3).

The Influence of Temperature and Moisture on the Uptake and Loss of Water in the Eggs of Gryllulus Commodus Walker (Orthoptera—Gryllidae)

ABSTRACT Eggs placed on moist plaster took up water slowly at first and then more rapidly until they had reached their maximum water content. The duration of the initial period of slow water uptake was dependent upon temperature, becoming shorter as the temperature was raised. Similarly the duration of the period of rapid water uptake decreased with temperature. No special water-absorbing structure was identified in the eggs of Gryllulus. The rate of water loss from newly laid eggs was more rapid at higher than at lower temperatures under conditions of constant atmospheric saturation deficiency. Newly laid eggs died if they lost more than about 20 % of their original weight. At 90 % relative humidity, eggs which had completed water uptake lost water at a higher rate at high than at lower temperatures. The rate of water loss was also dependent on the stage of development of the embryo. Eggs that had completed diapause development at 13° C. and had completed water uptake were able to lose about 30 % of their weight through desiccation without being killed, but many of those that lost more than about 20 % of their weight failed to develop when replaced on moist plaster; it seemed as if they had entered a state resembling diapause.

The Water Relations and Cuticle of Paradesmus gracilis (Diplopoda, Strongylosomidae)

ABSTRACT Water-loss in Paradesmus gracilis depends upon temperature and humidity, and is directly related to saturation deficiency. There is no evidence of any ‘critical point’ to indicate an epicuticular wax layer. Water is readily lost and taken up through the cuticle, the effect of the spiracles and of excretion being negligible. Despite great sensitivity to desiccation, there is nevertheless some degree of impermeability. The cuticle is in many ways similar to that of an insect: it is composed of a ‘cuticulin’ epicuticle, a ‘tanned’ chitinous exocuticle, and a laminated endocuticle of two optically distinguishable layers. The outer endocuticle is strongly calcified. The cuticle is penetrated by pore canals and the ducts of dermal glands. The latter are concerned with the production of exo- and endocuticle, and the secretion of the polyphenols which tan the protein of the exocuticle. The double hardening is probably a specialized condition of millipedes. Transpiration is almost quadrupled by extraction with hot, but not cold chloroform, as the exocuticle is impregnated with lipoids which reduce permeability. These are secreted by epidermal and dermal gland cells, and pass up the pore canals and gland ducts.

Climate and the Activity of the Kenya coastal Glossina

This paper deals with Glossina pallidipes, G. austeni and G. brevipalpis as they occur on the Kenya coast.Temperature and saturation deficiency both exert a generally inverse effect on the activity and apparent density of both G. pallidipes and G. austeni. True density also varies inversely as the s.d.

The Climatology of Blowfly Myiasis

The relationship of oviposition to weather, with a standard stimulus (4 per cent. ammonium carbonate), has been tested by a series of 134 exposures of a group of three sheep for one hour. 190 egg clusters were obtained, of which 169 were identified. With the exception of one cluster of L. caesar all were L. sericata.There was a fairly close correlation (0·586) of oviposition with air-temperature. When the possible effect of other factors was eliminated, the correlation was little affected, showing that temperature was in itself responsible for the variations in oviposition. The lower critical limit of temperature was in the neighbourhood of 56°–58°F.Solar radiation and oviposition had an apparent correlation, but this was found to be spurious, the effect being due to related air-moisture changes. At temperatures near the lower critical limit, insolation did not increase the oviposition rate, or lower its temperature threshold.Humidity had an apparent inverse correlation with oviposition, but this also was an indirect effect, due to related temperature changes. Humidity showed a closer correlation when expressed as saturation deficiency than as relative humidity. Evaporation rate from a white atmometer sphere had only a low correlation with oviposition.It is shown that the partial correlations of both saturation deficit and insolation with temperature effects eliminated, are insignificant, i.e. their apparent effects are due to related temperature changes. The multiple correlation coefficient for temperature humidity and insolation is practically identical with that for temperature.Independent variations of insolation, i.e. variations not simultaneous with related temperature changes, had a significant effect on oviposition; for a given temperature, a high intensity of insolation was less favourable than a low, and this appeared to be a true insolation effect, and not due to associated humidity changes.Independent variations of humidity had a suggestive but statistically insignificant effect, humidity tending to be inversely related to oviposition.The regression of log oviposition on temperature, within the limits of this experiment, i.e. at temperatures up to 72°F., was linear. The slope was not significantly affected by the insolation and humidity factors, the total coefficient being 0·, and the partial coefficient 0·0428. These coefficients are the increase of the log number of egg clusters (plus one) for 1 degree F. increase, i.e. the oviposition rate increased in geometric progression for unit increase of temperature, the value (N+l) for egg clusters doubling itself for 7° increase.

The water balance inIxodes ricinusL. and certain other species of ticks

The unfed tick gains water from humid air or from water in contact with the cuticle, and loses water by evaporation. Whilst attached to the host the tick is gaining water from the ingested blood and losing water in the excrement. The engorged tick usually lacks the ability to take up water from humid air.The exchange of water takes place mainly through the cuticle. Regulation of the water balance is therefore brought about by the activity of the epidermal cells.The cuticle comprises two principal layers, the epicuticle and endocuticle. The epicuticle is overlaid by a lipoid possessing important waterproofing properties. The pore canals, which traverse the endocuticle, are occupied by cytoplasm, and may in consequence play an important role in the active -transfer of water through the cuticle: they do not penetrate the epicuticle.Water loss from the unfed tick is not closely related to saturation deficiency, particularly at high humidities. This departure is due to a physiological cause, namely, to the ability to secrete water. The effects of this activity are such that a state of equilibrium is attained at a relative humidity of about 92%: at lower relative humidities the tick loses water by evaporation, while at higher humidities it takes up water. The retention of water at humidities below the point of equilibrium is due not only to the physical properties of the epicuticle but also to this secretory activity, for water loss increases when the tick is temporarily asphyxiated, poisoned with cyanide or injured through excessive desiccation. Near the point of equilibrium the loss or gain of water over a wide range of temperature is determined by the relative humidity.The uptake of water from humid air occurs when the tick is in a desiccated condition but ceases as the normal water content is restored. After previous exposure to saturated air the adapted tick at first loses water at relative humidities above the point of equilibrium, but later comes to retain water completely.Both unfed and engorged ticks possess the ability to prevent or to limit temporarily the entry of water in contact with the cuticle.The engorging female, originally weighing about 2 mg., ingests about 600 mg. of blood. About 300 mg. or two-thirds of the contained water are usually eliminated before the end of engorgement. Evaporation from the cuticle may account for a considerable fraction of this, for the temperature to which the attached tick is exposed (about 37°C.) is, inIxodes ricinus, above that temperature at which a marked increase in the permeability of the epicuticular lipoid takes place.The nine species of ticks examined differ considerably in their powers of limiting evaporation. This may reflect specific differences in the nature of the epicuticular lipoid. The order of their resistance is as follows:Ornithodorus moubata; Dermacentor andersoni; D. reticulatus; Rhipicephalus sanguineus; Amblyomma cajennenseandA. maculatum; Ixodes canisuga; I. hexagonus; I. ricinus. In dry air water loss throuǵh the cuticle is 10–15 times more rapid inIxodes ricinusthan inDermacentor andersoni. The more resistant species also take up water through the cuticle after desiccation; indeed, the rate of uptake over a unit area of cuticle is approximately the same in all species of Ixodidae. Uptake thus appears to be limited by the ability of the epidermal cells to secrete water.Stocks ofDermacentor andersoni, RhipicephalusandAmblyommaspp. were kindly supplied by Dr R. A. Cooley, Director of Entomology, Rocky Mountain Laboratory, U.S.A., through the good offices of Prof. P. A. Buxton, F.R.S. I am also indebted to Dr H. H. Green of the Veterinary Laboratory, Weybridge, for providing useful facilities, and to Dr V. B. Wigglesworth, F.R.S., for his generous help throughout the various stages of this work.

INSECT SURVIVAL IN RELATION TO THE RATE OF WATER LOSS

Summary1. If the relationship between rate of water loss from an insect and saturation deficiency is linear and survival is limited only by water loss, then the curve of longevity against saturation deficiency is hyperbola‐shaped. The position of this curve with respect to the co‐ordinates is, however, affected by other factors, which are discussed.2. The work of Bacot & Martin (1924) on the longevity of the rat flea, Xenopsylla cheopis, is reviewed in the light of the above hypothesis: errors in curve fitting are revealed which make it necessary to modify some of Bacot & Martin's conclusions.3. Selected data from various authors subsequent to Bacot & Martin are considered. The discrepancies, real and apparent, between Leeson's results with rat fleas and those of Bacot & Martin are analysed, and it is pointed out that these were to some extent due to errors in curve fitting. The hypothetical relationship between longevity, rate of water loss and saturation deficiency set out in the first part of this article are illustrated by Ludwig's (1937) data for Popillia japonica; the linear relationship between 1/longevity and the rate of water loss demonstrates the hypothesis that water loss limits survival with this insect. Other less complete data are also discussed.4. Various types of longevity‐saturation deficiency curves are described and the causes which probably account for such variations.5. The relationship of pupal or egg mortality to saturation deficiency cannot by itself throw light on the variation of rate of water loss with saturation deficiency; the time factor must also be taken into account. Lines of equal mortality may coincide with lines of equal saturation deficiency (the co‐ordinates being relative humidity and temperature), so that the product of mortality and saturation deficiency is constant at any one temperature; but this indicates only that temperature is without effect on mortality and does not throw light on the relation of mortality to different saturation deficiencies. Such coincidence of lines cannot, therefore, be taken as proof that Dalton's law operates.This work was done while I held the Avebury Studentship in the Department of Entomology, London School of Hygiene and Tropical Medicine, and I am greatly indebted to Prof. P. A. Buxton for his great kindness and ready help. I also thank Drs W. J. Martin and D. L. Gunn for their advice and criticism.

The Relation of Instar Length to the External and Internal Environment in Tegenaria atrica (Arachnida).

Summary.1. The influence of temperature, humidity, food, light, parentage, age, and sex on instar length and its variability in Tegenaria atrica was investigated.2. Only one factor of the external environment was varied at a time. The factors were:–20o C, 25‐5o C, and 30o C.; 8‐25, 16‐36, and 23‐13 mm. saturation deficiency; two food levels in excess of the needs for an instar and one at the minimum amount; light and darkness.3. A threshold amount of food was required for completion of an instar, and this quantity was greater when the rate of feeding was lower. It also increased as temperature fell and saturation deficiency rose, but illumination had no effect.4. Instar length decreased as temperature rose, as saturation deficiency fell, and when the animals were illuminated.5. Different amounts of food, provided that they were more than the minimum needed for instar completion, had no effect on its length. An instar which had only the minimum amount was slightly lengthened and the following one, with excess food, was considerably shortened.6. Variance of instar length increased at the lowest humidity, with minimum feeding, and with age, decreased with rise of temperature, and was unaffected by illumination.7. In regard to internal environment, the offspring of various parents fell into three groups, each with its own inherent mean instar length; instar length always increased with age; and the sexes of the animals were indistinguishable.8. The above results and their relation to work on insects are discussed, and it appears that the moult cycle of T. atrica is controlled in a similar manner to that of Rhodnivs and Cimex. However, this and other spiders show a striking variability in individual instar lengths.

STUDIES OF THE BIOLOGY OF THE DEATH‐WATCH BEETLE, XESTOBIUM RUFOVILLOSUM DE G.

Summary 1 This paper, the second in a short series on biological studies of the death-watch beetle, Xestobium rufovillosum De G., is concerned with the life of the adult, egg-laying and hatching. 2 Times of pupation and emergence in the natural habitat of the insect in decayed parts of willow, and in timbers in buildings are discussed and compared. 3 The rate of development of the reproductive organs after pupation and until egg-laying starts, is described. 4 The habits of the adults, activity, tapping, fight and mode of pairing are commented upon. 5 The major part of the paper deals with oviposition which has been studied out of doors and under controlled tempemture and humidity in the laboratory. 6 Out of doors forty to eighty eggs per female are laid during May and June over a period of 6–9 weeks. The egg stage lasts from 3 to 5 weeks, according to temperature conditions. 7 In the laboratory, oviposition and the hatching of the egg were studied at 15, 20, 25 and 30°C. and relative humidities of 23, 41, 53, 75, 86 and 95 %. The results are discussed in relation to the effect of temperature, relative humidity and saturation deficiency. 8 “Indices of suitability” for various combinations of temperature and humidity in relation to egg-laying and hatching are calculated from the experimental data. 9 It is shown that an increase in temperature to a maximum between 25 and 30°C. is favourable but that the optimum conditions must include a low saturation deficiency. The data demonstrate an increasing tolerance, by the insect, of dryness associated with a rise in temperature. 10 The application of these findings to the occurrence and spread of Xestobium in buildings is briefly discussed.

The longevity of the fasting bed-bug (C. lectulariusL.) under experimental conditions and particularly in relation to the saturation deficiency law of water-loss

1. With unfed 1st instars the relation between mean length of life and saturation deficit at constant temperatures between 7 and 15° C. at relative humidities between 7 and 90% is hyperbolic. The relationship becomes more linear at higher temperatures.At constant saturation deficits insects live longer at 15° C. than at lower temperatures. Longevity also decreases with rise of temperature above approximately 15° C.2. In general, the longevity curves, except for those at temperatures below 15° C., bear a very similar relation to saturation deficit and to each other as the reciprocal curves for rate of water-loss at the different temperatures.The influence of climatic factors on longevity at constant temperatures is discussed at length and it is concluded that over much of the temperature and humidity range survival time is limited by water-loss. At the higher humidities it is thought that either food, or perhaps an excessive accumulation of water within the insect, limits survival and causes a departure from the hyperbolic relation of longevity to saturation deficit.3. The effects on longevity of a single meal are discussed. The principal effect of a blood meal is to increase the time of survival. But the factors which limit survival at different humidities appear to be the same as with unfed bugs, except at high humidities below about 15° C.4.Mellanby's data on the rate of water-loss from fasting adult bed-bugs is analysed. It is found that the rate of water-loss is directly proportional to saturation deficit at constant temperatures between 8 and 37° C. and between 0 and 90%r.h. Although the rate may always be directly proportional to saturation deficit, the expressionis not always constant.Rate =K+b(saturation deficit), whereKvaries with temperature andbremains constant.5. Longevity in relation to host blood is discussed. Rabbit blood appears to be slightly less favourable to survival than human blood.6. If bugs are allowed to feed to repletion, longevity is not correlated with the size of the meal, nor with the weight of the unfed insect.Virgin females live longer than mated ones, but no effects of mating on survival were noticed with males. Mated males tend to outlive mated females except at very low temperatures: virgin females live longer than unmated males.7. The results of other workers and the possible causes of some discrepancies are discussed.8. The maximum survival times of bugs are listed. Adults and 5th instars live longer than other stages. In a house which has remained empty for a long time it is probable that 5th instars and adults, particularly unmated female adults, would predominate in the population.The longest observed survival was between 562 and 572 days.

The Humidity Behaviour of the Mealworm Beetle, Tenebrio Molitor L

ABSTRACT Humidity is well known to be a factor of great importance in the lives of many insects and other terrestrial arthropods. Apart, however, from the pioneer work of Shelford (1913) and his school on the rather complex stimulus of “evaporating power of the air”, there has been, until recently, little detailed and exact study of the behaviour of animals towards humidity as a stimulus. The recently developed methods of Gunn & Kennedy (1936) now make such study possible. By means of these methods, humidity behaviour has been examined in the wood-louse, Porcellio scaber, by Gunn (1937); in the locust, Locusta migratoria, by Kennedy (1937); in the mosquitoes, Anopheles gambiae and A.funestus, by De Meillon (1937); in the mosquito, Culex fatigans, by Thomson (1938) and in the cockroach, Blatta orientalis, by Gunn & Cosway (1938). The work described below deals with the reactions of the adult mealworm beetle, Tenebrio molitor L., towards humidity. This beetle lives in flour, bran and similar products and is moderately resistant to desiccation. It loses about 4% of its weight per day in dry air at 25° C. and dies in 8–9 days. Its resistance to desiccation is therefore less than that of its. larva, which was investigated by Buxton (1930).

The Reactions of Mosquitoes to Temperature and Humidity

Reactions to Temperature1. The reactions ofCulex fatigansto temperature were studied by means of a new type of temperature gradient apparatus based on the same principle as the humidity alternative chamber.2. Females at different stages were exposed to a range of 5°C. at different parts of the temperature scale, and it was found that the sensitivity was very much greater at high temperatures than at low ones.3. The most striking feature of behaviour at all stages was the strong avoidance of high temperatures. This was strongest in the hungry females, less strong in the blood-fed females and those with mature ovaries, and least strong in the newly emerged females.4. Newly emerged females showed avoidance of high temperature below 30°C., but not below 25°C. They also showed a weak avoidance of low temperature. At 29°C. they were sensitive to a difference of 1°C. or a gradient of 0·05°C. per cm.5. Hungry females showed a strong avoidance of high temperature below 25°C., the reaction still taking place below 15°C. There was no avoidance of low temperatures.6. Blood-fed females and those with mature ovaries showed a strong avoidance of high temperatures below 25°C., but below 20°C. they were unaffected by temperature differences. Blood-fed females were sensitive to a difference of 1°C. or a gradient of 0·05°C. per cm. at 23°C.7. Except in the case of newly emerged females there was quite a close relation between the reactions to temperature and the effects of temperature.8. Reasons are given for regarding the temperature reactions ofCulexas of first importance in determining the behaviour of the mosquitoes when seeking a resting place.Reactions to Humidity9. The reactions ofCulex fatigansto humidity were studied by means of the alternative chamber, in a dark constant temperature room at 25°C.10. At all stages the strongest humidity reaction was an avoidance of high humidities above 95 per cent. R.H.11. This reaction was strongest in the blood-fed females and those with mature ovaries, less strong in the newly emerged females, and weakest in the hungry females.12. The avoidance of high humidity was strongest when there was a difference of 20 per cent. R.H., such as a 78–98 per cent. R.H. gradient. Near saturation point all stages except hungry females were sensitive to a difference of 1 per cent. R.H. or a gradient of ·05 per cent. R.H. per cm. Hungry females at this point were not sensitive to a difference of less than 3 per cent. R.H.13. All stages showed a slight but regular avoidance of low humidities, provided a sufficiently large humidity range, not less than 40 per cent. R.H., was present.14. Hungry females, despite the onset of mortality due to desiccation, showed no sharp avoidance of low humidities which were rapidly fatal to them.15. Between 30 and 85 per cent. R.H. all stages were unaffected by humidity differences of as much as 40 per cent. R.H.16. The avoidance of high humidities was equally strong in blood-fed females at 20, 25, and 30°C. At 35°C., greatly increased activity eliminated the reaction.17. The reaction was much less intense in daylight and disappeared altogether after sundown.18. Reasons are given for considering that the measure of humidity which determines the behaviour of the mosquito is relative humidity and not saturation deficiency, even though the latter is the important one in the water relations of insects.

STUDIES OF THE BIOLOGY OF THE DEATH‐WATCH BEETLE, XESTOBIUM RUFOVILLOSUM DE G.

Summary This paper, the second in a short series on biological studies of the death‐watch beetle, Xestobium rufovillosum De G., is concerned with the life of the adult, egg‐laying and hatching. Times of pupation and emergence in the natural habitat of the insect in decayed parts of willow, and in timbers in buildings are discussed and compared. The rate of development of the reproductive organs after pupation and until egg‐laying starts, is described. The habits of the adults, activity, tapping, fight and mode of pairing are commented upon. The major part of the paper deals with oviposition which has been studied out of doors and under controlled tempemture and humidity in the laboratory. Out of doors forty to eighty eggs per female are laid during May and June over a period of 6–9 weeks. The egg stage lasts from 3 to 5 weeks, according to temperature conditions. In the laboratory, oviposition and the hatching of the egg were studied at 15, 20, 25 and 30°C. and relative humidities of 23, 41, 53, 75, 86 and 95 %. The results are discussed in relation to the effect of temperature, relative humidity and saturation deficiency. “Indices of suitability” for various combinations of temperature and humidity in relation to egg‐laying and hatching are calculated from the experimental data. It is shown that an increase in temperature to a maximum between 25 and 30°C. is favourable but that the optimum conditions must include a low saturation deficiency. The data demonstrate an increasing tolerance, by the insect, of dryness associated with a rise in temperature. The application of these findings to the occurrence and spread of Xestobium in buildings is briefly discussed.

Climate, the vital Factor in the Ecology of Glossina

The Seasonal Factor.1. The period of stress due to extreme climatic conditions is far shorter in the forest than in the meadow-pan.2. G. tachinoides is primarily dependent upon riverine vegetation or residual forest, and has only a very limited wet season spread. G. submorsitans is primarily an open woodland fly with great powers of dispersal which are annually checked by adverse climate that enforces a temporary dependence upon the riverine forests.3. G. tachinoides abounds within the stream-bed or residual forest ; G. submorsitans abounds on the forest fringe.4. The dispersal of both species is closely associated with evaporation and saturation deficiency. A mean monthly evaporation of 20–28 cc. and a saturation deficiency of 5 to 8 millibars results in maximal dispersal ; dispersal yields to concentration as the evaporative power of the air increases above these zones.5. The extent of the annual dispersal is indirectly governed by the duration of the wet season—the longer the season the more extensive the dispersal.6. The population of G. submorsitans steadily increases throughout the rains as the evaporation falls, and becomes maximal towards the end when the evaporation is in the optimum zone (20—25 cc.) ; density steadily falls throughout the dry season as evaporation rises, and becomes minimal at the end when evaporation is maximal. G. tachinoides is believed to behave similarly, but the optimum zone of evaporation is probably about 6 to 10 cc.