An attempt has been made to assemble the important factors concerned with the atomisation of a kerosene spray and to consider the behaviour of the spray alone, and in its relation with an insect flying through it.

The degree of atomisation attained by the spray gun is dependent upon nozzle design, the spraying pressure, the relative rate of liquid and air flow through the nozzle and such physical properties of the spray fluid as viscosity and surface tension.

All atomisers produce droplets of a variety of sizes which leave the gun with a considerable velocity and are carried along by the air current. At this stage they are at their maximum size and possess their maximum velocity so that the chance of impacting with an insect is great. Later on when the droplets have lost much of their velocity and volume (if they contained a non-volatile insecticide they will have become much more concentrated), they are much less likely to impact. It can thus be seen that a non-toxic carrier can increase apparent toxicity of a spray mist by assisting in the production of the momentum necessary for impaction (David and Bracey, 1944). On the other hand a reduction in the size of the droplet decreases the rate of loss from the air space by precipitation. In connection with evaporation the enormous change in concentration which occurs should be borne in mind. Within a few minutes the smallest droplets will have entirely evaporated, leaving only the non-volatile solute and before this stage is reached the droplet will have become super-saturated. Finally this non-volatile solute will exist as minute nuclei which, when they result from a fairly typical insecticidal formula containing up to say 5 per cent. nonvolatile material, may be so small as to settle only very slowly.

Regarding the response of the insect to the spray mist, it may be said that this depends upon the resistance of the insect and the dose of insecticide picked up. Since the spray mist takes some time to distribute evenly and since it is more easily picked up during or immediately after spraying and by insects in flight, it can be seen that the dose received by individual insects will vary a good deal unless the insects behave identically so that under these conditions there is little correlation between the comparative effect recorded on individuals and their relative innate resistance.

To the above remarks arising out of the theoretical discussion which formed the first part of this paper must be added the following conclusions from the experimental work.

For some years prior to 1935, the cotton Capsid of Uganda, which is now known to occur in several other parts of Africa also, was called Lygus vosseleri, Popp. In 1935, Mr. W. E. China of the British Museum (Natural History), after examining type material, published a note synonymising vosseleri of Poppius with simonyi of Reuter and concluded that the cotton Capsid previously identified by him as L. vosseleri, Popp., must therefore be called L. simonyi, Reut. This latter name has been in general use for the cotton Capsid since 1935 and is very widely known because of the great importance of the insect as a pest of cotton.

1. During the past eight years field investigations on the epidemiology of yellow fever have been carried out in Bwamba County, a small heavily-forested area in the extreme west of Uganda.

During the course of this work yellow fever virus has been isolated from a human case, from Aëdes (Stegomyia) simpsoni, Theo., and from a mixed lot of Aëdes spp. taken in uninhabited rain-forest.

As it is known that yellow fever is endemic among the monkeys of the Bwamba forests and that a high rate of immunity is shown by certain species that rarely descend to the ground, it was concluded that an arboreal mosquito was the most likely transmitter of yellow fever among monkeys, and an investigation of the arboreal mosquito fauna was begun.

Kashmir valley is the largest deciduous fruit-growing tract in India, and, owing to the increased demand for fruit, many orchards, both large and small, have recently been laid out with technical assistance from the Department of Agriculture. Apple is the first choice of the fruit-growers, but as the trees begin bearing later than many stone-fruits, it is a common practice to plant peaches and nectarines between the rows of the young apple trees; this produces early returns without disturbing and exhausting the soil by ploughing and growing annual crops. With the rapid development of road transport, it is hoped that peaches will find a market outside the valley. The peaches of Kashmir are not so well known as are its apples and pears but with better attention to the trees an increase in production and an improvement in quality should be possible.

The best way of preventing locust outbreaks is to control them at their source. After a description of those habits of the Red Locust that have a direct bearing on control, an account is given of some of the results of three years' experience of control of this species in an outbreak area. The methods used in Rukwa are described in detail, and attention is drawn to their relative efficacy, advantages and difficulties. A number of other methods are also referred to and their values discussed. Control methods are considered with special reference to conditions in Rukwa and other outbreak areas, and in relation to large scale campaigns against large outbreaks. Conclusions on the relative values of the various methods are summarised below. These values depend both on the habits of the locusts and on the ecological, economic and social conditions of a country in which a campaign is to be waged. It must not be assumed that conclusions about control methods reached with one locust or in one country necessarily apply to another locust or another country.

The yellow tea-mite should in future be known as Hemitarsonemus latus (Banks) Ewing.

The males have a curious habit of carrying female pupae to younger leaves. The pupa is held above the male's body by means of a sucker-like organ near the tip of his tail-like, posterior terminus. Thus the male plays an active part in distributing the pest.

Mating has not been observed, but normally it occurs as soon as the female becomes adult. It may also occur later in her life.

Unfertilised females give rise to male offspring only, but mated females have mixed families.

Four insect parasites of the carrot fly have been encountered and of these three have been identified as Dacnusa gracilis, Loxotropa tritoma, and Aleochara sparsa. The fourth is a member of the genus Kleidotoma, species not yet ascertained.

The life-history, development and host relations of D. gracilis and L. tritoma are given together with data on their distribution and range of parasitism.

L. tritoma also occurs as a hyperparasite of D. gracilis but this appears to be quite fortuitous.

Experimental evidence is brought forward to show that the first instar larva of L. tritoma goes into a diapause in early July, that a period at a low temperature is necessary to break this and hence development is only completed in the following spring.

A description is given of the three larval stages of A. sparsa.

Brief reference is made to the Kleidotoma sp. which appears to be of rare occurrence.

The following discussion is devoted chiefly to toxicological studies, but in order to make the picture more complete, other methods of control of Capnodis and their practicability in the light of the studies presented in three earlier papers will first be referred to briefly.

Since December, 1942, a supply of adult house-flies, averaging approximately 2,500 a day, has been required in this Laboratory for experimental work on fly sprays. The production in quantity of insects suitable for such use is by no means a straightforward matter and a considerable amount of work was undertaken before a satisfactory technique was established.

1. Two huts were selected which were surrounded by dense bush, which varied in height from 4 to 15 feet. After elimination of this bush within a radius of 140 yards from these huts, the proportion of the population of ♀ A. funestus and ♀ A. gambiae attracted to them was at least 90 per cent. of the proportion which reached them before the bush clearance. This result indicates that bush clearance is not a justifiable measure for reducing Anopheline infestation. Even the male Anophelines entered the cleared area in the same proportion as they did before clearance.

2. While bush clearance was in progress there was a temporary sharp reduction in the infestation, and the proportion of the female Anopheline population attracted declined to between one-third and one-quarter of the proportion attracted before clearance. The proportion of males dropped by one-half. At the same time the ♀ Anophelines were apparently much less able to discriminate between quantitative olfactory stimuli. Both effects are attributed to masking by an obscuring odour from the cut and withering bush.

3. There was no difference between the numbers of A. melas attracted to a hut screened by a 10 yard thick belt of dense high bush and the numbers attracted to an identical hut in the adjacent clearing 25 yards away.

(1) Spraying of huts with 0·1 per cent. pyrethrum in kerosene, at the rate of 25 c.c. per 1,000 cubic feet, deterred 90 per cent. of A. minimus from entry on the following night. The repellent effect persisted in diminished degree for at least four days.

(2) A rather larger quantity of pyrethrum, sprayed in a Freon aerosol, produced a similar but somewhat less marked effect.

(3) The repellent effects of Lethane 384 in kerosene were much less marked, and not discernible on the second night.

(4) Different species varied greatly in their sensitivity to the repellents.

(5) Two types of repellent effects were indicated. Repellent effects proper were the most important, and males were more sensitive to these than females of the same species. There was also a masking effect, to which anthropophilic females were sensitive, which resulted from obscuring of attractive human scents by the chemical odours.

Simple tests have been done to appraise the value of garments impregnated with DDT as an anti-louse measure. It is very toxic to the body louse but its action is relatively slow. Woollen and cotton fabrics can be impregnated with it.

No estimate can be given of the precise length of the insecticidal life of a garment because of variations in wear, etc. But garments have been found to have o·1 per cent. DDT remaining after five launders and three weeks' wear. The minimum safe degree of impregnation as an anti-louse measure appears to be o·1 per cent. By the use of simple biological tests reasonably accurate assessments of the chemical content of garments can be made.

DDT is to some extent resistant to high temperatures. It is suggested that the chief cause of loss of insecticidal power is due to loss of nap. Impregnated garments transfer their insecticidal power to uriimpregnated garments kept in contact with them.

Results show that subsequent mortality is complete or very high among lice which have crawled for three hours on fabrics impregnated with DDT at rates between o·1 and 1·75 per cent. Signs of poisoning may take six hours to be manifested in the louse.

The results obtained with a sleeve test compared very closely with those obtained by the box method used chiefly in this work.

The theory is put forward that DDT enters the body louse through its tactile sense organs.

A previous investigation by the writer established that the minimum deposit of DDT on both cement and old painted wood necessary to ensure complete mortality of bed-bugs 48 hours after spraying was 0·2 mg. per sq. cm., and that the toxicity of both surfaces diminished slowly over a period of six months. The present work has shown that when DDT is applied with an adhesive agent, the minimum deposit of insecticide necessary on these surfaces to yield a similar kill at the same interval after treatment was only 0·06 mg. per sq. cm. The addition of an adhesive agent to a DDT spray thus effects considerable economy in the use of DDT. Campbell and West (1944), investigating the toxicity to houseflies of coumarine resin and DDT dissolved in white spirit, have reported an increase in the efficiency of DDT when sprayed with an adhesive agent. These authors confined their investigation to a resin only and they suggested that the coumarone resin conferred, in a manner not specified, increased insecticidal activity on the DDT. The present work has shown that relatively small doses of DDT are highly effective in the presence of at least three dissimilar adhesive agents. It is possible, therefore, that instead of increasing the activity of DDT, the agent merely blocks the minute pores of the treated surface, and by minimising absorption of the solution, increases the amount of DDT available to the insect. Alternatively, the adhesive agent may have some affinity for substances on the surface of the bug's tarsi and so assist transfer of DDT from the sprayed surface to the insect.

The residual toxicity of cement treated with DDT and an adhesive agent, like that of a similar surface treated with DDT alone, diminished slowly. The toxicity of painted wood, on the other hand, treated with doses of DDT and adhesive agent ranging from 0·005 to 0·02 mg. per sq. cm., decreased within one month, irrespective of the adhesive agent used. This fall in toxicity is puzzling, particularly in view of the fact that no such decrease occurred when the surface was treated with a heavier deposit of DDT and adhesive agent, namely 0·06 mg. of each per sq. cm. This latter observation seems to preclude a possible interference with the efficiency of DDT by a slow interaction of the paint constituents and the adhesive agent.

The application of these adhesive agents, even in relatively high doses, failed to prevent the removal of DDT from a smooth surface by repeated washing and rubbing; furthermore, an increase in the deposit of adhesive agent does not markedly increase the insecticidal efficiency of a given deposit of DDT.

There has been much confusion in the taxonomy of the species of Dysdercus in the Caribbean area, and this contribution is an attempt to straighten out the nomenclature and synonymy for future workers on these important pests, until such time as a complete revision of the Neotropical Dysdercus can be attempted.

1. Owing to the history of yellow fever in the Sudan and the lines of communication which traverse the country, a general survey of the mosquitos, particularly the Culicines, is of practical interest. The known potential vectors of yellow fever are listed.

2. Observations on the man-biting or man-ignoring habits of some species are recorded and also notes on the bionomics of Taeniorhynchus africanus, T. uniformis, Aëdes aegypti and other species.

3. For purposes of description the country is divided into sub-divisions of the recognized faunal areas and notes are given on the Culicines of each.

4. Brief observations are made on mosquito control measures against yellow fever.

Experiments are described which illustrate some of the ways in which the results of exposing insects to a spray mist are dependent on either their behaviour or physiological state. Only fine mist dispersals with particles below 10 microns in diameter are considered and in such mists the flight movements of the insects are very largely responsible for the collisions between the insect and the droplets. An increase in flight activity leads to the accumulation of a greater number of spray droplets and consequently a higher kill. Under these conditions insecticides such as pyrethrins which exert a strong irritating action on Aëdes in minimal amounts are superior to DDT which does not have this effect. For practical applications under circumstances where the sprays depend on mist action to produce their effect it is advantageous to mix pyrethrum with the DDT sprays so that the maximum dose of the latter may be accumulated.

Further experiments are concerned with the relative resistance of male and female Aëdes aegypti to insecticides and the change of resistance which takes place on ageing or as a result of a blood meal. It is shown that the male is innately more susceptible to both pyrethrins and DDT and that the resistance of the females is higher after a blood meal. There is also a progressive decrease in resistance with age.

The yellow fever mosquito, Aëdes aegypti, has been employed in experiments on the mode of action of adjuvants. With this insect under the test conditions employed, a decrease in the rate of knockdown was observed as a result of adding sesamin, sesame oil or IN 930 to a pyrethrum spray. It was shown that the increment of kill produced by activators was not attributable to any effect on either the particle size of the mist or the behaviour of the insect which led to an increase in the dose accumulated during the test exposure. No activating effect was apparent with DDT. The final mode of action remains unexplained.

(1) Sprays that are atomised and mingled with air were unsuitable because they were wasteful in both material and labour.

(2) Knapsack oilers and modified stirrup pumps were efficient. Where labour is cheap hand-operated sprayers of this type are more economical than motoroperated machines.

(3) The relation between mosquito mortality and infectivity is discussed and it is calculated that any treatment which produces 75 per cent. mortality among the mosquitos exposed to it should effect malaria elimination.

(4) The effects of a DDT emulsion were much more durable than those of a similar quantity of DDT in kerosene solution. The difference was much less marked when Gammexane was used. Emulsions will probably displace kerosene solutions as vehicles for the application of residual films.

(5) The persistence of the effects of DDT was markedly increased by increased dosage, so that it will be most economical to apply DDT in heavy doses at long intervals.

(6) The persistence of Gammexane effects was not greatly increased by increased dosage because it evaporates much more quickly than DDT. It is not likely to be effective for much longer than 10 weeks.

(7) Treatments with DDT were usually effective against A. minimus for about twice as long as against A. vagus or Culicines, but treatments with Gammexane had similar effects on all species.

1. The taxonomy and bionomics of the eight species and subspecies of Eretmapodites found in the Bwamba County, Uganda, are discussed.

2. Experiments are described to show that the larvae of these species are predatory, sometimes very actively so.

3. The females of all the Bwamba species bite man by day, sometimes in large numbers.

4. The larva and pupa of E. inornatus, Newst.; larva, pupa, and adult female of E. penicillatus, Edw.; larva, pupa, and adults of E. ferox, sp.n.; and larva and pupa of E. leucopus subsp. productus, Edw., are described for the first time, special reference being paid to the range of individual variation. The taxonomy of the various previously described species is also treated briefly.

5. Keys are provided for the identification of the known larvae, pupae, males, and females. These keys have been prepared in order to include the new material, and to delete certain unreliable characters previously regarded as of specific value.