For a study of population trends of Diarthrothrips coffeae, Williams, on coffee in Kenya, and to assess the value of sprays for its control, a method of sampling the population was required. This insect feeds on the leaves, green shoots, and berries ; but by far the greater part of the population lives on the leaves. Attempts to count the insects on the leaves in the field, by naked eye or with a hand lens, proved difficult and inaccurate. Two other methods were tried, those of E. A. McGregor (Pan-Pacif. Ent., 3, 1926, p. 29) and J. W. Evans (Bull. Ent. Res., 24, 1933, p. 349).The former proved unsatisfactory for several reasons such as the failure of the insects to forsake the leaves, the collection of debris on the grease tending to conceal insects, and the difficulty of counting the insects in the grease. The method of Evans was designed originally for obtaining the thrips from blossoms, but an examination of the method was made with the coffee thrips. Fairly consistent results were obtained with it, but the figures were low and it was clear that the sampling was not complete. It was found that a number of insects remained among the leaves and failed to reach the paper at the bottom of the cylinder. There are other disadvantages ; for example a large number of the cylinders are necessary to carry out a single spraying experiment, they are usually difficult to come by and the work of counting has to be done in the field, requiring a rather elaborate provision of tables and microscopes. Further it was found that even a light breeze was apt to cause loss of thrips during the counting.

Since 1930 the European spruce sawfly (Diprion polytomum) has caused severe damage in eastern Canada and is a meance to the spruce woodlands in the northeastern part of the United States. In 1938 under a cooperative arrangement with the Division of Entomology, Canadian Department of Agriculture, and the Farnham House Laboratory, Imperial Institute of Entomology, England, which were to continue their collecting in the central European area for the recovery of natural enemies, the Bureau of Entomology and Plant Quarantine, United States Department of Agriculture, made a survey of the Scandinavian and eastern Baltic countries to extend the knowledge of the distribution of the sawfly ; to discover, if possible, better or equally as good collecting grounds, where new species of parasites might be found ; and to obtain additional information regarding the better known species of parasites. The most favourable habitats of the sawfly are found in the one-generation areas under conditions which provided a maximum of sun-heat, moisture, and shelter, near to the upper limits of spruce growth. The least favourable habitats are the drier, less humid places, especially in the two-generation areas. The sawfly was found everywhere on spruce, from the German-Danish border in Denmark and in southern Norway to the northernmost spruce forests of Sweden, and from far above the Arctic Circle in Finland southward through the eastern Baltic countries of Estonia, Latvia and Lithuania. The range of distribution covered in this survey was from approximately latitude 55° in Denmark and Lithuania to latitude 68° 45′ in Finland. This includes most of the Scandinavian Peninsula, Fennoskandia, and the eastern Baltic countries. Spruce comprised from 25 to 40 per cent. of the productive forest land of this large area. Large collections of the European spruce sawfly were made in Sweden and Finland for the Canadian Division of Entomology.

1. When spring migrants of Aphis fabae, Scop., from Euonymus europaeus and Viburnum opulus are given a choice of summer host plants enclosed by a muslin cage, they colonise bean first and then sugar beet. Dock, poppy and Chenopodium are colonised in no very definite order, but thistle is the last plant to be infested.

2. Further infestation of surrounding plants takes place rapidly by means of apterae, which migrate actively and colonise plants at random.

3. The total number of Aphids on the summer host plants is of the same order as the times of colonisation.

4. The effect of parasites and predators is proportional to the numbers present and to the size of the colony.

5. The results do not indicate the presence of biological races on the chosen host plants.

I should like to thank Mr. F. G. W. Jones, of the School of Agriculture, Cambridge, for his help in preparing the ground for the plots and in rearing some of the plants, and Dr. W. H. Thorpe for his criticism.

In 1939 I dealt with two species of Ephialtes which parasitise the codling moth, Cydia pomonella, L., namely E. caudatus, Ratz., and E. crassiseta, Thoms., and showed how they differed from E. punctulatus, Ratz. (=extensor, Tasch.) with which species they had formerly been confused. During 1940, F. J. Simmonds, of the Imperial Institute of Entomology, sent to the British Museum a series of a species of Ephialtes bred from this same host in the south of France. This is a new species, and it was previously known to me only from a single female which had been bought by D. S. Wilkinson from O. Schmiedeknecht, who had incorrectly named it Pimpla roborator, F. In fact, this new species does not belong to the same species group as Ephialtes (Exeristes) roborator, F., Grav. (=Pimpla roborator, F., Schmied.), which is a well-known parasite of the European Corn Borer (Pyrausta nubilalis, Hb.) and also parasitises many other hosts.

As there has been considerable confusion in the synonymy and interpretation of E. roborator, F., it seems advisable to give notes on the relevant type specimens seen by me. It has been impossible to find the type of Ichneumon roborator, F. Gravenhorst's interpretation of this very variable species has therefore been taken. The description given by Schmiedeknecht in his Opuscula Ichneumonologica is adequate for the recognition of the female ; the male is very distinct, having a tubercle in the middle of the clypeus, a character unknown in any other described European species. Pimpla cicatricosa, Ratz., which is given by Schmiedeknecht as a synonym of P. roborator, F., is a distinct species.

One or two errors in “ Mosquitoes of the Ethiopian Region ” have recently come to light, and several new Culicine larvae have been discovered since the first volume was published. I take this opportunity to correct the errors and to indicate the positions which the newly-described larvae should occupy in the keys. I have not seen most of these larvae and have inserted them in the keys solely from the descriptions.

A number of the mosquitos of which I described the larvae in Vol. I of the monograph were incorrectly determined, but Edwards has already pointed out these misdeterminations (in Vol. III) and I have not thought it necessary to mention them here.

At the request of Mrs. G. R. Cunningham van Someren (formerly Miss E. C. C. Macdonald), I have added her description and figure of the larva of Aëdes adersi and her description of that of Culex adersianus as an appendix (see p. 178).

The examination of mosquito larvae collected by house searchers on Stegomyia control measures in Dar-es-Salaam, the largest town in Tanganyika, shows that Aëdes aegypti is definitely the predominant domestic breeder there. The other species of the genus Aëdes encountered are listed with their frequencies.

Similarly the examination of larvae found in tree-holes in Dar-es-Salaam shows that Aëdes aegypti, A. simpsoni and A. metallicus are regularly encountered.

A system of obtaining an index of the population density of both A. aegypti and A. simpsoni, unconnected with the actual process of Stegomyia control measures, is described. This would be applicable to smaller towns, rural districts, and towns in high rainfall areas where A. aegypti is not the only danger as a yellow fever vector, and where extra-domestic breeding is important. The system is based on the exposure of a fixed number of bamboo pots containing water for a constant period of days, to attract egg-laying female mosquitos. Species of mosquitos and their frequencies obtained in a series of trials are given.

In practice it is seldom found that any one of the numerous grades of oil prepared from crude petroleum is a satisfactory mosquito larvicide. The final product is almost invariably blended from several constituents. Each of the constituents may confer beneficial properties on the larvicide directly, or it may serve to correct undesirable features possessed by the other ingredients.

Even under world conditions which might be regarded as normal compared with those prevailing to-day, difficulties have confronted those who wished to select or blend a larvicide. There was firstly, in the remoter parts of the world, a lack of any specialised chemical apparatus ; secondly, the lack of a complete minimum performance specification by which to judge laboratory blends ; and thirdly, the difficulty arising from the fact that the specifications supplied for commercial grades of oil do not include data on many properties of the oil which are important to the malariologist but unimportant to the usual consumer.

This paper represents an endeavour to overcome the three difficulties outlined above. It provides a minimum performance specification for a larvicide and describes a series of simple tests which may be applied (a) to a commercially blended larvicide in order to establish whether this meets the requirements of the specification, and (b) to the individual ingredients (e.g., kerosines, Diesel oils or fuel oils) so that, in each case, the one with the most desirable combination of properties may be selected for incorporation in the final blend.

Insects immersed in oils exude minute droplets of water over the surface of the cuticle. These appear more rapidly in light oils than in heavy oils and are more numerous in young insects than in old. Immersed in mixtures of oils and alcohols which separate in contact with water they show a vigorous effervescence at those regions where the mixture penetrates most rapidly to the water in the cuticle. There is a delay of varying duration at different parts of the cuticle when heavy oils are used. This delay disappears if the lipoid layer is first extracted with petroleum ether.

Factors controlling the rate of entry of pyrethrum through the cuticle have been studied by the application of pyrethrum in oil to a restricted area of the abdomen of nymphs and adults of Rhodnius. Entry is more rapid in light than in heavy petroleum oils ; it is much accelerated if the cuticle is first treated with petroleum ether. Entry is very slow in vegetable oils. There are great individual variations, due chiefly to the thickness of the endocuticle, which in turn is determined by the size of meal taken before moulting, the age and the amount of food taken after moulting, and the degree of stretching of the cuticle. There is evidence that the pore canals are important in the passage of pyrethrum through the endocuticle.

Histological examination of the integument shows that in the nymph of Rhodnius oils are taken up first by the epidermal cells in the zone around the bristles ; later by the general epidermis. The uptake of oil is greatly increased if 5 per cent. of oleic acid is added to refined petroleum. In the adult Rhodnius soon after moulting, oils are taken up by the general epidermis. In old insects they are taken up solely by the dermal glands. There is no visible entry through the sockets of the bristles.

The entry of pyrethrum in oils is accelerated by the addition of oleic or other fatty acids.