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, in Ixodes 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 cajennense and A. maculatum; Ixodes canisuga; I. hexagonus; I. ricinus. In dry air water loss throuǵh the cuticle is 10–15 times more rapid in Ixodes ricinus than in Dermacentor 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 of Dermacentor andersoni, Rhipicephalus and Amblyomma spp. 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.

An account is given of a composite virus disease of tobacco for which the name tobacco rosette has been suggested.

The two component viruses, named the mottle and vein-distorting viruses respectively, have been separated, and their symptomatology and methods of transmission described. The mottle virus is both sap and aphis-transmitted, but the vein-distorting virus is aphis-borne only.

The symptoms and histopathology of the complex disease in the tobacco plant are dealt with in some detail. There are three main types of symptoms: (1) intense rosetting, (2) splitting of the tissues, (3) formation of enations on the under-surface of the leaves.

The splitting of the tissues has been examined microscopically, and a number of photomicrographs are given illustrating the formation of the fissures. It is suggested that there is a concentration of virus in the cambium which prevents the formation of the normal xylem. Abnormal tissue and giant cells are formed in the cortex and pith. This appears to set up stresses which cause the splitting.

The insect vector of the complex disease is the aphis Myzus persicae Sulz. Another aphis, M. pseudosolani Theob., is also a vector but is less efficient than M. persicae.

The writer's thanks are due to Prof. F. T. Brooks, F.R.S., with whom he discussed the histopathology of the rosette disease, to Dr Roy Markham for taking the photographs illustrating Pl. I, figs. 2–5, and to Mr Charles Harpley of the Molteno Institute for his assistance in taking' the photomicrographs.

1. Experiments are described on the transmission of mottle and vein-distorting viruses by aphides.

2. A quantitative analysis is made of Watson's experiments on the aphis transmission of persistent and non-persistent viruses. The principal features of the experimental results can be explained in terms of different rates of inactivation of the virus in the insect.

3. It is shown that these considerations explain why non-persistent viruses are more commonly sap-transmissible than persistent viruses.

The nematodes recorded in the present paper were obtained from fishes caught on some of the deep-sea fishing grounds to the west of Ireland, namely, the Porcupine Bank and Irish Atlantic Slope, and from the Irish Sea. The writer is indebted to Mr S. J. Hughes and Mr J. Llewellyn for the collection of material from the deep-sea fishing grounds, excursions having been made for the purpose in a commercial trawler in August 1938 and July 1939. The majority of the nematodes were collected by Mr S. J. Hughes during the 1939 excursion, the first excursion being devoted mainly to the collection of trematodes and cestodes. A record of the trematode and cestode parasites obtained during both years has already been published (Rees & Llewellyn, 1941). Nematodes from fishes in the Irish Sea have been obtained at intervals from 1936 onwards.

Life histories of the Trombidiidae, or, for that matter, other families of mites have, with few exceptions, not been studied and very little information is available as a guide. This is not entirely the fault of the acarologists, as not one of them was employed in any country of the world entirely upon the study of mites. Most of them, if official, were entomologists who could spend only a fraction of their time on mites. Their time was spent therefore on systematics, an essential branch of acarology.

During five months spent in Colombo (April–August 1944) a large number of small mammals and birds were examined for larval Trombiculinae.

From November 1944 to February 1945 similar investigations were carried out on the Maldive Islands. The number of species obtained included three species of the genus Gahrliepia Oudemans, 1912 (not included in this paper), Trombicula deliensis Walch, 1923, Trombicula acuscutellaris Walch, 1923, Neoschöngastia indica Hirst, 1915, Walchia turmalis Gater, 1932 (a second species of Walchia not included in this paper), a new species of Schöngastia herein named ‘maldiviensis’ n.sp., and Womersleyia minuta n.g., n.sp.

Montgomery (1917) showed that Nairobi sheep disease, a tick-borne gastro-enteritis of sheep and goats in East Africa, was caused by a filtrable virus transmitted most commonly by the brown tick, Rhipicephalus appendiculatus. The transmission of the disease by this tick was confirmed by Daubney & Hudson (1931, 1934), who demonstrated also that ticks infected in any instar usually transmitted the disease when fed in the succeeding stage on susceptible sheep; and that the virus ingested by a female fed on an infected sheep passed through the eggs to the larvae of the next generation.

The incidence of east coast fever in areas where the disease is endemic is associated with the common occurrence of the usual tick vector, Rhipicephalus appendiculatus. Outbreaks of the disease in other areas are sporadic or seasonal, and although, in many such instances, R. appendiculatus has been found either in isolated localities, or very sparsely distributed over the whole of an infected area, in other cases it has not been possible to collect even a single specimen of this tick. Collections of ticks from infected farms in these sporadic areas comprise such species as Boophilus spp., Rhipicephalus capensis, R. simus and R. evertsi or Hyalomma spp., Amblyomma spp., Rhipicephalus pulchellus and R. neavei.

1. A new monocystid gregarine is reported from the seminal vesicles of brandling worms, Eisenia foetida Sav. The morphology and movements of the trophozoite are described, and the stages of gametogony and sporogony are outlined.

2. The chromosome cycle is investigated. The haploid number of chromosomes is four, and the reduction division is shown to be zygotic.

3. The systematic position of the gregarine is discussed, and it is placed in the genus Apolocystis as A. elongata n.sp.

Further details of tick distribution (Ixodes ricinus L.) on the pastures of a typical hill farm are given.

Sheep paths (tracks) and their borders have tick populations similar in density to that of the general pasture.

Sheep lairs (night resting places) have significantly lower nymphal densities than the general pasture. With males and females, though the difference is in the same direction, it is less well marked.

On the typical hill, the vegetation layer generally becomes thinner, and nymphal density decreases, with altitude. The case is not so clear with males and females. Provided vegetation conditions are suitable (i.e. sufficiently thick), altitude has no limiting effect on nymphal density distribution within the levels studied. Tick density distribution is therefore a function of vegetation conditions, the latter usually being affected by altitude.

Distribution of ticks on apparently uniform areas of vegetation is studied. Sampling methods are discussed. Probably no more than twelve nymphs per square yard of ground are present on the average in heavily infested pasture. The numbers of nymphs per 100 sq. yd. differed significantly. The number per acre (approx.) also differed significantly in rather more than half the cases observed. Larger areaunits within uniform stretches of pasture probably carry similar numbers of ticks.

Practical applications of the findings to the study of tick control are indicated.

Thanks are due to Dr V. B. Wigglesworth, F.R.S., Director, Agricultural Research Council Unit of Insect Physiology; Dr A. E. Cameron, Edinburgh University; and Captain the Hon. Claud Lambton, on whose farm the work was carried out.

The life history of Ornithodorus delanoei acinus under laboratory conditions at 30°C. may be summarized thus:

Thanks are due to Prof. P. A. Buxton, F.R.S., in whose departmemnt the work was done, and to Dr A.D. Lees of the Insect Physiology Unit, Agricultural Research Council, who made final observations on some of the later nymphal stages and adults.

1. Membranes prepared from chicken skin provide a suitable medium through which Aëdes aegypti females may be induced to gorge.

2. Under suitable conditions the proportion of female A. aegypti which will gorge through membranes, though more variable than when a living chick is offered, is great enough for experimental purposes.

3. It is shown that the gorging reaction in A. aegypti is provoked by a heat gradient between the environment and the food-limiting membrane.

4. The feeding reactions of A. aegypti towards whole blood, fractions of blood, and other sub-stances have been studied. It was found that (a) whole blood, and red corpuscles in saline when ingested through membranes go directly into the stomach which becomes fully distended; (b) haemoglobin in plasma or distilled water is ingested to a lesser degree than whole blood or red corpuscles in saline, and plasma alone is rarely ingested, but all these pass to the stomach; (c) sweet solutions containing glucose or honey are seldom imbibed through membranes and pass to the stomach or diverticula, but only the diverticula are fully distended.

When offered as open drops (a) blood is seldom ingested, but if ingested passes to the stomach; (b) haemoglobin in plasma or water, or plasma alone, are very rarely ingested, but pass mainly to the stomach; (c) sweet solutions containing honey or glucose, or mixtures of blood and honey are readily ingested and pass mainly to the diverticula which become fully distended, though traces may be found in the stomach.

5. Aëdes aegypti may be infected with Plasmodium gallinaceum by allowing them to gorge on drawn infected chicken blood through a membrane. Infection rates comparable to those obtained when the mosquitoes are fed directly on living chickens may be obtained by this method.

6. If infected mosquitoes are allowed to gorge upon uninfected blood through membranes they eject viable sporozoites into the blood. When young chicks are injected intravenously with blood so infected, infections are produced which in period of incubation and intensity are comparable with those resulting from the bites of infected mosquitoes.

7. The ejection of sporozoites through membranes in this manner provides a ready means of obtaining sporozoites free from glandular tissue.

8. Sporozoites collected by this method will be suitable for in vitro experiments upon the action of drugs on sporozoites, and also as a source of material for studying in tissue cultures the developmental stages of the malaria parasite arising directly from the sporozoite.

We wish to thank Prof. D. Keilin, F.R.S., and Dr P. Tate for helpful criticism and advice during the course of the work.

1. The oxidation-reduction potential in cultures of E. histolytica has been studied. It was found that a close relation exists between the potential in the culture and the growth, encystation, and excystation of E. histolytica. The small differences observed in different cultures indicate the reproducibility of the potentials and the reliability of the potential condition in explaining these biological activities of the amoebae in culture.

2. The best growth of trophozoites was observed at potentials between −350 and −425mV., less at potentials between −275 and −350mV., still less at between −200 and −275mV. At potentials between −150 and −200mV. a bare increase in number of trophozoites was noticed. Between potentials of −50 and −150mV. no increase of growth was observed. Trophozoites were found dead in 12 hr. or more at potentials between −50 and +50mV., and died and disintegrated in a couple of hours between potentials of +100 and +200mV.

3. The life span of a culture of E. histolytica was also found to be related to the potential condition. The longer the potential in the culture remains below −150mV., the longer the culture remains positive.

4. Excystation of E. histolytica, no matter in what media, took place only at potentials of −115mV. or lower.

5. Mass encystation of E. histolytica was induced in cultures in which a ‘sharp rise of potential’ occurred after a greatly increased rate of multiplication of trophozoites after 22–24 hr. of incubation. Maturation of cysts occurred during this period of ‘sharp rise of potentials’. If the potential went down shortly after its sharp rise, the cysts failed to become mature and disappeared in about 12 hr.

6. Trophozoites and cysts observed in cultures with very low potentials (below −300mV.) and a rapid multiplication were definitely smaller than those in cultures having higher potentials and trophozoites multiplied slowly.

7. In cultures of amoebae in which mass encystation failed to occur, or mass encystation occurred but cysts failed to become mature, some success was observed in a few trials by artificially creating the ‘sharp rise of potential’ by oxygenation.

8. The pH values noticed in cultures at times of encystation ranged from 6.4 to 6.8. This pH range was a little lower than that observed previously. Mass encystation failed to occur at the change in potential if the pH was 7.2 or higher.

9. It is felt that the process of encystation requires, at least, two extrinsic and one intrinsic factors. The first two are the potential change from more favourable to less favourable conditions and the pH. The last one is the tendency to encyst possessed by the amoebae.

10. The low potential condition in a culture of E. histolytica was found chiefly provided by the activity of the accompanying bacteria. It is, therefore, felt that the indispensability of a bacterial flora for cultivation of amoebae may be explained, partly if not entirely, by the potential relation. To verify this statement, it is hoped to propagate E. histolytica in cultures in which a strongly reducing condition is provided without the presence of bacteria.

The author wishes to acknowledge the kindness of Dr David Weinman of this department in making many invaluable corrections of this manuscript, and he is very grateful to Mrs Elizabeth B. Keller and Mrs Wanda Reed for their help in preparing the graphs accompanying it.