(1) The medical importance of trombiculid mites is indicated and reference is made to their geographical distribution.

2. Methods used by investigators for assessing the distribution of the mites in the Far East are discussed as well as methods used in the study of Trombicula autumnalis.

3. A detailed account is given of a light-trap and the method of using it; the reaction of the mites to the trap is discussed.

4. The results of a detailed survey are presented to show the significance of the method in relation to the distribution and bionomics of trombiculid mites.

The main results of this survey are as follows:

(a) The mites are characteristically distributed in pockets of infection.

(b) A study of the ecoclimatic conditions favourable to mites shows that humidity is a limiting factor.

(c) The free-living larval stage disappeared from the soil in October, and nymphs were found in the soil in September. These facts, together with the results of laboratory breeding experiments at room temperatures, which showed that it was normal for nymphs to appear in October, indicate that the nymphal and adult stages over-winter.

(d) Mites are not associated with fruit bushes if the physical conditions of the soil are unfavourable. A tentative explanation is given to account for the frequent association of fruit bushes with miteirritation.

(e) The gregarious habit of clustering is primarily due to a tactile response.

5. It is concluded that the light-trap method offers a new approach to the problem of surveying an area in order to diagnose trombiculid mite infection. It is also of value in obtaining abundant material for laboratory investigations.

Since it is concerned with filling in certain details of a picture, long since sketched in outline, the new work in this paper permits only a somewhat disjointed summary.

From a study of Ixodes ricinus L. in nature, the main new facts emerging are:

(I) Unfed adults in a deep vegetation layer such as presented by rough hill or moorland pasture dominated by bents, heather or bracken. (i) Practically all inactive unfed ticks are in the underlying mat, and nearly all of these in the upper portion of the mat. This distribution does not change, summer or winter. (ii) When the tick is active its progression is practically confined to the vertical, i.e. between its niche in the mat and the vegetation tips immediately above. Random undirected movements due to certain circumstances may achieve a 0–8 in. (average 2 in.) change of position in the horizontal plane; but there is apparently no ruling urge to move in the horizontal plane. (iii) The unfed tick comes to the vegetation tips to await a host for only a limited time during the 3-month activity season. Commonly this amounts to about five periods of 4–5 days each if no host is forthcoming. Between the periods of activity it returns to the upper mat. If it stayed constantly at the tips its survival time would be much shorter than normal. (iv) From nymphs engorged in spring, adults emerge in autumn but remain inactive for a considerable period. About half of them may perish within the first 100 days after emergence. Survivors become active during the following March–June. Effective life ends when the tick, if still unfed, finally stops coming to the vegetation tips, the only position from which a host can normally be achieved. After emergence, the effective life of the unfed adult deprived of a host is less than one year; and the actual life is not much more than one year at best. Thus adults not finding a host in spring will be ineffective, i.e. die without reproducing before the next activity season.

Spatial distribution is understood to connote distribution of density (including zero density) over the land surface at a given time. Combining fact and conjecture, the following is a theory of spatial distribution of Ixodes ricinus in Britain. The tick passes practically the whole of its 3-year life span on the ground, spending a total of only 3 weeks on hosts. A considerable amount of vegetational cover is necessary for efficient survival on the ground; the more important limiting circumstances in lesser cover seem to be humidity (conditioned by temperature) in summer, perhaps temperature (alone) in winter, and the activity of predators. A host is necessary for mating as well as food purposes. Though small at best, the tick's chances of achieving a host are greater the more hosts on the ground. In practice this means the more sheep, since the latter are by far the main hosts. For the most part, spatial distribution to-day can be explained adequately in terms of interplay between amount of ground-cover and of host-potential, with cover the master factor. Thus poor cover keeps the heavily stocked lowland pastures free of permanent tick population even when opportunity of colonization is offered. The tick is confined to the hills and moorlands with their deep vegetation layers. There, within limits, it is more numerous on a pasture the more universal the distribution of adequate cover (because it drops at random from the host) and the higher the host-potential. Among hill and moorland pastures, however, there are some cases where, despite ample cover and hostpotential, either (1) no ticks are present, or (2) the tick population is lower than it ought to be. These cases can be explained on grounds of lack of opportunity for colonization (1); or colonization so recent that population has not attained the level of ‘dynamic equilibrium’ with the environment (2). The question of ‘spread’ (colonization) is discussed fully in the text. The sheep flock is the main agent in spreading ticks. Spread is comparatively slow and relatively small in extent nowadays. There has very probably been considerable increase both in tick population densities and in infested areas since the advent of intensive hill and moorland grazing with farm stock some centuries ago.

1. Two specimens of Coenurus serialis in the rabbit are described.

2. The duration of infection is estimated at 18–24 months.

3. Secondary external daughter coenuri had developed within the same adventitious cyst of host tissue as the primary coenuri.

4. No internal daughter coenuri were found within the primary coenuri.

5. Both primary coenuri contained some scolices which had developed vesicles (ranging from 3 to 30mm.) in their ‘neck’ region. These vesicular scolices remain attached to the germinal layer until the vesicle is 4–5 mm. in diameter, when they become detached. These would appear to be the internal daughter coenuri of many authors but are here shown to be degenerating scolices.

Trombicula tamiyai and T. fujigmo n.sp. are described and figured from Japan and northern Burma respectively; the holotypes are in the U.S. National Museum. The first, unnamed or referred to European autumnalis, has previously been illustrated and discussed by some Japanese authors with T. japonica Tanaka et al., under the term Japanese harvest mites (‘akidani’). Figures of T. japonica are also provided here for comparison. Reared nymphs and adults are reported by the Japanese as unusually elongate for trombiculid mites. Figures of the elongate nymphs are comparable with that given by Hirst for T. autumnalis, the European harvest mite, the adult of which is likewise elongate. Should further data, including rearing, indicate the need for generic separation of this group of ‘harvest mites’ from Trombicula, then Neotrombicula Hirst would apply. Specific characters for other apparently related species are tabulated and discussed.

It is generally accepted that the various species of Leishmania of men and dogs are morphologically indistinguishable. Therefore it has been argued that, according to the present rules of taxonomy, they should all be included in one species, L. donovani (Laveran & Mesnil, 1903), Ross, 1903. A review of the literature indicates, however, that this view is not generally acceptable, while strict adherence to the morphological rule would require the inclusion of the Leishmania of lizards (L. hemidactyli, L. agamae, etc.) in the same species as those of men and dogs.

The specimens of this new species of mite were collected by Mr Gordon Thompson, formerly curator of the Science Museum of the Institute of Jamaica, from the legs of a specimen of the golden thrush, Turdus aurantiacus Gmelin, taken at Morant Bay Road, 14½ miles east of Kingston, 14 January 1947. The bird was a male, and in a letter dated 11 March 1947 he says the mites were ‘extracted from a “growth” after macerating it for a while in caustic potash. This is the only “growth” of its kind I have found among approximately two hundred birds examined in Jamaica. The bird seemed quite healthy.’ I have to thank Mr Thompson for his kindness in handing me this, together with other acarine material, for study and determination.

The present paper is mainly concerned with the records and descriptions of new species of parasitic acari obtained in Ceylon and sent to me during 1947 by Mr Gordon B. Thompson, of whose collection they form a part. The study of the specimens of the tropical rat mite, Echinolaelaps echidninus (Berl.), represented in this material led me to re-examine both the literature and the specimens of this species contained in my own collection, and I have included in the present paper the results of this investigation and have established various forms of the species which this necessitates besides showing the probable relationships existing between them.

Most of the nine species dealt with in this paper were collected before the outbreak of World War II. Library facilities in Ceylon, meagre enough even in normal times, were altogether unavailable during the war years owing to the removal of the Colombo Museum Library to safe areas inland. It was only after the library was brought back to Colombo at the beginning of 1947 that this paper could be got ready for publication. Hence it appears 10 years after the last contribution in the series.

A method for comparing the lethality effect of contact insecticides on mosquitoes and house-flies is proposed.

It is based on the comparison of the death-rate and the length of survival of insects after a contact for a very short and exactly measured time with a surface sprayed with a known concentration of the insecticide.

It is suggested that the same method may be used to compare the susceptibility of various species of mosquitoes and flies to the toxic action of a contact insecticide.

Some of the methods and apparatus were worked out and constructed by the Staff of No. 1 Malaria Field Laboratory under the command of the senior author. The following Officers and N.C.O.'s of the Royal Army Medical Corps were especially active in this work, and acknowledgement is made of their valuable contributions: Major T. T. Macan, Major Fox, Lt. MacDonald, Staff-Sgt. (later Major) Iles, Cpl. Mortimer.

We are also indebted to Prof. F. S. Bodenheimer and Major T. T. Macan for their help in preparing this paper.

Many workers have described abnormal forms and structures in ticks. Neuman (1899) was the first who recorded some abnormal Ixodidae. Brumpt (1934) reviewed the literature and recorded some new interesting abnormalities. The posterior region of the body of one of the specimens described by Brumpt had two ani and four stigmata. Brumpt considered that only the Ixodidae display abnormal forms, but since then Robinson (1943–4) described abnormalities in laboratory-bred Ornithodorus moubata. He also noticed a ‘partially twinned’ tick which mated, and laid eggs which developed into normal ticks.

A critical review is presented of the species of Dinobothrium and their probable synonyms. D. septaria v. Beneden and D. planum Linton are accepted as valid species. D. keilini Sproston and D. plicitum Linton are regarded as of doubtful status, the former being probably a synonym of D. septaria, while D. plicitum may be a synonym of D. planum. The type-specimens of D. keilini and of D. paciferum Sproston have been re-examined. A new interpretation is offered of the structure of the scolex of D. paciferum, and some other details of its anatomy are redescribed. A small species from Cetorhinus maximus is described, which is very similar to, and may prove to be identical with, Dinobothrium paciferum. As it apparently differs in having the neck and parts of the bothria covered with spines, it is treated as a distinct species and named D. spinosum. The question of the specific identity of the known larval forms of Dinobothrium is briefly discussed.

Five main types of harvest mite larvae have been found in Great Britain. These may be distinguished according to the following key:

(1) Galeal setae ciliated

Galeal setae not ciliated (2)

(2) Scutal setae duplicated

Scutal setae not duplicated (3)

(3) Humeral setae duplicated Humeral setae not duplicated (4)

(4) Eight setae in both the second and

the third rows of dorsal setae Normal type a

Six setae in both the second and

third rows of dorsal setae Normal type b

Eight setae in the second dorsal

row but only six in the third Normal type c

Type a and type b may be further divided into groups based on differences in scutal width and setal length. Nevertheless, scutal width, setal length and setal number are not correlated with each other. Were larvae to be separated into two species on the hypothesis that one of these characters (e.g. differences in setal number) is genetically determined, then the range of variation with respect to the other two characters (namely scutal width and setal length) would be the same for each species. Forms with duplicated humeral setae do not differ otherwise from the normal type. The ciliation of the galeal setae and the duplication of the scutal setae is not consistent and larvae of mixed or intermediate type are known. It seems to me that these morphological differences may not be genetically determined but are solely phenotypic (and in the case of setal number may be influenced by rainfall and temperature). This, together with the lack of points of distinction among nymphs and adults, suggests that there is only one British species which was first described as Acarus autumnalis (Shaw, 17 90). As no type is known to exist, a lectotype has been selected from British material and deposited in the British Museum.

The systematic position of this species is difficult to determine. Until larvae of known ancestry can be reared under controlled conditions the systematic significance of the variations described above must remain in doubt.

Nymphs and adults do not appear to differ greatly from those described by André (1930). The adult is recorded for the first time in England. The male genitalia are described.

Previous work is discussed and described. The numbers of larvae present in a square yard of soil can be estimated by the use of caged guinea-pigs or by light traps. The infestation observed on the ears of wild rabbits gives a rough index of the population present in a given area.

Using a modified light trap, gorged larvae can be collected from their hosts.

Adults can be extracted from light, friable soils by flotation in water and from any type of soil by the Salt-Hollick soil-sampling machine.

The harvest mite occurs throughout the British Isles. It seems possible that climate and the geological origin of the soil do not affect harvest mite abundance. They are, however, less numerous on heavy clays.

Larvae are most abundant in late summer and autumn. Adults remain constant in numbers throughout the year.

The larvae appear to infest most warm-blooded vertebrates. They do not attack at night. Nymphs have been observed sucking the contents of nymphophanes.

Adults have been found at a depth of 3 ft. They appear to come nearer the surface in warm, moist weather, but are driven down by drought, heavy rain and ground frost.

1. The cycle of vertical migration of the infective larvae of Haemonchus contortus on experimental plots of Lolium perenne S 24, kept out of doors, has been related to meteorological and micrometeorological factors.

2. The three main factors concerned with vertical migration are temperature, humidity and light intensity.

3. In the majority of cases the greatest number of larvae are on the grass blades in the early morning and in the evening, less being recovered at night and during the middle of the day. The time of the morning maximum becomes progressively earlier passing from winter to summer, and the time of the evening maximum progressively later. The reverse is true in the second half of the year.

4. Low and high temperatures accompanied by low humidity inhibit vertical migration on the grass.

5. High humidity (continuous precipitation) does not prevent migration, the two daily maxima are apparent, but the numbers climbing are less than in the absence of rain.

The literature on mosquito larvae as intermediate hosts of larval Trematodes is scanty. Porter (1938) states that it is well known that anopheline larvae act as intermediate hosts for Xiphidiocercaria, but she cites no instances. Harper (1929) experimentally infected twenty-five larvae of Culex pipiens with the Xiphidiocercaria, X.I., whilst Eckstein (1922) and Ruge (1903) record finding the encysted stage in mosquito larvae.

1. The development of exo-erythrocytic parasites of Plasmodium gallinaceum is more rapid and intense in chicks inoculated with blood from birds heavily infected with exo-erythrocytic parasites than in those inoculated with blood from birds in which no exo-erythrocytic parasites can be found.

2. If parasitized blood from a bird heavily infected with exo-erythrocytic parasites is inoculated intravenously into a normal bird and withdrawn 24 hr. later, it produces infections in normal birds characterized by a slow development of exo-erythrocytic parasites.

3. The proportion of birds which survived after inoculation with P. gallinaceum and daily treatment with suppressive doses of quinine, mepacrine or pamaquin, was greater when the inoculum was derived from donors in which no exo-erythrocytic parasites could be found (48%) than in those inoculated with blood from donors in which exo-erythrocytic parasites 'were numerous (15). No birds inoculated with parasitized blood from donors heavily infected with exo-erythrocytic parasites were sterilized of their infections by the action of the drugs, whereas in eleven out of sixty-six of the birds inoculated with parasitized blood from birds in which no exo-erythrocytic parasites could be found the infections were eradicated by the drugs.

4. Blood-inoculated infections of P. gallinaceum may in some cases be eradicated by short courses of treatment with mepacrine, quinine or pamaquin.

5. Whereas blood-inoculated infections of P. gallinaceum derived from donors in which no exo-erythrocytic parasites could be found were sometimes eradicated by prophylactic treatment with quinine, mepacrine or pamaquin, it was not possible to eradicate the infections by curative treatment with similar doses of these drugs.

6. The difference in type of infection produced by parasitized blood from birds in which no exo-erythrocytic parasites could be found at the time of withdrawal of the blood and those in which exo-erythrocytic parasites were plentiful appears to be due to the presence of exo-erythrocytic parasites in the circulating blood of the latter type of donor.

7. Sterilization by the action of quinine, mepa-crine or pamaquin of blood-inoculated infections from donors in which no exo-erythrocytic forms could be found, appears to depend upon whether the erythrocytic parasites are killed before they can become converted into exo-erythrocytic forms.

8. No evidence has been obtained of the intensification of development of exo-erythrocytic parasites by the suppression of erythrocytic parasites with drugs.

Acquired resistance to paludrine in Plasmodium gallinaceum was retained undiminished after passage at frequent intervals through untreated chicks for more than 1 year.

1. A thirty-two-fold increase in resistance to sulphadiazine has been induced in Plasmodium gallinaceum in chicks by treatment with that drug.

2. No loss in resistance to sulphadiazine occurred in the resistant strain during cyclical passage through Aëdes aegypti.

3. The sulphadiazine-resistant strain was resistant also to sulphathiazole, sulphanilamide and sulphapyridine, but not to mepacrine, quinine or pamaquin. An increase in sensitivity to pamaquin was observed.

4. The sulphadiazine-resistant strain was resistant to paludrine and its methyl homologue M 4430.

5. In strains maintained in a state of acute infection and treated with sulphadiazine, resistance to paludrine developed more rapidly than resistance to sulphadiazine, and in one strain a high degree of resistance to paludrine was obtained before any increase in resistance to sulphadiazine could be detected.

6. Resistance to paludrine as induced by sulphadiazine, develops rapidly and extends at once over the full range of doses which the chick will tolerate, whereas resistance to paludrine as induced by that drug itself, develops more slowly and by stages.

7. Whereas resistance to paludrine is induced readily by treatment with sulphadiazine, resistance to sulphadiazine is induced by paludrine only after treatment with high doses of the drug for a prolonged period.

8. In latent infections of Plasmodium gallinaceum resistance to paludrine or sulphadiazine was not induced by sulphadiazine during the period of the experiment (49–75 days), though the aggregate dosage of drug used was much greater than that with which resistance was induced in strains maintained in an acute state.

9. It is not considered probable that cross-resistance between sulphadiazine and paludrine is due to a similar mode of action of these drugs, as whereas sulphadiazine is antagonized by p-amino-benzoic acid paludrine is not.

1. Resistance to sulphanilamide, of a low order, has been developed in a strain of Plasmodium gallinaceum by the treatment of patent infections with the drug.

2. The sulphanilamide-resistant strain was resistant to paludrine over the whole range of doses tolerated by the host; it also proved to be resistant to sulphathiazole, sulphapyridine and sulpha-diazine, though the degree of resistance shown was much slighter than to paludrine.

3. Resistance to paludrine developed in the sulphanilamide-resistant strain before any resistance to sulphadiazine could be detected.