(1) An account is given of a serious infestation in sheep and lambs by Ixodes ricinus in the uplands of Denbighshire, North Wales.

(2) This infestation was accompanied by serious disturbance of health resulting in the general deterioration of the flock and the death of lambs.

(3) Experimental dippings were commenced in 1924, using a special “short-interval” arsenical dip supplied by the Cooper Technical Bureau, London. Serial dippings were made each spring at five-day intervals. When possible an early autumn series was conducted.

(4) The objects were (a) to control the infestation, (b) to control the diseased condition, and (c) to attempt to eradicate ticks from the land.

(5) The control of both the ticks and the disease were easily effected. A few sporadic cases of disease did occur during the course of the experiment.

(6) Eradication has not yet been accomplished, but the average number of ticks was reduced from 121 per animal in May 1924 to 0-96 per animal in July 1926.

(7) It is therefore suggested that short-interval dipping for serious tick infestation in sheep is practicable on British hill farms, and that resultant mortality from disease can thereby be reduced.

1. Introduction: We have little understanding of the true rôle of parasitic insects in the maintenance of the natural equilibrium chiefly because few efforts have been made to interpret the available data.

2. This is especially true concerning the problem of host relations which is in many ways of fundamental importance.

3. To solve the problem of host relations we ought to be able: (a) to deduce from a study of the host the composition of its parasitic fauna; (b) to deduce from a study of the parasite the composition of its host list.

This would necessitate (1) the definition of the “typical” or “ideal” parasite of any given host; (2) the definition of the typical or ideal host of any given parasite; (3) the establishment of a correlation between such definitions and morphological or systematic data in such a way that conclusions could be drawn concerning species whose habits had not yet been the subject of direct observation.

The object of the present paper is the examination of this problem.

4. The problem of the host relations is simply the problem of a particular kind of animal association. From analogy with other problems of this type, we may deduce that the idea of formulating a definition of the “typical parasite” of a given host is fallacious. A consideration of the data in regard to the parasitic fauna of various hosts confirms this as does experimental evidence. The attraction exerted by the host on the various members of its fauna is not necessarily identical or even similar.

5. The only feasible method of attacking the problem is therefore the analysis of the behaviour of parasites with the object of defining their “typical hosts.”

6. An experimental study of the host relations of Melittobia acasta Walk. was conducted in the laboratory and an attempt made by the analysis of the characters of the various species accepted or refused by the parasite, to formulate a definition of the normal host.

7. The nature of the hyperparasitic and gregarious habits are discussed in connection with the habits of Melittobia. An effort is made to discuss the true causes of the phenomena.

8. A study of the variations in oviposition habit in relation to the particular characteristics of various hosts shows that these variations, though complex and very difficult to explain in relation to the efficient causes involved, are easily explained if considered in relation to the object of the behaviour.

9. A general definition of the host of Melittobia acasta was formulated on the basis of the observations and experiments performed.

10. Attempts to simplify the definition and discover for the phenomenon of attraction some simple stimulus, completely failed.

11. A critical examination of the definition of the normal host of Melittobia shows that though it indicates the general trend of behaviour in the species studied, it is at once too narrow to include all possible hosts and too broad to exclude all species which might be refused.

12. A detailed study of the extensive data available on the host relations of the Tachinid Compsilura concinnata Meig. confirms the above results and shows that the typical host cannot be defined in relation to either morphological or physico-chemical characters.

13. It is shown further (a) that the fauna of systematically related parasites though sometimes similar are not necessarily so and may be very different; (b) that parasites so similar as to be morphologically inseparable in most or all stages of development may have different host relations.

14. A scientific treatment of host relations along ecological lines whether (a) by the establishment of an empirical correlation between systematic position and habitat or (b) by the definition of the parasite in terms of its habitat is thus impossible.

15. It follows that the acts of the parasite are not strictly determined if considered simply in relation to the physico-chemical and morphological properties of the host.

16. The reason for this is the fact that the choice of hosts is primarily a phenomenon of the psychological order which can be understood only by analogy with a similar process, such as the choice of food, in regard to which direct knowledge can be obtained by introspection.

17. A comparison along these lines leads to the conclusion that the choice of hosts, like the choice of food, is not a phenomenon which can be referred to any constant and definite efficient cause exterior to the organism; in certain cases the substances selected differ entirely in their properties and are identical as causes of behaviour only in that they are all agreeable to the organism concerned. In other words, the cause of behaviour is one only upon the psychological plane. Both food and hosts may be considered in relation to their suitability in relation to a given organism, but again the only category in which all types of food and hosts can be grouped is that of assimilable substances. The cause of behaviour considered from this angle is one only upon the physiological plane.

18. Pursuing the analysis of the process of food or host selection one finds, that this process occurs as if the suitability of substances as food or as hosts, obviously not in itself perceptible, were deduced by the animal from the perceptible qualities, though we have no reason to suppose that any conscious judgment occurs in the animal mind. This conception which corresponds exactly to the classical idea of instinct is the only one which seems to fit all the facts of the case.

19. An apparent objection to the views here advanced, based on the fact that the choice of the habitat in many animals seems to have been expressed with sufficient adequacy in terms of simple factors having certain quantitative values owes what force it possesses to the fact that the factors considered are those affecting the tactile sense in various ways, this sense being that in which the qualities of the object perceived affect the sentient subject most acutely so that his sensory impressions of pain and pleasure, ultimate sources of his actions, are more nearly measures of the qualities perceived than is the case with other senses. It is to be noted further that the objects of the tactile sense are essentially less varied than in such senses as taste, smell and sight, and that many stimuli classed as gustatory or visual are in reality tactile.

In view of the undeniable adaptive power possessed by organisms, it is however doubtful whether tactile impressions really constitute a valid measure of tangible qualities, or that movements induced by tactile pleasure and pain can be considered for a given organism as due to certain constant values of these qualities as measured by physical instruments. The results obtained up to the present do not constitute by any means a sufficient answer to this difficulty and have in any event but little bearing upon the problem of host selection which there is no reason to consider as determined by impressions of the tactile order.

20. There is therefore no reason to suppose that the accumulation of data on the host relations of entomophagous parasites will ever lead to the discovery of laws of parasite distribution permitting us to correlate the properties of parasites with those of their hosts so that we can predict in advance the host list of a given parasite or the parasitic fauna of a given host. The data collected may suggest interesting hypotheses or facilitate greatly operations of the practical order, but they have in themselves, only an empirical significance. The laws underlying the problem of host relations are not capable of expression in scientific terms nor discoverable by scientific methods.

21. From this it follows that in problems involving the practical utilisation of entomophagous parasites, as for example, when these species are transferred from one country to another, we have no means of predicting results because it is impossible for us to foretell the behaviour of the parasite with regard to the possible host insects of the new area. It would therefore be futile to make the execution of such projects dependent upon investigations in the native home of the parasite and designed to supply advance information as to the results obtainable. Such studies, while no doubt of great interest and value, cannot lead to any certain conclusions in regard to the question at issue, which experiment alone will solve.

Under the name of Zografia notonectae n.g., n.sp., Bogoyavlensky (1922) has described a new organism which he found parasitic in the body cavity of Notonecta (Hemiptera) collected in ponds in Russia (government of Moscow).

On February 8th, 1923, I received three examples of living larvae of Musca domestica, from Dr James A. Stephen (Head of the Child Welfare Department of the Public Health Service, Aberdeen) with a statement that they “came from the bowel of a child, 4 months old, breast-fed, but having sugar on a teat.”

A single specimen 141 long, 3 broad. Genital pore in centre of proglottis margin, vaginal pore posterior to cirrus sac. Testes approximately 100, in a broad band posterior to female glands and extending laterally to excretory vessels: between ovaries testes extend more anteriorly and are in six transverse rows. Cirrus sac 0·31–0·56 x 0·04–0·06 long, extending just past excretory vessels, containing a small vesicula seminalis. Ovaries practically touching the respective excretory vessels. Vitelline gland rather aporal and posterior to ovary. Gravid segments absent.

The “Arapaima,” “Pirarucú” or “Pirá-Urucú,” of the large rivers of Brazil and the Guianas is the largest strictly freshwater fish known. It is said to reach a length of 15 feet, and a weight of more than 400 lbs. It is known to science under the name of Arapaima gigas (Cuvier).

The insect fauna of tree wounds with sap-exudations or “slime flux,” as well as that of the water-filled hollows of trees, has already attracted the attention of entomologists and several interesting life histories of insects have been recorded from these media. In the present paper I propose to deal with the fauna of only one tree, a horse-chestnut standing on the Downing site between the School of Biochemistry and the Molteno Institute. For several years this tree has supplied me with very rich material for the study of Dipterous larvae and their parasites. Unfortunately, this study recently came to a natural end when this tree, with several others, was cut down to make place for a new building. This fact justifies the publication of the present records of observations collected during several years, although I do not consider them as complete. The most interesting feature of our horse-chestnut tree was the presence of two kinds of lesions each harbouring a different fauna: (A) a sap-exudating or “ slime flux ” wound, and (B) a waterfilled hollow or reservoir.

Having at our disposal material from various sources obtained from the intestines of monitor lizards, it appeared useful to prepare drawings and descriptions of the less known species of parasites.

Some years ago we recorded the occurrence of entamoebae in the stools of patients showing diarrhoea of obscure origin (1915). The symptoms of these patients were not suggestive clinically of acute, nor even of chronic, amoebic dysentery. The entamoebae from these cases proved, on inoculation into kittens, to be pathogenic, producing, however, only a chronic mucous diarrhoea without blood.

1. T. raiae and Bodo caudatus are investigated using Feulgen's nucleal reaction in addition to the usual nuclear stains. The nature and value of this method is discussed.

2. In T. raiae the parabasal body or kinetonucleus is found to consist of chromatin as shown by the nucleal reaction. An achromatic element in its composition is suggested in the division by the desmose between the two halves being achromatic.

The blepharoplast is an achromatic structure. It seems probable that it persists in the aflagellar phase of the trypanosome. In T. raiae the division of the blepharoplast precedes but does not apparently orientate the division of the parabasal body. The division of the parabasal body does not present a mitotic figure.

3. The nucleus in T. raiae consists of an achromatic karyosome body with some chromatin surrounding it, there is also chromatin arranged upon the inner surface of the nuclear membrane.

4. At division the karyosome apparently draws out into an achromatic spindle figure, the chromatin collects in the prophase into a loose granular mass and then becomes arranged at the equator. The mass divides into two portions; it is not possible to distinguish chromosomes with sufficient certainty. The nuclei are finally reconstructed from the achromatic ends of the spindle and the chromatin which has travelled to either pole.

5. In Bodo caudatus the parabasal body (kinetonucleus) is composed of chromatin and of an achromatic substance, the blepharoplasts or basal granules are achromatic bodies which play an orientating part in the division of the parabasal body: this division does not show an equatorial plate stage. The nucleus is made up of an achromatic nucleolus-karyosome surrounded by a hollow sphere of chromatin; there is a very definite division process which takes place apparently within the nuclear space. There is no equatorial plate formed and the achromatic figure which is derived directly from the karyosome is a division column which inserts itself into the already dividing chromatin and is not of the spindle fibre type found in the mitosis of the metazoan cell and of many protozoa.

This paper contains a revision of the morphology and classification of three species of holotrichous ciliates, Lembus pusillus, Cyclidium glaucoma and Uronema nigricans (=Uronema marina Dujardin, 1841=Cryptochilum nigricans Maupas, 1883=U. marina et U. marinum auctorum, pro parte), together with various observations on their biology. The revised data on these species are summarised in the diagnoses preceding their description.

The close resemblance between U. nigricans and the other species described has led to its transfer from the family Chiliferidae to the family Pleuronemidae.

Cyst formation in L. pusillus is described for the first time. The cysts, when fully formed, are provided with a double-layered wall, to which the body of the ciliate is more or less closely adherent. When preserved in dry condition, the cysts retain the same structure and are able to hatch after several months. Excystation is brought about by active rupture of the cyst-wall on the part of the ciliate. The primary cyst does not, however, afford adequate protection to the ciliate in a fluid medium. After remaining in the latter about three weeks, the animal secretes a cyst of the second order, in which it remains viable some time longer, but if left in the fluid, the ciliate ultimately dies, gradually breaking up into a number of granules.

In order to establish the relationship of the marine and fresh-water forms of the same species, experiments on their acclimatisation to the opposite environment were conducted. The fresh-water U. nigricans adapted itself to pure sea water within 12 days and developed for 12 months in this medium, in which it is still being maintained. The marine U. nigricans was gradually transferred to fresh water, in which it was kept for 60 days. Observations on the morphological changes undergone by this ciliate in the course of acclimatisation show that the fresh-water forms living in sea water gradually acquire the habitus of the marine ones, and that the latter undergo the reverse change in fresh water. The characters distinguishing the two strains of U. nigricans being insignificant and transitory, the two forms are regarded as constituting a single species.

The fresh-water L. pusillus required eight days for acclimatisation to sea water, in which it was maintained for 163 days without exhibiting any morphological changes.

Attempts to acclimatise the fresh-water C. glaucoma to pure sea water failed, but in 50 per cent, sea water it grew for 130 days. The morphological and biological differences between the fresh-water and marine strains of C. glaucoma are probably sufficient for them to be regarded as sub-species or races.

Experiments were conducted on the cultivation of these ciliates in faecal matter from normal and diseased human subjects, and varying widely in consistency. Tests on the influence of the reaction of the medium upon the growth of the ciliates showed that they prefer a slightly alkaline medium (pH 7·4–8·0), but are able to tolerate an increase of alkalinity up to pH 9·0–10·0, whilst the acidification of the medium beyond pH 5·0 inhibits their growth.

As regards the facility with which these ciliates grew in faecal cultures, Lembus occupied the first place, having developed in the majority (64 per cent.) of samples of pure (i.e. undiluted) faeces, and in all those slightly diluted with water. The second place belongs to Uronema, which grew in all the diluted specimens and in one undiluted, whereas Cyclidium was unable to grow in pure faeces, but developed in about half of the diluted samples. All these ciliates were successfully cultivated in suspensions of human and animal faeces.

The coprozoic habits of the ciliates in question are compared with the records for “Uronema caudatum,” comprising a number of ciliates described by some authors as human intestinal parasites. It is shown that the conditions under which these ciliates were observed indicate that they were coprozoic forms that had contaminated the material examined. Though described under the name “U. caudatum” these ciliates represent at least two distinct genera, those recorded by Martini being similar to Cyclidium or Uronema, the ones described by Yakimoff et al. probably representing Lembus pusillus.

The last-named ciliarte having been originally found in a sample of canine faeces, experiments were carried out with the view of ascertaining its origin. It was demonstrated that the ciliate was unable to withstand incubation at 37°C. for 24 hours, and, when fed to mice in the form of cysts, did not survive the passage through their intestine. These facts indicate that the faeces in which Lembus was found must have been contaminated by this ciliate from outside.

Authors almost invariably figure the shell-gland of Cestodes in a form resembling that of a daisy's corolla. This may serve to indicate the position of the shell-gland but it should be remembered that the shell-glands are not all of this form. Thus, the shell-gland shown in Fig. 1 is somewhat shrub-like in form, consisting of irregularly fusiform elements which grow filamentous at one end and they are clustered on one side of the ootype of the uterine duct. The nucleus is situated in the fusiform portion and the filamentous portion extends toward the parenchyma of the cortical layer.

The infected termite was collected in Samoa by Dr P. A. Buxton and was identified by Mr G. F. Hill, Melbourne, who noticed “foreign bodies” attached to the forelegs. These bodies proved to be fructifications, in the form of small apothecia, of a fungus to which the name Ectomyces caloterni n.g., n.sp., has been given. The two infected legs comprised all the available material, and, as they were preserved in alcohol, cultivation of the fungus could not be attempted.

The host of the Coccidian described in this paper is Hydroporus palustris L., a small Dytiscid beetle common in stagnant ditches. The infected material was collected from two ditches in the neighbourhood of Grantchester (Cambridge). From one ditch 25 per cent. of the beetles were parasitised, and from the other ditch 15 per cent. were parasitised. The infection with Legerella, which is confined to the malpighian tubules, is often very intense. Heavily infected beetles are readily recognised on dissection because the malpighian tubules are colourless and do not contain the brown excretory granules characteristic of the normal tubules. The parasite has been observed at all seasons of the year, and the life cycle does not appear to follow a seasonal course.

The purpose of this note is to record the occurrence of a haemogregarine parasite in the red blood corpuscles of the South American lung-fish, Lepidosiren paradoxa.

During recent investigations on Endamoeba blattae (Bütschli) Leidy of the common cockroach, Blatta orientalis, there were observed in the hindgut of the host, two other distinct amoebic species. Both appear to be truly entozoic, but contrast strikingly with E. blattae both in morphological features and life cycle. They are interesting not only in being among the comparatively few species yet reported from insects, but also in their relationships to certain of the amoebae of vertebrates.

In his experiments upon the toxicity of oxygen for protozoa Cleveland (1925, a, b, c) was able to kill the intestinal protozoa of Termites, without injuring the host, by increasing the pressure of oxygen for varying lengths of time. He found that at one atmosphere pressure of oxygen the intestinal protozoa of Leucotermes were killed in 24 hours; those of Termopsis in 72 hours; whilst those of Reticulitermes and Cryptotermes were not all killed in ten days. At 3·5 atmospheres of oxygen he found that the intestinal protozoa of all four species were killed in less than an hour. At this pressure the intestinal protozoa of cockroach and frog were killed in 3½ hours and 28 hours respectively, whereas it took 90 hours to kill the cockroaches and 65 hours to kill some of the frogs. He also experimented upon warm-blooded vertebrates (rats), but he found that these animals survived only 5–6 hours at 3·5 atmospheres of oxygen and their intestinal protozoa were not killed in that time. Trichomonas, grown in cultures, however, from frog, rat, and man, were killed in 6, 10, and 11 hours respectively when subjected to 3·5 atmospheres of oxygen.

1. Six new furcocercous cercariae from the north-western United States are described and comparisons with similar forms are made.

2. On the basis of the structure of Cercaria tuckerensis spec. nov. and three cercariae reported in the recent literature it is possible to characterise more precisely the Elvae group, which now contains eight members.

3. The execretory system of Cercaria elvae Miller, 1923, was re-studied and found to differ from the original description. The bearing of this sort of error on Faust's use of excretory system formulae is touched upon.

4. Cercaria burti Miller, 1923, originally described from northern Michigan, is reported from San Juan Island.

5. Certain features of the morphology and biology of the furcocercous cercariae are discussed in the light of this and other recent studies.

6. Check List II of the furcocercous cercariae, with eighteen larvae, is given.

Alaria arisaemoides, n.sp., is described from the red fox, Vulpes fulva. The attachment of this parasite to its host and the host reaction are discussed in detail.

In a recent paper1, written jointly with Dr P. P. Laidlaw, methods were described for cultivating Entamoeba histolytica, and various other entozoic amoebae of man and monkeys, from the encysted forms. We there showed that cultures of all the species studied can be readily obtained by merely incubating such cysts in any of the media suitable for the cultivation of the corresponding active forms—provided that certain precautions are taken.