It is intended that this review shall give an account of what is known of the physiology of the parasitic stages of nematodes parasitic in animals, with particular reference to those species which are found in the alimentary tract of their hosts. An account will also be given of attempts which have been made to keep the adult and larval phases of parasitic nematodes alive in vitro and of the bearing of the available physiological data upon this problem.

Anopheles elutus females are attracted by human beings. The human body eliminates certain substances which exert an attraction. The expired air alone is sufficient to attract the mosquitoes. Mild air currents may play an important part in carrying and directing these attracting factors towards the mosquitoes.

1. The longevity of the fasting larvae and nymphs of Hyalomma savignyi is investigated under different conditions of temperature and humidity.

2. It is found that both larvae and nymphs are much more sensitive to humidity than to temperature. Their longevity at low humidity is short as compared with that at high humidity at the same temperature. The longevity of the fasting larvae varies between very wide limits. The mean lenght of life varies between 1·25 and 162·6 days, whereas the maximum longevity obtained varies between 2 and 241 days, according to temperature and humidity.

3. The mean longevity of the nymphs varies between 6·8 and 149·2 days. The maximum varies between 10 and 246 days.

4. The relative dispersion of longevity varies within restricted limits under different experimental conditions.

Up to the present very few authors have directed their attention to factors influencing the emergence of larval trematodes from their snail hosts. Cort (1922) has made a study of the escape of cercariae from their hosts and the writer (1931) has examined the effect of light and temperature on the emergence of three species of larval trematodes from three species of fresh-water molluscs. More recently, Giovannola (1936) has investigated the effect of a reversal of light and darkness on the periodicity of emission of cercariae from three species of host snails. All these studies have been made on freshwater molluscs. In the case of molluscs living between tide-marks on the shore, in addition to variation in temperature there exists another variable factor, namely, the salinity of the water. Many rock-pools situated in the intertidal zone are small and will therefore be subject to considerable variations in temperature and salinity at different times of the year and even for short periods of up to 12 hr. during individual days. In the summer months the temperature of the water in these pools may rise to a high degree and the high temperature would be maintained for a longer period near high-water mark than near low-water mark, where the period of exposure is less. Similarly, the decrease in temperature would be very marked during the cold winter months. As the water in the exposed pools evaporates the salinity increases and high up on the shore, between successive tides on hot summer days, it may become twice that of normal sea water. The pools are also exposed to rain and in the same period the salinity of the water may decrease considerably if the precipitation is heavy. The light factor would be the same for molluscs living in fresh and in salt water, but the reaction of various species of larval trematodes to the presence or absence of light varies. Some emerge from their hosts in larger numbers during the daytime, others during the night and some indiscriminately during periods of light and darkness.

In a former paper (Baylis, 1944) an account was given of some observations on the life-history of Mermis nigrescens, and of its occurrence, naturally and experimentally, in earwigs (Forficula auricularia). As stated in that paper, the material then available did not seem to warrant an attempt to describe the larval development. Further material has now been obtained and studied, and it seems desirable to place on record some of the observations made, although many points still remain obscure.

1. In an alkaline inorganic solution containing borate, no bacterial disintegration of tissues occurs in a week.

2. Survival is prolonged by adding sugars to this solution, but not by the addition of bile salt or peptone. Monosaccharides are more effective than disaccharides, and both may enter the worm through its body wall.

3. The effect of variations in certain physicochemical conditions was studied. Wide ranges of osmotic pressure, of the K/Naratio, and of the Ca/Na ratio, have little effect on the worms. The optimum pH is c. 8.4, and t h e optimum temperature c. 36° C. Approximately anaerobic conditions decrease the survival time.

4. The effect of various bactericides, bacteriostatics and anthelmintics upon the worm were investigated in a preliminary manner. Many bactericides are toxic, certain anthelmintics are not. Survival is prolonged by 1/5000 trypan blue.

5. A satisfactory medium for large-scale ‘culturing’ is: NaCl 150 mM., KC1 10 mM., CaCl2 1 mM., borax 6 mM., glucose 30 mM. (pH 8.6). In this the worms survive for c. 60 hr. at a temperature of 36° C.

Both species of tick which are described in the present paper are members of insular faunas and demonstrate remarkably well the tendency, shown by these parasites in geographical isolation, to species formation. In this connexion it is perhaps worth quoting what Cooley & Kohls (1945) say concerning this subject in their recent work on the genus Ixodes: ‘These studies’, they write on p. 1 of their introduction, ‘have revealed in considerable measure the more useful specific characters and the importance of a real appreciation of the significance of geographical range and variation in the differentiation of species.… In determination of species limits we have considered not only morphologic but also biologic factors such as host relationships and geographic distribution. No subspecies are recognized.’ There is little doubt but that geographical distribution will be found to play an almost equally important role in the genus Amblyomma.

All the Mesostigmatic mites which have been found as ecto- or endoparasites of snakes are listed and it is suggested that the latter are probably derived from the former. The homologies of the dorsal shields are established and a progressive fusion is shown to have taken place. On this evidence a new genus is erected for O. bedfordi (Radford) to include most probably the lung mites of African snakes. The life history and general biology are discussed. It is shown that there is no experimental evidence that these acari are vectors of disease or that they induce any change in the tissues of the host. Evidence is brought forward to show that the parasites of the more nearly related snake hosts are themselves more nearly related: some consequences of this are discussed. A key to the females of all the known lung mites of snakes is given together with the diagnosis of the new genus Hammertonia and of a new species, Ophiopneumicola americana. The view that the genus Ophiopneumicola is not a natural group is expressed. O. hammertoni Radford is redescribed and some points of significance discussed. The species incertae sedis of Keegan (1943) are discussed.

1. The adults of Fasciola hepatica feed mainly, if not exclusively, on blood.

2. The oxyhaemoglobin of the blood is converted to haemoglobin, and then to acid haematin, the pH of the empty gut being about 6·4.

3. The blood is concentrated by the removal of fluid, and haemolysis eventually occurs in the lumen of the gut. Some of the haematin is absorbed by the epithelial cells, and the remainder is concentrated as faeces.

4. The gut epithelium is responsible for extracellular digestion and absorption, but there is little evidence that the major changes in its appearance are associated with particular phases of this process.

5. Attempts at increasing survival times in vitro by using blood as food have failed to date because of bacterial contamination.

The idea has been mooted that the sheep tick (Ixodes ricinus L.) is active at different times on different host species. Tick-activity curves for sheep, deer, hare, rabbit, skylark and meadow-pipit are similar in form and timing. With these animals, therefore, there is no indication of any marked individual species effect on activity.

Where spring activity occurs, males, females and nymphs usually have similar and concurrent activity curves, but nymphs may occasionally have their peak 2–3 weeks later than adults. Larval activity is always later, the peak being several weeks after the female peak. The data for nymphs amend the general statement on activity in a previous paper by the present author.

Ticks are active during the hours of darkness but the activity-density then is considerably less than in daylight. This diurnal fluctuation in activity seems to follow fairly closely the diurnal fluctuation in atmospheric temperature.

1. The egg shells of Fasciola are derived from globules or granules contained in the, vitelline cells. These globules contain an orthodihydroxyphenol and protein.

2. The anatomy and histology of the Mehlis's gland region have been described, and the site of liberation of the above granules has been isolated.

3. Mechanical forces, due to the churning action of the uterus, are believed to be responsible for freeing most of the granules. This liberation can be correlated with the presence of spermatozoa in the uterus. The spermatozoa can penetrate the vitelline cells, but this does not free the globules.

4. Active spermatozoa, in alkaline solutions, tend to form aggregates, and incorporate into these aggregates any solid material in the vicinity. This is apparently responsible for the arrangement of cells within the egg, and for the formation of a shell round it from fused vitelline cell globules.

5. The polyphenol of the newly formed egg shell is oxidized into a quinone in the anterior uterine coils. Polyphenol oxidases have not been conclusively demonstrated. The relative uterine coils are surrounded by an especial concentration of a generally distributed tissue haemoglobin.

6. The final egg shell is composed of a quinone tanned protein, similar to the sclerotin of the cockroach ootheca.

7. Suggestions are advanced as to the possible functions of Mehlis's gland and Laurer's canal.

1. Cyclical development of Spirochaeta duttoni in lice was observed in seven experiments. In the last four, the same strain was transmitted through four successive batches.

2. The resultant infections in animals behaved exactly like S. duttoni even after the fourth transmission through the abnormal host.

The comparative nutritive value of dried horse blood, highly milled wheat flour devoid of bran, a mixed diet of blood and wheat flour and that of blood and yeast for the larvae of Xenopsylla cheopis, X. brasiliensis and X. astia was ascertained experimentally. Pure blood proved inadequate, probably because it is deficient in accessory food factors. The growth of their larvae on wheat flour alone was erratic; only partial success was obtained, and the adults emerged after long and irregular intervals. This I attribute to the association of micro-organisms, possibly fungi, with this food. A mixture of blood and wheat flour quickened their larval development, but it was not a satisfactory larval diet. Blood and yeast form an ideal food for all flea larvae.

It is concluded that larval diets, containing blood or wheat proteins and vitamins of the B group, are essential for the successful rearing of these rat-fleas, and that the proper sclerotization of the adult is due to the presence of haemoglobin in the larval food. The available data on the effects of diverse diets on the growth of flea larvae lend strong support to the conclusion that successful development depends on the presence of these vitamins in the food; it also leads me to think that their source in nature may be the association of micro-organisms with the food.

The larval food appears to be an important factor that governs the distribution and host preferences of different species of flea. The larvae of X. astia require the most nutritive diet. If a rich larval food is present in a rodent burrow, X. astia flourishes, as in the burrows of Tatera indica and Bandicota malabarica, and even in those of the domestic rats in certain regions. In contrast, the nutritional requirements of the larvae of Xenopsylla cheopis and X. brasiliensis are simple; thus they prosper readily in a burrow of the domestic rat, even where the nutritive value of the larval food is very low. As the temperature tolerance of X. brasiliensis is the lowest, this species is confined to some of the cooler regions.

The irregular distribution of the three species of rat-fleas inside India may be related to differences in the nutritional value of the varied organic sub stances found in rat burrows in different plaoes. The fact that the distribution of X. cheopis and X. brasiliensis is wider than that of X. astia is attributed to the ability of the larvae of the first two species to grow better on flour alone; this possibly enables them to survive transport in grain, even without rats, to places far from their original home.

1. Careful search has failed to disclose the presence of flame cells in the excretory system of the adult liver fluke.

2. Fat globules are produced in the walls of the excretory system, and are freed into the lumen, and thus to the exterior.

3. These globules do not contain appreciable quantities of cholesterol, cholesterides or lipins, but may contain some unsaturated fatty material.

4. The aqueous medium in which the globules float is either neutral or slightly alkaline.

5. The distribution of reduced ascorbic acid has been investigated by the silver nitrate technique. This indicates that the main concentration occurs in the smaller vessels of the excretory system, and in the submuscular tissues of the body wall.

In general, female ticks infest the hairy areas of sheep: head and legs; the (comparatively) bare areas: axillary and inguinal; and certain woolled areas: chiefly the short-woolled margins of the hairy and bare areas; they do not attach in the long wool of withers, back, rump and flanks.

More female ticks attach on the head than on the axillary region, and more on the axillary than on the inguinal region, obviously on the principle: first come best served.

Different tick stages tend to attach in different zones of the same regions; two theories are put forward to account for this.

On a hill pasture where the main tick activity occurs in spring only, the average totals of female ticks carried per annum by ewe, hogg and lamb were in the proportions 8:6: 1. Host size is thought to be the chief governing factor.

Under existing British hill pastoral conditions, the upper limits to female tick infestation appear to be of the following order: average spring peak total count per ewe reaches 100–150, with extreme individual limit about 300; and about five times as many ticks are carried per ewe over the entire spring season.

On heavily infested hill pasture, stocked at one sheep to 1–5 acres, typical flocks feed from 100 to 600 female ticks per acre per annum. The densest sheep stocking results in the highest tick density.

For every one female tick, about eight nymphs were fed by sheep in one season.

The better the condition of a sheep, the less bare of wool and apparently (to touch and sight only) the more greasy, and the lower the tick infestation.

In any age group of sheep on one grazing, there is considerable range in weekly infestation weights but individuals tend to keep their relative positions consistently in this range. Sheep differing markedly in their infestation weights show a remarkable lack of difference in the majority of physical and physiological characteristics of body form and body surface. (Fleece humidity was omitted from study.) Notably, there is no indication of significant differences in fleece greasiness. Apparently only two characteristics go with the lighter infestations in Blackfaced sheep. These are coarser fleece hair and less clearance between fleece tips and ground, having doubtful and minor effects respectively.

The most important factor in individual differences of infestation within an age group is very probably individual activity, i.e. the more ground covered in unit time, the more ticks picked up.

Sporozoon Tincae Volf & Dvořak, 1928, is the cause of a fatal disease of the tench (Tinca tinca) in fish-ponds in south Bohemia. We can distinguish in its life cycle two different stages, multiplication by repeated binary fission, resembling schizogony and by sporogony with the formation of elliptical spores measuring 1.5–2 by 0.5–1 μ. The development of all these stages occurs in the cytoplasm of histiocytes. A detailed description of the life cycle is given. This parasite should be placed provisionally in the order Haplosporidia.

That quinine has little or no destructive action against gametocytes of Plasmodium Falciparum (crescents) has been known for a long time. Marchiafava & Bignami (1900) pointed out that quinine (grains xxx daily) over a period of time did not perceptibly affect the sexual parasites of P. falciparum. Darling (1914), working in the Panama Zone, showed that Anopheles albimanus readily became infected when fed upon carriers of M.T. crescents who were taking xxx grains of quinine daily. These and other experiments showed that quinine, even in large doses, has no direct destructive action on crescents, and does not prevent mosquitoes from becoming infected once the.crescents are formed and are present in the peripheral circulation. However, quinine given in heavy doses during the first day or two of a primary attack, restricts and reduces the asexual source of supply.

In Britain the genus Psychoda Latr. contains sixteen species and two subspecies (Kloet & Hincks, 1945), the larvae of which occur in cow dung, decaying vegetation, and the bacteria beds of sewage purification works. A knowledge of these larvae would be useful both to the sewage biologist and the sanitary inspector, but for only three species have the larvae been fully described. Two of these, Psychoda alternata Say, and Ps. cinerea Banks, are inhabitants of the bacteria bed, and the larva of the former has been described on at least six occasions, the classical account being that of Dell (1905). The larva of Ps. cinerea has been described three times, the most complete account being that of Malloch (1917). The third Psychoda larva of which we have a full account is Ps. albipennis Zett. (Keilin & Tate, 1937), a species inhabiting dung and decaying vegetation. In addition to the foregoing there is a brief and inadequate description of the larva of Ps. severini Tonn. (Johnson, 1914), a species which is commonly present in the bacteria bed.

In recent work on the deposition of the epicuticle, Wigglesworth (1946) has shown that, in Rhodnius prolixus, not only is the epicuticle a composite structure comprising several distinct layers, but that the pore canals, which hitherto have been believed to terminate at the base of the epicuticle, do in fact penetrate its innermost regions. Wigglesworth extends the term epicuticle to cover the four outer layers of the cuticle in Rhodnius. The innermost of the four is a cuticulin layer which is followed by a polyphenol layer, a wax layer and an outer cement layer respectively. The pore canals penetrate the cuticulin layer.

In 1941 work was started in North America with a view to introducing into South Africa parasite species which might act as factors in the control of the insect pest, Loxostege frustalis Zell. (Pyraustidae), which destroys large areas of Karroo bush (Pentzia incana), an important sheep-fodder plant in South Africa (Taylor, 1940). Several nearly related species of moths occur in North America, one of them being Loxostege sticticalia L., a serious pest of sugar-beet in the Great Plains region. A number of parasite species have been recorded from this host both in America and Europe (Pepper & Hastings, 1941), and it was thought that several of these species occurring in North America might be suitable for introduction into South Africa after their hiolocv and life histories had been investigated.