The palps of all stages of Ixodes trianguliceps are provided with a ventral plate below the basal article; formerly this plate was thought to be the first palpal article. This article in the larva and nymph is produced forward into a spur, but in the female tick this spur is incorporated into the basis capituli as the sella. The hypostomes of I. trianguliceps and I. canisuga are redescribed to clear up existing inaccuracies.

Ticks in which the rostrum does not extend beyond the apex of the first palpal article are found on birds, and this probably represents a primitive condition. Those with palpal spurs, which may or may not be fused with the basis capituli, are found on birds (not in Britain) or on mammals of the mouse size group, and those where the rostrum is produced beyond the first palpal article occur on a wide range of large and small animals. The longer and more heavily toothed digits of ticks appear to be associated with a wide host range and vice versa. The structure of the digit may also influence the choice of attachment sites by ticks on their hosts because the microstructure of the skin varies in different parts.

Variations in size and form of the scuta of some British ticks are described, and the mean growth rate is ascertained from this data. The information shows that the material of I. ricinus and I. hexagonus is homogeneous, and that specific differences occur in size, shape, the position and type of dermal ducts and in the relation between scutal and alloscutal bristles.

The morphology of Gené's organ in I. hexagonus is described. It consists of a basal sac-like portion surmounted by four horns and lined with a cuticle beneath an epithelium. The gland is a proliferation of the epithelium and located near the bifurcation of the base into the horns. A watery refractile fluid, secreted by the gland, accumulates between the epidermis and the cuticle in the horn-like extensions. Proximally the thick endocuticle and epidermis lie close together, and two cuticularized rods penetrate the endocuticle for about half-way along the stalk. The rods arise from the postero-dorsal margin of the basis capituli. Muscles, arising from the free ends of the rods, pass back to near the hind-margin of the scutum; they retract the basis capituli after egg laying and indirectly assist in the retraction of Gené's organ. A suggested mechanism for everting Gené's organ in I. hexagonus is outlined.

During feeding the opisthosomatic cuticle of all developmental stages of the tick is much stretched. In the larva, where the cuticle is thin, this is effected by the flattening of the epicuticular pleats, but nymphs and females have, in addition, two longitudinal folds alongside the body which stretch to a far greater extent than do the epicuticular pleats. Similar folds are present between the hard ventral plates and the scutum and between the epimeral and median plates in the male. Coupled with the large size of the male digit, this suggests that morphologically, at least, males can imbibe blood.

The growth of the leg segments of I. hexagonus is not constant from stage to stage, and as a result the shortest ‘leg-length’ of the female exceeds the longest ‘leg-length’ of the male.

1. The action of the mouthparts of Ixodes hexagonus during the penetration of artificial membranes is described.

2. Above the hypostomal groove, the chelicerae are closely apposed in the mid-line, but on the inner lower surface each inner cheliceral sheath is drawn into a ventral, toothed lobe, which overlies the hypostomal groove. This lobe disappears in front of the labro-hypostomal groove.

3. A lamellate mass of homogeneous material surrounds the rostrum in the skin and at the tip of the hypostome there is marked cytolysis of the dermal cells. There are some pyknotic nuclei visible in this region, and the endothelial walls of the capillaries are swollen, and some contain many polymorphs.

In April 1950, numerous ticks of the genus Ornithodoros were found in two coral caves, several hundred yards from the sea, near Diani about 20 miles south of Mombasa. Most of the ticks were found in the larger cave, which was irregular in shape, about 42 ft. long, and from 2 to 7 ft. high. Some of the ticks were crawling on the fine brown earth which covered the floors and others were just below the surface; none were seen on the walls.

In 1950 the author published a paper entitled ‘The Mites (Acarina) parasitic on Mammals, Birds and Reptiles’ in which it was stated that the subject of feather mites (Analgesidae) was too big to be included in that work, and though they were parasitic throughout their life upon the host or in the quills of the feathers they should be made the subject of a separate paper.

Prior to 1939 our knowledge of the mites of the sub-family Trombiculinae was limited to about a dozen species, whose life history had been the subject of investigations by Nagayo and his associates in Japan, Walch in the Dutch East Indies and Gater, Fletcher, Lesslar and Lewthwaite in Malaya.

In this paper four new mites are described and figured, one being a new species of the subfamily Macronyssidae, one in the subfamily Laelaptinae and two in the family Trombiculidae.

The development of Ascaris devosi, a parasite of the fisher and marten, was followed from the egg to the adult stage using the white mouse and the ferret as the intermediate and final hosts respectively. The eggs contained motile 1st stage larvae 6 days after cleavage and were infective at 12 days, the 1st moult having already occurred. The eggs remained infective for at least 1 year. The 2nd stage larva after hatching from the egg in the intestine of the mouse passes through the intestinal wall to the liver and mesenteric tissues. At 3 days after infection they were recovered from the heart, lungs, brain, kidneys and from the carcass. The larvae grow and store food material during the 2nd stage and between 8 and 12 days after infection they undergo the 2nd moult. The mouse shows the most severe pulmonary symptoms on the 3rd and 4th days after infection, the lungs showing complete red hepatization at this time. The 3rd stage larva is relatively inactive and becomes encapsulated in various tissues, particularly in the muscular and subcutaneous tissues of the neck, shoulders and thorax. The chief developmental changes, apart from growth, which occur in the 2nd and 3rd stage larvae are: (i) the intestine develops from a single row of cells to a multi-cellular tube; (ii) the body cavity appears; (iii) the excretory lobes appear, the nucleus on the left side becoming prominent at the end of the 2nd stage; (iv) the cuticle shows transverse striations at the end of the 2nd stage; (v) the lateral lines become prominent.

The encapsulated 3rd stage larvae remained alive for at least six months in the tissues of mice and at 25 days after infection of the mouse they were able to develop in the young ferret following killing and ingestion of the mouse. No infection of ferrets was obtained through oral administration of embryonated eggs or 3rd stage larvae digested from mouse tissues.

The 3rd moult occurred in the intestine of the young ferret 3–4 days after infection; in adult ferrets the 3rd stage larvae were evidently unable to gain a hold and were passed out in the faeces. In the next 2–3 weeks the larva grew from about 2 to 16 mm. the 4th moult occurring between 2 and 3 weeks after infection. During the 4th stage the lips develop into the adult form and sexual differentiation occurs. In the female the genital rudiment moves forward and becomes differentiated into the vagina, uteri and ovaries. The vulva remains closed throughout the 4th stage.

The adult parasites had developed to sexual maturity by 56 days after infection, but they continued to grow and were considerably longer at 6 months after infection. The position of the vulva relative to the body length was found to move from about midway along the body in the 4th stage larva to a position at the junction of the anterior and middle third of the body in the mature adult.

The life history of this parasite is discussed in relation to that of A. lumbricoides and other species. It is considered that the life history of A. devosi, requiring as it does a true intermediate host for its completion, provides further information on the evolutionary development of the ascaris group. This work accordingly supports the hypothesis that the earliest members of this group utilized an intermediate host and does not support that which supposes that ascaris parasites are descended from skin-penetrating forms.

During this investigation the writer has benefited considerably from correspondence with Dr J. D. Tiner, Department of Zoology, University of Illinois, Urbana, Illinois, U.S.A. His thanks are also due to Dr H. B. Speakman and Dr A. M. Fallis for their encouragement, guidance and help.

This work was supported by the Province of Ontario on the recommendation of the Research Council of Ontario.

Grateful acknowledgement is made to Mr Cliff Smith of the Connaught Medical Research Laboratories of the University of Toronto for photographic work.

A recent editorial in Nature (Anon., 1951) on current problems in parasitology has emphasized the importance of the cultivation of parasitic organisms free from their hosts and from other species. The increased interest of workers in this type of problem, and their expanded efforts with helminths (as reviewed by Read, 1950, for helminths in general, and by Hobson, 1948, for the nematodes), which have so far almost completely resisted in vitro cultivation through whole life cycles,‡ make highly desirable an assessment of the nomenclature to be used in describing various relationships between organisms, both in nature and in the laboratory.

1. Mepacrine has a damaging action upon both vegetative forms and cysts of Giardia intestinalis. The action upon the former is rapid; but the action upon the cysts is gradual, and it takes 4 days of incubation in a 0·06% mepacrine suspension before all normal cysts disappear. The degeneration of cysts is accelerated by raising the concentration of mepacrine to which they are exposed.

2. Mepacrine produces a characteristic degeneration of the cyst which can be seen both in vitro and in vivo. This appearance develops gradually. In affected cysts the wall appears unaltered; but the protoplasm becomes shrunken and yellow, occupying only a small part of the internal cyst space. Whether shrunken or not, protoplasm stained by mepacrine is probably too much injured to survive, although the intense yellow colour makes it difficult to demonstrate eosin staining.

3. When parasites showing the characteristic abnormality are seen in the faeces they should be recognized as degenerate cysts, and further specimens should be examined before the patient is pronounced free from infection.

4. The usual method of treating a Giardia infection with mepacrine in a dosage of 0·05–0·3 gm. daily according to age, given by mouth for 6 days successfully eradicates the parasite, which is killed in both cystic and vegetative stages, but this dose is near to the minimum effective and neither time nor dosage can safely be reduced if a reliable cure is wanted.

5. Caustic soda destroys the whole cyst fairly rapidly, while a detergent had a damaging action on the protoplasm. Alkaline detergent solutions may therefore be recommended for the cleansing of infected utensils and premises. Dettol and Lysol kill cysts within 30 min. (judged by eosin staining), and 1% solutions of these disinfectants appear as effective as 5%. Utensils should be left to soak in the disinfecting solution since some cysts are slower to be affected than are others.

6. Cysts of Giardia are fairly resistant to acid, and up to 50% of them survived 4 hr. incubation in hydrochloric acid at a concentration of 0·26 gm.%, which corresponds to the upper limit of normal human gastric acidity (Harrison, 1947).

7. The cysts were damaged in different ways by different substances. In some cases the cyst was unchanged in appearance and yet rendered susceptible to eosin staining, in others the nuclei alone stained with eosin. Sometimes the protoplasm shrank within an intact cyst wall, and sometimes the wall was attacked directly.

8. In each sample of faeces studied in these experiments the Giardia cysts present appeared to be of unequal vitality, since, whatever the test substance to which they were exposed, and whatever form of degeneration developed, some cysts were affected hours or days before others showed any changes. It is conjectured that the age of the parasite since encystment may influence its resistance to damage.

Grateful acknowledgements are made to the practitioners and Medical Officers who co-operated in the following-up of cases of Giardiasis after they had been treated by mepacrine.

An account is given of the methods used to produce radioactive oocysts of P. gallinaceum. The latter are demonstrated in stained sections and in autoradio-graphs. The photographs show oocysts in mosquitoes fed on solutions of radioactive phosphorus and radioactive sulphur.

1. 2:4-diamino-6:7-diisopropylpteridine has an antimalarial index of 1:40, when tested against Plasmodium gallinaceum.

2. 2:4-diamino-1′-methylindolo-(2′:3′-6:7)-pteridine showed no activity against P. gallinaceum in doses tolerated by the chick.

3. Proguanil- and sulphadiazine-proguanil-resistant strains of P. gallinaceum were cross-resistant to 2:4-diamino-6:7-diphenylpteridine.

4. Proguanil- and sulphadiazine-proguanil-resistant strains of P. gallinaceum showed no cross-resistance to 2:4-diamino-6:7-diisopropylpteridine, but were slightly hypersensitive to this compound.

I wish to thank Dr A. Bishop for her interest in this work, and for advice and criticism. The work has been financed by the Medical Research Council.

1. An increase in resistance to metachloridine of more than 100-fold was obtained within a few weeks in a strain of Plasmodium gallinaceum treated with gradually increasing doses of the drug and maintained in young chicks by blood-inoculation at intervals of 2–3 days.

2. There was no evidence that the rapid development of resistance arose by the selection of highly resistant individuals present in the normal population.

3. Two strains of P. gallinaceum passaged through chicks treated with 0·025 mg. doses of the drug gradually became resistant to greater concentrations than that to which they had been exposed, though their growth rate decreased when they were inoculated into birds receiving higher doses of the drug.

4. In both strains maintained in birds treated with 0·025 mg. doses of the drug, resistance reached a maximum beyond which it did not increase.

5. Cross-resistance tests failed to show any relationship in mode of action between meta-chloridine and pamaquin, mepacrine, quinine or chloroquine. A strain of P. gallinaceum, highly resistant to metachloridine, showed slight resistance to sulphadiazine, sulphapyridine and sulphathiazole, but none to sulphanilamide or proguanil.

We are indebted to the Cyanamid Products Ltd., London, for the gift of the Folvite used in these experiments.

During June 1951 a quantity of eggs of the privet hawk moth were obtained in Cambridge. On emergence from the egg it was noticed that a number of the young larvae had died, some immediately after emerging, others after a period of a few days. In some cases the young larvae died before they had got completely clear of the egg shell. Examination of these larvae showed that all had died of a polyhedral virus disease.