A simple method is described for eliminating the symbionts from the body louse during the egg stage, and thus producing symbiont-free larvae. The behaviour of lice freed of their symbionts by this method is described and compared with that of lice freed of their symbionts by extirpation of the larval mycetome.

The complete accord of the results obtained by the different methods warrants the conclusion that the symbionts play an essential role in the life of the louse, so much so that without them death of the larvae results. It is, therefore, justifiable to consider this relationship true symbiosis.

The two subjects considered in this paper are of practical importance to those interested in research of the kind carried out at the Human Trypanosomiasis Research Institute, Uganda. Their discussion would seriously encumber a report on experimental work and would distract the reader's attention from the main theme. The subjects are therefore dealt with separately, since they bear on all work, past and present, carried out at the Institute.

The insect parasites bred out of Psyllid hosts, in the course of studying their biology in Scotland, belong to four families of Hymenoptera and one of Diptera, the total number of species encountered being seven. Of these one and possibly two more of the hymenopterous species are hyperparasites. The hosts involved include four species, of which only one, Psyllia mali race peregrina, may be said to have been fairly heavily parasitised. As observed by Ferrière (1926), species of Psyllidae, in spite of their nymphs leading a comparatively sedentary life, are less susceptible to attack by parasites than are the other Homoptera. As a rule the hymenopterous parasites attack them in the nymphal stage, while the dipterous species parasites the adults as well.

Two species of Nematodes belonging to the genus Trichostrongylus, namely, T. tenuis (Mehlis 1846) Railliet and Henry 1909, and T. pergracilis (Cobbold 1873) Railliet and Henry 1909, have been described from birds, the location of both species being the caeca. A third species, described as Strongylus douglasi Cobbold 1882 emend. Gedoelst 1911, from the proventriculus of the ostrich, is considered by some authors as belonging to the genus Trichostrongylus, and by others, notably Lane (1923) and Nagaty (1932), as necessarily referred to a distinct genus, Libyostrongylus, with which latter opinion we are inclined to agree, as a result of examination of specimens of this species.

Clinical trichostrongylosis, caused by T. tenuis in the caeca, previously known in epizootic form among gallinaceous birds in Great Britain, is here reported for the first time in this country, in native domestic geese in the District of Columbia and in imported pheasants in Michigan.

In cultures made from caecal contents of infected pheasants, eggs hatched in 36–38 hours; the infective larval stage might be reached as early as the sixth day (fourth day after hatching). Moisture being present, migrating larvae, retracted within the cuticular sheath, resisted an average temperature of about −3° C. (range +3° to −20° C.) for 4 months, proving infective for a bird host at the end of that period. The chicken, turkey and guinea-fowl were successfully infected with cultures derived from pheasants; attempts to infect the duck and pigeon with cultures from pheasants, and to infect the duck with cultures from geese, were unsuccessful. Development to adults in bird hosts took place in a minimum of 7 days after infection; the size of adults varied in different birds, being greater in the chicken than in the pheasant, and still greater in the turkey. In early stages of infection diarrhoea from the caeca, with passage of mucus and blood, occurred; later all discharge from the caeca ceased. The pathological picture was that of acute typhlitis, with the trichostrongyles lying in close contact with or penetrating the mucosa. Reinfection being prevented, symptoms were of fairly short duration, and the infestation tended to die out in about 2 months.

The following is a description of a new species of Accacoeliid Trematode found in the intestine of a sun-fish (Orthagoriscus mola Bloch), caught by a fisherman in June, 1933, at Salcombe, Devon. The fish was examined by Mr J. W. Poulton of Downing College, Cambridge, to whom my thanks are due for bringing me the parasites for identification. In addition to the species described in this paper the host contained the following forms: on the skin, Tristoma molae and Lepeophtheirus nordmanni; in the intestine, Ancistrocephalus microcephalus.

Of the ten species which have been referred to the genus Rhadinorhynchus four (pristis, horridus, tenuicornis and exilis) are considered valid members of that genus. Selkirki is considered a synonym of pristis. Katsuwonis is placed in a new genus, Nipporhynchus, and it is suggested that ornatus may possibly belong with it. Medius is transferred to the new genus Gorgorhynchus of which a new species, G. gibber, is made the type. Wheeleri and johni are both referred to the genus Aspersentis.

The unusual occurrence of an infestation of Fimbriaria fasciolaris in the proventriculus of a swan is recorded.

The worms were associated with the presence of bipolar staining bacteria and ulcer formation near the site of their attachment.

Sixteen species of mites belonging to nine different families were found on Mus musculus and Apodemus hebridensis and in the nests of the former. Of these, eleven species may be regarded as parasitic in habit, two species have previously been recorded in association with rodents and moles, and three species may be described as accidental visitors.

I wish to thank Mr Elton and Prof. Munro of the Imperial College for their help, and I am also greatly indebted to Dr Caiman and Dr Finnegan for allowing me to make use of the facilities afforded by the British Museum (Natural History).

The many factors governing loss of water from the egg of Lucilia sericata are discussed, and this loss of water is shown to be the probable cause of mortality and failure to complete development at different combinations of temperature and humidity.

1. This paper comprises a study of the larval morphology and biology of the dipterous parasites of the terrestrial Isopods or woodlice.

2. Porcellio scaber L., Oniscus asellus L., Metaponorthus pruinosus Brandt., and Armadillidium vulgare Lat., were found to be attacked by one or more of the parasites.

3. Seven dipterous parasites have been reared and identified from woodlice: Plesina maculata Fall., Melanophora roralis L., Phyto melanocephala Meig., Styloneuria discrepans Pand., Frauenfeldia rubricosa Meig., Cyrillia angustifrons Rond., and Rhinophora lepida Meig. The biology and larval forms of all of these except R. lepida are described and figured, together with those of two undetermined species, “A” and “B.”

In theory, a parasite whose effective reproductive rate remains, under practically all conditions, equal to, or greater than, the effective reproductive rate of its host, ought eventually to annihilate it, or, at least, to produce an abrupt reduction in the host population, so great, that it is almost impossible for the vast majority of the emerging parasites to find any prey; so that the parasite population suffers, in its turn, an abrupt and catastrophic reduction, after which the host again increases, followed in due course by the parasite. The numerical fluctuations occurring as a result of this interaction, under various conditions, have been the subject of much theoretical study, but relatively little is known about what really happens in nature. The following brief account of the development of a colony of the Chalcid parasite, Aphelinus mali Hald., at the expense of its host, Eriosoma lanigerum Hausm., may therefore be of interest.