The following paper as a preliminary study of the acarine parasites of South African lizards attempts to furnish as complete a list as possible of the pro-stigmatic mites of two families of lizards peculiar to South Africa, the Zonuridae and Gerrhosauridae.

1. A strain of T. rhodesiense, isolated from man and readily transmissible by tsetse, was passed by direct inoculation through a series of fourteen guineapigs over a period of 18 months. At the end of that time it had lost its transmissibility by Glossina palpalis, and it also failed to infect a volunteer.

2. Another line of the same strain, after 98 days in a bushbuck, 30 days in a fowl and 294 days in oxen, proved still readily transmissible by G. palpalis and also readily infective to man.

3. A second strain underwent seven consecutive cyclical passages through tsetse, then two passages by the syringe, and finally another cyclical passage, all save one in monkeys. When tested on man at the tenth and eleventh passages it was found to be non-infective.

4. A strain of T. gambiense, isolated in November 1920 from a patient from Fernando Po, was found in February 1934 to be readily infective to man. The strain was entirely non-transmissible by and almost completely non-infective to G. palpalis.

5. Three strains of T. brucei, one from the west, one from the north and one from the south of Uganda Protectorate, were found to be incapable of infecting normal healthy man. All the tests were carried out with cyclically infected tsetse.

6. A freshly isolated strain of T. gambiense from a Uganda native was transmitted to man by cyclically infected laboratory-bred G. palpalis.

7. A strain of T. rhodesiense, shortly after its recovery from a native of Tanganyika Territory, underwent three successive cyclical passages by laboratory-bred G. palpalis from monkey to monkey; at each passage the strain was tested on man and found to be readily infective.

8. A strain from Nigeria showing points of resemblance to both T. gambiense and T. rhodesiense was found to be pathogenic to man, on subcutaneous inoculation of infected blood, three years after its first isolation.

1. Evidence is produced to show that T. rhodesiense may retain its cyclical transmissibility by Glossina for at least 600 days in an antelope.

2. Prolonged residence in these animals tends to impair the power of a strain to infect man, when infection is attempted by the cyclical route. Thus of six volunteers exposed to cyclically infected tsetse carrying T. rhodesiense that had been for many months continuously in antelope, only one became infected. All these flies derived their infections from the antelope itself. As the cyclical method is the one operating in nature, these observations suggest that although certain species of antelope are admirable hosts for T. rhodesiense, yet as a reservoir from which tsetse can become infected with trypanosomes pathogenic to man these animals do not constitute so great a menace as has hitherto been supposed.

3. In contrast to (2), in every instance where the trypanosomes from the antelope, before being tested on man, were inoculated by the syringe into a monkey, every volunteer exposed became infected.

In the single experiment where the monkey was infected by tsetse instead of by direct inoculation from the antelope, one volunteer became infected and another did not. There is thus a suggestion that the passage through the monkey prepared the trypanosome for survival in man. On the other hand, the behaviour of strain Tinde I shows that T. rhodesiense may lose its power to infect man in spite of repeated passage through monkeys.

4. Some further indirect evidence is produced to show that T. rhodesiense may owe its origin to T. brucei, and that pathogenicity to man may be a property possessed in different degrees by different strains of T. brucei in nature.

5. These researches suggest, also, that human beings differ in their susceptibility to T. rhodesiense, but that these differences only operate within a narrow range of variation in the power of strains to infect man. In other words, strains strongly pathogenic to man will infect anyone, but strains whose power has been weakened will only be able to use individuals of subnormal resistance.

6. Two different strains of T. rhodesiense, have been shown to behave differently in the same man. One did and the other did not infect him.

7. There may be a difference in the suitability of the various species of antelope to act as hosts to T. rhodesiense. Thus the bushbuck seems to be a better host to the trypanosome than the oribi.

8. A young hyaena infected with T. rhodesiense for 180 days remained in excellent health. Flies infected from a monkey sub-inoculated with the blood of this hyaena infected a volunteer: the trypanosome had then been 80 days in the hyaena.

9. Clean flies that had taken their first two meals off monkeys infected with a strain of T. rhodesiense non-pathogenic to man, were nourished entirely on human blood during the first 3 weeks of the cycle of development of the trypanosomes in their interior. These flies, on the completion of this cycle, were still unable to infect man.

10. A strain of T. rhodesiense, after a series of cyclical passages through a reedbuck and six monkeys, was found to be non-pathogenic to man. The possibility has to be borne in mind that this strain owed its original association with man to meeting an abnormally susceptible individual. This strain since its arrival at the Institute has been tested on nine different volunteers, and none of them became infected. There is therefore nothing to distinguish it from T. brucei, save the fact of its isolation from man.

11. It is recorded that a single cyclically infected fly infected a volunteer at a single feed.

12. It is shown that the appearance of a tender indurated swelling at the site of the bite of an infective fly is a not uncommon symptom of an infection with T. rhodesiense. Local tenderness and swelling were also noted where the glands of an infected fly had been inoculated subcutaneously, even in cases where no infection ensued. Where inoculations of blood were employed on man the local disturbance disappeared rapidly when no infection resulted, but when infection took place the local symptoms steadily increased during the last few days of the incubation period, and before trypanosomes were recognised in the peripheral blood.

13. The only infection of an antelope with T. gambiense carried out during this research was that of oribi III with strain LI. This strain at its first isolation was not very transmissible by G. palpalis, and after a few months in the antelope showed signs of failure to adjust itself of this particular host. Another strain, mentioned in a previous paper (Duke, 1935), failed to infect an adult female situtunga, although two sheep exposed to the same strain at the same time duly became infected.

The life history of Panopistus pricei, as determined experimentally, has been described, together with the life history stages, including the sporocyst, cercaria and metacercaria. The snail Zonitoides arboreus has been determined experimentally to be a first and second intermediate host of the parasite.

Two adult females bearing males, on the gills of Trigla cuculus at Plymouth, collected by Miss Miriam Rothschild in November 1933.

ON 19. v. 1934, while trawling from S.S. Salpa, kindly placed at my disposal by the Marine Biological Association, Plymouth, the following Lernaeidae were taken:

The two species of bed-bug, Cimex lectularius and C. rotundatus, are found in temperate and tropical regions respectively. They are very similar anatomically, and have the same habits.

Physiologically, the two species have many resemblances. Their water balance is similar; they are very little affected by humidity if there is plenty of food available. The thermal death-points are identical.

At temperatures below 30° C., C. lectularius is able to breed more rapidly and more successfully than C. rotundatus. Between 30 and 35° C. both species behave alike, and above 35° C. C. rotundatus may be the more efficient species. Under most conditions, C. lectularius is the more resistant to starvation and desiccation.

Results indicate that although C. lectularius is more fitted for life in temperate countries, if introduced into the tropics it should be able to carry on at least as well as C. rotundatus.

Studies of geographical distribution indicate that C. rotundatus has spread all over the tropics, but has never established itself outside them. C. lectularius has spread all over the temperate world, and into the ports in some tropical countries. Experimental work suggests that it may become widespread in those parts of the tropics into which it is introduced.

In a previous paper (MacLeod, 1934) the different physical factors in the environmental complex of the sheep tick were discussed in their relation to the development of the gorged tick, and an experimental analysis was made of the effects produced, under laboratory conditions, by their action and interaction. The present paper deals with the active unfed ticks. These present rather a different problem from the gorged ticks, for, whereas in their case the climatic optimum resolved itself into that combination of conditions under which development proceeded favourably and with a low attendant mortality, the unfed tick, on the other hand, is faced with two desiderata, survival and hostparasitisation, the physical optima for which may not be coincident. It is necessary, therefore, to consider not only the climatic optimum for survival and activity, but also the factor or factors which influence parasitisation, and those conditions under which host-parasitisation is facilitated.