A study, begun in 1966 at Edinburgh University, and completed during the ensuing 30 months at Muguga, Kenya, of the developmental periods of Rhipicephalus appendiculatus Neum. under controlled temperature and humidity was carried out to provide basic background information for an intensified FAO/EAVRO programme of East Coast fever research. Quantitative data are given on the preoviposition, pre-eclosion and, for larvae and nymphs, premoulting periods at three constant temperatures, 25, 21 and 18°C. The rate of development at all stages was accelerated by raising the temperature and retarded by lowering it. A "pre-immobilization period" which precedes the entry of engorged larvae and nymphs to the true moulting process also appeared to be temperature dependent. Development became very prolonged as temperatures fell below 18°C, being negligible at 15°C and having apparently ceased altogether at 9°C. Continuous exposure for more than two or three days to temperatures of 4"C and below were lethal to all engorged instars. Humidity (controlled at levels between 18 and 87% r.h.) had no influence on the rate of development. It is concluded that within limits of tolerable aridity, developmental periods would be completed within the times dictated by temperature regardless of the level of humidity or changes in the level of humidity. No suggestion of any diapause mechanism was discerned. IntroductionThis paper records the findings from routine observations on the developmental periods of the brown ear tick, Rhipicephalus appendiculatus Neum., under controlled conditions, and also describes experiments set up to examine the influence of temperature and humidity on development. The work formed part of a general life-cycle study of R. appendiculatus which was begun during a preliminary year at Edinburgh University in 1966 and which was continued at the East African Veterinary Research Organization (EAVRO) over the following thirty months. Until recently it was not possible to isolate, store or transmit infections of Theileria parva, the protozoan parasite of cattle which is the causal organism of East Coast fever (ECF), without using the vector, R. appendiculatus. Accordingly, a laboratory colony of this tick species has been maintained since 1952 at EAVRO using Bailey's (1960) method. Greatly increased demands were made of this colony when an intensified programme of ECF research was commenced in 1967 at Muguga under the joint auspices of EAVRO and FAO, and some of the measures taken to meet these demands have already been described (Branagan, 1969).

Results are recorded from observations over a period of two years on the development and survival of all three instars of R. appendiculatus placed under quasinatural conditions in two contrasting types of East African grassland. Hypothetical life-cycles computed from the results demonstrated the dominance of temperature over the rate of development, so that only one life-cycle could be completed each year at the cooler, more humid locality while two were certainly possible at the warmer, more arid locality.Results on survival demonstrated that the desiccation of eggs was probably the ultimate limiting factor at the drier margins of the R. appendiculatus distribution, and that the ability to exploit transpirational microclimates gave emerged instars a certain immunity from general low humidities. The marked disparity between conditions within dense grass cover and those recorded by conventional climatological instrwnents was also clearly illustratedThe influence of temperature, water relations and host presence are considered as acting in an order of precedence when discussing the manner in which population dynamics are determined by these variables.

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A study was made between 1967 and 1971 at Edinburgh University and at EAVRO, Muguga, of the variability in feeding performance of all three instars of Rhipicephalus appendiculatus Neum., with a view to better prediction of transmissions of Theileria parva. Variability could be definitely ascribed to factors within the ticks only when vigour was reduced after prolonged storage and when mating opportunity was withheld from attached females.Lower environmental temperatures prolonged engorgement, but it was arguable whether this effect was produced through the tick or through the host. All other sources of variability arose from influences within the host. The most significant of these were recognised in responses by rabbits in which resistance had been induced by successive infestations. Resistance was manifested by prolonged engorgement, by a reduction in numbers engorged and in degree of repletion, and by an increasing severity in host reaction. Three resistance mechanisms were suggested by host reactions-one appearing at the earliest stages of attachment, a second with the deposition of cement, and a third at the start of salivation. Each varied in efficacy, and each was capable of arresting an infestation. Cumulative and synergistic responses were ascribed to interactions between these mechanisms. Storage periods, sizes of infestations and sex ratios of R. appendiculatus were standardised within routine procedures, and abundant records of previous and subsequent performances on rabbits and cattle are presented for comparison and reference. Comparison of feeding performances on different animals suggests that cattle, buffalo, eland, waterbuck and oryx are as satisfactory as hosts as the rabbit, whereas Thomson's gazelle, wildebeest, sheep and goat are less so. Immature instars successfully completed engorgement on mongoose, cane rat, genet, domestic fowl and spur-fowl.

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