(5) _Mode of Transmission and Reservoir._--Kinghorn has shown that _T. rhodesiense_ is transmitted by _Glossina morsitans_ in which it undergoes development. Kinghorn and Yorke[82] found that about 16 per cent. of the wild game examined in Northern Rhodesia was naturally infected with _T. rhodesiense_. The wild game examined included waterbuck, hartebeest, mpala, bushbuck and warthogs. One native dog near the Nyasaland border was found infected, but not domestic stock. Taute doubts whether _T. rhodesiense_ really occurs in wild game. Approximately 3·5 per cent. of the tsetse flies fed on infected animals may become permanently infected with _T. rhodesiense_, and capable of infecting clean animals. Furthermore, a tsetse fly when once infective probably remains infective for the rest of its life. [82] _Annals Trop. Med. and Parasitol._, vii, p. 183. Kinghorn and Yorke, however, have shown that climatic conditions, namely, those of temperature, also affect the infectivity of the tsetse fly, as the ratio of flies capable of transmitting _T. rhodesiense_ to those incapable of transmitting the virus is 1 : 534 in hot valley districts (_e.g._, Nawalia, Luangwa Valley, temperature 75° to 85° F.), while on elevated plateaux (_e.g._, Ngoa, on the Congo-Zambesi watershed, temperature 60° to 70° F.) the ratio falls to 1 : 1312. Mechanical transmission by the tsetse fly does not occur, if a period of twenty-four hours has elapsed since the infecting meal. _Developmental Cycle in the Fly._--The period which elapses between the infecting feed of the flies and the date on which they become infective varies from eleven to twenty-five days in the Luangwa Valley, according to Kinghorn and Yorke. Attempts carried out at laboratory temperature on the Congo-Zambesi plateau, during the cold season, to transmit _T. rhodesiense_ by means of _G. morsitans_ were always unsuccessful. The developmental cycle of the trypanosome in the fly is influenced by the temperature to which the flies are subjected (as stated above). The first portion of the developmental cycle proceeds at the lower temperatures (60° to 70° F.), but higher temperatures are necessary for the completion of the development of the trypanosome. Kinghorn and Yorke found that the trypanosomes may persist in the fly, at an incomplete stage of their development, for at least sixty days when the climatic conditions were unfavourable. The first portion of the developmental cycle of the trypanosome takes place in the gut of the fly. Invasion of the salivary glands of the tsetse is secondary to that of the intestine, but is necessary for the infectivity of the fly. A relatively high mean temperature, 75° to 85° F., is essential for the passage of the trypanosomes into the salivary glands and the completion of their development therein.