_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.
Kinghorn and Yorke[83] state that the predominant type of trypanosome in the intestine of infected _G. morsitans_ was a large broad form, quite different from that which is most common in the salivary glands. The trypanosome in the glands resembles the short form seen in the blood of the vertebrate host. The authors quoted state that both the intestinal and salivary gland forms of infective _G. morsitans_ are virulent when inoculated into healthy animals.
[83] _Annals Trop. Med. and Parasitol._, vii, p. 281.
Bruce and colleagues[84] have quite recently (June, 1914) published an account of their investigations of _T. rhodesiense_ in _G. morsitans_ in Nyasaland. (Incidentally it may be remarked that Bruce considers _T. rhodesiense_ to be identical with a polymorphic strain of _T. brucei_--see pp. 83, 94). The development of _T. rhodesiense_ takes place in the alimentary canal and salivary glands, not in the proboscis, of the tsetse fly. In feeding experiments with laboratory bred flies, as well as with a few wild flies, fed on infected dogs or monkeys, only 8 per cent. of the flies were found to be infected on dissection. Of such infected flies, however, only some allow of the complete development of the trypanosomes within them, in other words only about 1 per cent of the flies become _infective_. The length of time which elapses before a fly becomes infective varies from fourteen to thirty-one days, averaging twenty-three days, when kept at 84° F. (29° C.). The dominant intestinal type of flagellate in the fly is that seen in the proventriculus, which contains many long, slender trypanosomes. These proventricular forms find their way to the salivary glands, wherein crithidial and encysted forms are seen. They change into “blood forms,” which are short, stumpy trypanosomes and are infective. “The infective type of trypanosome in the salivary glands--corresponding to the final stage of the cycle of development--is similar to the short and stumpy form found in the blood of the vertebrate host.” The cycle is thus very similar to that of _T. gambiense_ in _G. palpalis_ (fig. 30).