316 
Trypanosoma leivisi 
development of the trypanosomes in the transmitting insect, thereby 
increasing the chances of successful transmission to the vertebrate host. 
It is extremely likely for example that if the trypanosomes undergo 
a developmental cycle in the flea or louse there is an optimum tempera¬ 
ture at which such development proceeds. It will be remembered that 
Grassi (1906) and others have shown experimentally that there is an 
optimum temperature (77°—86° F.) for sporogony of the malarial para¬ 
site in the mosquito, and that Ruge (1906) has given similar proof of 
the influence of temperature in the case of the Proteosoma of birds. 
Up to the present time, however, developmental stages of T. lewisi have 
not been described in the rat flea, and the observations of Prowazek 
(1905) on the development of T. lewisi in the rat louse have not been 
accepted by other workers as established. 
In Chart IV the temperature curve 1 for Bombay is presented 
for the year under review together with the trypanosome curve for 
M. rattus 2 ; the mean monthly temperature is 79° F. 
It will be noticed that the trypanosome curve closely follows the 
temperature curve. This relationship is indeed so marked as in our 
view strongly to suggest that temperature is an important factor in 
determining the prevalence of trypanosome infections in rats. More¬ 
over, since temperature can conceivably exercise an influence only by 
its effect upon the development of the trypanosomes in the transmitting 
insects, the relationship between temperature and trypanosome pre¬ 
valence must be regarded as explicable on this theory. As to the exact 
significance of the interval separating the two curves it would be prema¬ 
ture at present to speculate. 
There is yet another influence, however, which climatic conditions 
may have upon the life history of T. leiuisi in the agents of transmission. 
Judging from present knowledge of the conveyance of trypanosomes 
from insects to the vertebrate host it would seem very probable that in 
the case we are considering direct or mechanical transference of the 
trypanosomes takes place. If this is so—and future experiments will 
doubtless settle the point—it must be supposed that an important 
1 The curves in Charts IV, V and VIII have been constructed by taking the mean 
temperature and the mean humidity for the year and expressing the respective deviations 
in each month as percentages in terms of the mean. In Chart VII the “ mean line ” 
corresponds to a hypothetical optimum temperature of 79° F., the monthly deviations 
being grouped in relation to the “mean line” as percentages in terms .of the optimum 
temperature. 
2 The rattus curve has been chosen in preference to the decumanus curve as being 
based on larger figures and therefore probably more accurate. 
