202 Annals Entomological Society of America [Vol. X, 



disappears from the stomach more quickly than at lower 

 temperatures, i. e., 70° to 80° F. They found fleas remained 

 infective for longer periods at lower temperatures. A tem- 

 perature of 50° or less, may directly influence plague prevalence. 

 Fewer rats were found with developed septicaemia at low 

 temperatures than at higher ones. 



From an explanatory standpoint these facts mean that due 

 to heat, frequent evacuations take place in the flea, and as a 

 result of this, the bacillus in the digestive tract of the flea is 

 filtered out with greater frequency. 



Another excellent observation was that high temperatures 

 retarded both egg deposition and development, and that low 

 temperatures prolonged the life cycle. This should be humidity 

 because high temperatures in India were associated always with 

 high humidity. Humidity, then, is inimical to the flea. The 

 chart shows fleas at their greatest abundance from February 

 to May, their numbers dropping off rather sharply after May, 

 and the cause of this is the humidity which is on the increase 

 from June to August. 



Nearly all reports on. plague show that its maximum coin- 

 cides with the period of maximum numbers of fleas. Kitasato 

 (1909) finds that the absolute and relative abundance of X. 

 cheopis is much increased during the autumn, i. e., during the 

 plague season. Tidswell (1910) gives a table of the flea popula- 

 tion per month per one thousand rats, the average mean monthly 

 temperature, and the average mean monthly humidity; the 

 flea abundance corresponds with the plague season. 



The chart for Bombay shows the fleas on M. decumanus 

 increase in numbers from June to August, outnumbering those 

 on the black rat. This period is one of heavy rains, and these 

 drive the brown rat from its subterranean burrows, cellars, etc., 

 and force them into dwellings, i. e., into drier situations. Rat 

 breeding increased at this time, due to the ravages of plague 

 among them during the previous months. This influx of rats 

 into a drier habitat is most favorable to the rapid development 

 of fleas. 



Quite naturally, the houses in the barrios which are near 

 wharves, etc., may show slight recrudescences of plague at a 

 time when plague in general had declined. 



