LENGTH OF LIFE OF THE FLEA 465 



case of the rat flea it is suggested that the site in which the fleas are 

 found may be related to the usual site of the plague bubo. 



The seasonal prevalence and reproduction of fleas are markedly 

 affected by conditions of temperature and humidity. The conditions 



have been carefully studied in connection with the 



.... ,. . . . , Seasonal prevalence 



seasonal incidence of plague, since it was found that 



the period of the greatest prevalence of the rat flea is also the season 

 of the maximum intensity of the disease. The average number of 

 fleas per rat (cheopis, almost without exception) in Bombay varies 

 from 6'7 and 6'8 in March and April, the height of the plague season, 

 to 2'9 in October; in the Punjab an average of 12'6 has been found 

 in April, which is the beginning of the hot weather and the time of 

 the heaviest mortality from plague, as against an average of 2*0 in 

 August, when the disease is dying out. Similar results have been 

 obtained in Australia. 



The prevalence of Ceratophylliis fasciatus is the reverse of the 

 foregoing, so far as India is concerned. It is found only rarely at 

 any time, and seldom amounts to more than two per cent of the total in 

 the Punjab districts, and it only occurs in the early part of the cold 

 weather, disappearing entirely as the hot weather comes on and the 

 percentage of cheopis increases. Ct. felis is present in the greatest 

 numbers in Madras in November and December, two fairly cool 

 months. Echidnophaga gallinaceus has been found breeding in enor- 

 mous numbers in the hot weather in the hills in South India, but 

 could not be found in the same place in the cold weather. For every 

 flea, apparently, there is an optimum temperature for reproduction, as 

 is the case with so many other insects. 



The length of life of the flea, and the duration of the several 

 instars, are still matters of dispute, and the observations so far re- 

 corded suggest that these insects are very remarkable 



, . r i i Length of life 



in this respect. It has been frequently observed 



that fleas (P. irritans?) have appeared in enormous numbers in places 

 which have not been visited by man for very prolonged periods, and 

 that, even in towns in Europe, they are often very numerous and 

 voracious in houses which have not been occupied for some time. 

 Some exact observations have been made with Ceratophylhis fasciatus, 

 Nicoll found that the average length of life at ordinary temperatures was 

 six to eight days, with a maximum or over forty days in one experi- 

 ment. Ten per cent lived over fourteen days. The average duration 

 of life is much greater in winter than in summer, and at very low 

 59 



