258 Morris Rockstein 



plotted against time), shown in Fig. 3 (Rockstein and Lieber- 

 man, 1959), portrays in more dramatic fashion the actual 

 course of the survival (or, conversely, the mortality) trends 

 of Fig. 2, above. From the similar flat slopes of the log-probit 

 curves of both males and females during the first ten days, a 

 similar low mortality rate is apparent. After the tenth day, 

 however, a marked shift upward of the male curve, which con- 

 tinues as a straight line through the fortieth day, represents a 

 high and uniform mortality rate during 93 per cent of the male 

 population's existence. Thus a single (or at least relatively 

 simple) mortality factor appears to be operating in the great 

 majority of the male houseflies of this strain. In the females, 

 however, one sees that the same log-probit curve follows a 

 more irregular course, with a slight upward trend at the tenth 

 to 21st days, a steep shift upward at the 21st day, another more 

 gentle upward rise in slope at the 30th, and a final shift upward 

 at the 40th day, which persists unchanged as a steep, straight 

 line from the fortieth to final day of the female population's 

 existence. One might infer from this kind of plot that, for the 

 female, mortality (and therefore survival) is a much more 

 complex phenomenon, with more factors for mortality becom- 

 ing effective w4th increasing age. These inferences have been 

 further substantiated in the life-tables which have been recently 

 completed by Rockstein and Lieberman (1959) from mortality 

 data for this population of male and female flies. Despite the 

 wide distribution of the ubiquitous housefly, only a few other 

 studies have been made on its longevity. The study of Wilkes 

 and co-workers (1948) employed the Peet-Grady method (in 

 which some of the dietary components are not standard) and 

 obtained average longevities of 12 days for males and 20 

 days for females of a laboratory strain and about 12 days 

 and 24 days, respectively, for male and female flies of a 

 wild strain, kept at 80° f and 50 per cent relative humidity. 

 Aside from dietary variation, the authors also describe major 

 difficulties of overcrowding, especially in the larval stage and 

 also for adults. In a recent letter, Rollins (1959) has supplied 



