The Biology of Ageing in Insects 261 



was undertaken last year, with four types of matings as 

 follows: young males by old (about 29 days) virgin females, 

 old (about 23 days) males by young virgin females, old males 

 by old virgin females, and "modified old-old crosses" (in 

 which, as in the original study of parental age, males and 

 females were allowed to mate freely from emergence, but 

 where eggs were collected only from parents at an advanced 

 age). A fifth set of cages involved the usual young-by-young 

 matings employed in routine stock breeding and in other 

 studies in which parental age was not a variable ; this was the 

 control series. Offspring longevity data were obtained for 

 over 1,000 flies of each sex for each type of cross, for three to 

 four generations in each case, in order to test the possible 

 presence of a Lansing-like, cumulative ageing factor related to 

 parental age, in the housefly. The results obtained indicate 

 that the slightly lowered 30-day mortality for males from 

 oldest parents, seen in our earlier study (Table III), might 

 indeed have been significant. Statistical analysis of data from 

 our current study indicates that, for two generations, the 

 average male longevity (20-6 days) for offspring from " modified 

 old-old parents" is higher, by three days, than the mean 

 longevity for young-by-young crosses (P is less than 0-01). 

 That this result may be due to the effect of the female parent 

 upon male offspring is supported by the observation that a 

 similar increase in male offspring longevity was obtained for 

 crosses between young males and old virgin feynales and for 

 two cases of crosses between old males and old virgin females. 

 Thus, the mechanism involved may be one of selection of a 

 long-lived strain of male offspring from the longer-lived 

 female parents surviving to almost 30 days of age and still 

 sexually functional at that advanced age. As for the effects of 

 parental age on female offspring longevity, data obtained in this 

 more recent and extensive study suggest that the parental 

 ageing effect on this sex is more complex: it is considered 

 essential at this point to continue these studies with single- 

 pair crosses in order to follow the longevity of the adults as 



