The Biology of Ageing in Insects 249 



humidity. Except for specific experiments designed to test 

 the role of parental age in determining the longevity of the 

 offspring, all flies were reared from eggs laid by parents as 

 soon as they were capable of oviposition, i.e. between the 

 fourth and fifth days of adult age. A well-regimented strain 

 under the conditions of rearing and maintenance described, 

 these flies emerge as adults exactly two weeks following the 

 emergence of adults of the previous generation and young 

 females begin laying eggs on exactly the fourth day after they 

 have reached the imaginal state. 



Longevity and diet 



During the course of collecting adult male and female house- 

 flies for biochemical study (Rockstein, 1956), it was observed 

 that there were relatively fewer and fewer males available for 

 enzyme determinations, especially by the end of the second 

 week. Thus, from a sex ratio of one to one, the male to female 

 population composition fell to a one to two ratio by the end 

 of two weeks and to a less than one to three ratio by the end 

 of the third week. A pilot follow-up study was made of six 

 cages of about 125 flies each in which mortality (rather than 

 survival) counts were made; the results (Rockstein, 1957) 

 clearly confirmed the fact that male longevity was consider- 

 ably smaller than that of the female housefly. However, in 

 both of these cases, although the larvae were reared on a 

 standard laboratory medium of powdered whole bovine milk 

 (KLIM, Borden's) dried brewer's yeast and agar, the adults 

 had been maintained on sucrose and water alone (in order to 

 eliminate extraneous factors of diet and egg-production in the 

 females particularly, in the age-dependent enzyme study). 

 Under such conditions of restricted diet, it was thought that 

 the observed sex-related diff'erential in longevity, favouring 

 the female of this species, might have resulted from the pos- 

 sible availability of nutrient reserves (such as oocytes or fully 

 developed ova in the ovarioles of the young female at emerg- 

 ence) to individuals of that sex exclusively. Figs, la and lb 



