Growth and Senescence 



22nd instars, without reference to the chronological age which 

 these may represent. (Figs. 37, 38, 39.) In D. magna, Anderson 

 and Jenkins (1942) found a mean life-span of 960 hours or 17 

 instars — the number of pre-adult instars varied from 4 to 6 and 



Well-fed 

 (Normal) 



Starved until 

 6th instar 



Starved 

 throughout life 



1015 



Fig. 38. — Effect of restricted food upon the duration of instars in Daphnia 

 longispina (from Ingle et al., 1937). 



the differences in longevity between individuals represented 

 differences in the length of the pre-adult period. (See also 

 Dunham, 1938.) The finding of Fritsch (1953) that the pan- 

 tothenic acid content of the medium is a major factor in 



145 



