The Biology of Senescence 



determining the life-span of Daphnia complicates the interpre- 

 tation of some of these studies of dietary retardation, however. 



Birth 3 6 9 12 15 18 21 



Fig. 39. — Effect of restricted food upon the rate of senile change in the 

 heart rate of Daphnia longispina (from Ingle et al., 1937). 



5-2-2 INSECT METAMORPHOSIS AND SENESCENCE 



A great many insects are capable of very long pre-imaginal 

 life, the duration of which is largely determined by food supply. 

 The 'rate of living', as a simple quantity treated apart from 

 morphogenetic processes, does not give an entirely satisfactory 

 picture of insect development. We might possibly make an 

 experimental approach to the study of insect senescence on the 

 following lines. Senescence, of course, occurs in the ordinary 

 course of events only in the imago. The larval or nymphal 

 stages must be regarded as a system which is self-maintaining 

 but which tends towards ultimate metamorphosis. They are 

 analogous to the young growing period of non-metabolous 

 metazoa. The question arises how long, if metamorphosis could 

 be indefinitely prevented, the metathetelic larva would remain 

 self-maintaining as an equilibrium system. It might presumably 

 do so indefinitely, or it might ultimately undergo a specialized 



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