The Distribution of Senescence 



to the cuticle (Blunck, 1924; Wigglesworth, 1945), depletion of 

 reserves both in feeding and non-feeding imagines, accelerated 

 in some cases by reproduction (Krumbiegel, 1929a, b; Bilewicz, 

 1953), accumulation of urates (Metchnikoff, 1915), deterior- 

 ation of the nervous system (Hodge, 1894-95; Pixell-Good- 

 rich, 1920; Schmidt, 1923; Weyer, 1931, etc.) and 'general 

 senile decay' have all been demonstrated by more or less 

 satisfactory evidence. The vast majority of holometabolous 

 imagines give every evidence of having a sharp specific age, and 

 this is a group in which we are unusually well-equipped with life- 

 tables. The nature of the processes which limits imaginal life 

 seems, however, to vary widely, but they have the common 

 property of being processes operating in a cellular system where 

 little or no renewal, and no further morphogenetic develop- 

 ment, are occurring. 



One of the best general descriptions of insect senescence is 

 given by Blunck (1924) for Dytiscus marginalis: he describes the 

 main signs of advancing age as diminution in activity and 

 deterioration of the epicuticle, with the growth of colonial 

 protozoa on the dorsal shield, legs and mouth parts, which the 

 animal cannot any longer clean effectively. The cleaning secre- 

 tions seem to be reduced, and the chitin appears brittle, whole 

 legs or antennae being occasionally snapped off in swimming. If 

 pygidial gland secretion fails, air enters the subalar air chamber 

 and the beetle drowns. In the beetles dissected by Blunck, the 

 gonads had almost disappeared during the third year of life, the 

 fat-body was increased in size, almost filling the body cavity, 

 but chalky and full of concretions. In some individuals there 

 was almost complete atrophy of the wing muscles. The extreme 

 life-span is under 3 years, females living longer than males: 

 sexual activity usually ceases in the second year but may persist 

 in individuals into the third. A number of senile processes, 

 which may not be mutually dependent, can be detected in this 

 description. The balance between mechanical, depletive and 

 'morphogenetic' senescence must vary considerably from species 

 to species, and even from individual to individual. Blunck's 

 description is of interest in providing not only an account of 

 such a mixed senescence, but one of the very few instances 

 where the 'change in inert structures', so popular with colloid 



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