The Biologp of Senescence 



chemists investigating senescence, really seems to occur — in the 

 progressive hardening and weakening of the chitin of Dytiscus 

 elytra, which Blunck found to be a reliable rough measure of 

 the age of specimens taken in the wild. On the other hand, a 

 considerable part of this change, as Blunck himself suspected, 

 may represent failure to secrete the normal lubricant coat — a 

 cellular rather than a mechanical deterioration. 



In many insects, especially lepidoptera, there is evidence that 

 the fat-body contains a definite reserve of materials, which are 

 not replaceable during imaginal life. In females of the moth 

 Ephestia elutella, longevity and fecundity are both functions of 

 body weight at eclosion (Waloff, Norris and Broadhead, 1947). 

 Longevity is also greater in virgin females, possibly owing to the 

 sparing of reserves through egg-rudiment resorption (Norris, 

 1933, 1934). Exhaustion of the fat-body is characteristically 

 found in Ephestia which appear to have died of old age. Norris 

 (1934) found evidence that the fat-body contains two types of 

 store, one needed for the maintenance of the ovaries and the 

 other for the maintenance of life. The second appears to be 

 supplemented by feeding the imago, but not the first (Norris, 

 1933). Similar deterioration of the fat-body has been described 

 as a sign of senescence in Carabus and Drosophila (Krumbiegel, 

 1929) and Sitodrepa panicea (Janisch, 1924) in which the period 

 of depletion is apparently hastened by exposure to G0 2 . This 

 type of 'depletion senescence' is, in fact, in one sense an exten- 

 sion of morphogenetic senescence, if in the transition from larva 

 to imago the organism loses the power of synthesis or assimila- 

 tion of some material which it is able to store during larval and 

 pupal life. How far depletion of larval reserves is a general 

 feature of insect senescence it is difficult to say. The non-feeding 

 or the starved imago is necessarily dependent upon what stores 

 it has, although Metchnikoff (1915) from a careful study of 

 Bombjx, favoured an 'accumulative 5 rather than a 'depletive' 

 mechanism to account for imaginal death. Other imagines prob- 

 ably vary a great deal in their biochemical accomplishments. 

 Some lepidopteran imagines feed on nectar and are known to 

 absorb water and sugars. Frohawk (1935) kept Nymphalis antiopa 

 alive for three months from eclosion by feeding sugar solution. 

 On the other hand, robust Coleoptera, such as Blaps, are fully 



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