The Biology of Senescence 



reserves. In winter bees, then, absence of brood leads to reple- 

 tion, which in turn induces both quiescence and inherently 

 greater longevity at a time when both are beneficial to survival. 

 The expectation of foraging life decreases in proportion to the 

 age of the bee when it begins to forage — endogenous senescence 

 therefore appears to play a part in limiting the life of workers, 

 and they do not all die from accident alone (Ribbands, 1952). 

 Whatever the facts concerning 'Gehirntod', this process of sen- 

 escence appears to contain a major depletive element, com- 

 bined, in all probability, with an element of mechanical 

 damage. In this respect the senescence of worker bees conforms 

 to a pattern which seems to be widespread in insects. 



2-5-6 MOLLUSCS 



Pelseneer (1934) divides molluscs in the wild into annual 

 species, pluriennial species with a short reproductive life, and 

 pluriennial species with a long reproductive life. In some 

 members of this last group, indeterminacy of life-span cannot be 

 excluded. Most of the evidence is obtained from wild material. 

 The combination of patterns appears analogous to that found 

 in fish. Like fish, molluscs include short-lived forms, forms with 

 a longer but apparently determinate life, and forms, expecially 

 among the larger pelecypods, which appear to have no 

 maximum size. 



The annual forms include many nudibranchs (Pelseneer, 

 1934, 1935) and probably most of the smaller freshwater species 

 (Paludestrina jenkinsi — Boycott, 1936; Ancylus — Hunter, 1953). 

 According to Boycott (1936) Planorbis corneus is the only British 

 fresh water pulmonate which is not normally an annual. Many 

 of these annual forms die immediately after reproduction. In 

 Viviparus contectoides (van Cleave and Lederer, 1932) and V. 

 bengalensis (Annandale and Sewell, 1921) the wild males live 

 one, and the females up to three years. In captivity Oldham 

 kept male V. contectus for 4| and female for 5 years (Oldham, 

 1931), and living embryos were present at the time of death. 

 Growth, judged by length, ceased in the second or third year 

 of fife. A number of other forms live for a maximum of 2 or 3 

 years in the wild, breeding during one or two seasons (Lioplax 

 — van Cleave and Chambers, 1935; Bithynia — Boycott, 1936; 



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