The Biology of Senescence 



2-5-5 ARTHROPODS 



Senescence in arthropods is widespread and probably uni- 

 versal. Those forms which have wings, jaws, bristies, and other 

 chitinous tegumentary structures not renewed by moulting are 

 particularly liable to genuinely 'mechanical' senescence. In the 

 forms which moult as adults, the time of ecdysis is a particu- 

 larly arduous one, judged by the mortality, and many of these, 

 such as Daphnia and large spiders, appear very often to die in 

 the attempt to carry out a final moult. 



' Physiological 5 senescence, in the sense in which nineteenth- 

 century biology used the term, also appears in a convincing 

 form for the first time in arthropods, since, as MetchnikofT first 

 pointed out (1907, 1915), a non-feeding imago must be regarded 

 as expendable from the evolutionary point of view. The evolu- 

 tion of short sexual life as a modification in some groups is 

 balanced by the evolution of a very long sexual life in specialized 

 individuals of other, social, species, as part of the adaptive 

 development of a group-existence — the longest life-span being 

 reached in one or both sexual forms among true ants and 

 termites. 



There is no known case of arthropod indeterminacy com- 

 parable with that of actinians. Growth in most insect imagines 

 is more or less rigidly limited, although the capacity for con- 

 tinued cell division persists in varying degrees. According to 

 Harms (1949), somatic mitosis in many arthropod imagines is 

 virtually confined to the mid-gut. Mitotic capacity has not 1 been 

 shown to bear any relationship to longevity, except perhaps in 

 forms producing queens, where the relation of continued repro- 

 duction to long life might be either a direct example of cause 

 and effect, or the result of two parallel adaptations. Some 

 solitary arthropods are capable of very long life (20 years in 

 tarantulas, Baerg 1945, possibly 50 years in lobsters, Herrick, 

 1896). 



Crustacea. The small Crustacea (Cladocerans, Copepods, Iso- 

 pods) generally show very sharp specific age. In Daphnia 1 this 



1 There is a striking lack of unanimity in the literature over the 'normal' 

 life-span of various species of Daphnia, even when grown in apparently 

 similar media. Fritsch (1953) has shown that this variation depends to some 

 extent upon the amount of available pantothenic acid. Where Daphnids are 



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