THE LIFE-HISTORIES OF LESTES SPONSA (HANSEMANN) AND SYMPETRUM STRIOLATUM (CHARPENTIER) (ODONATA) by PHILIP S. CORBET Department of Zoology, University of Cambridge, England Contents Introduction Lestes sponsa (Hansemann) Larval growth Emergence and longevity .. Sympetrum striolatum (Charpentier) Larval growth Emergence and longevity Discussion Acknowledgements Summary References Page 217 218 218 220 222 223 226 226 228 228 228 Introduction With regard to their life-histories, it is meaningful to recognise two types amongst British dragonflies: spring and summer species. Spring species, typified by Anax imperator Leach and Pyrrhosoma nymphula (Sulzer), have been defined as those possessing a diapause in the final larval instar; completion of this dia-pause in autumn results in a synchronised emergence early the following year. Although summer species may have a diapause stage in the life-history, it is by definition absent from the final larval instar (Corbet 1954). It has been sug-gested (Corbet 1952) that in summer species the final larval instar is entered shortly before emergence and that, since growth is not interrupted at this stage, emergence is temporally dispersed. The peak of emergence may therefore occur in the middle of the emergence period instead of at the beginning, as is the case in spring species. An implication of a normally distributed emergence curve is the demand it imposes upon adult longevity. In summer species it is doubtful whether the reproductive potentialities of a population could be realised adequately unless the average expectation of life exceeded half the flying season. Thus, summer species might be expected to live longer than spring species. In the present paper the life-histories of two, univoltine summer species are described with the view of testing these hypotheses. 217
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