The age-composition of populations of Calathus melanocephalus L. analysed by studying marked individuals kept within fenced sites

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Abstract
Investigations on the age-composition of the carabid beetle Calathus melanocephalus are discussed in a number of papers. The overall aim of this study is to estimate the significance of the variability in the age-composition in time and space of field populations of this species. This variability is thought to contribute to some degree to the stabilization of the fluctuations in animal numbers from year to year by which, for this species, the “chance to survive” in some area will be increased. In an earlier paper the occurrence of this variability was demonstrated by comparing three populations of this species living in three different localities at Schiermonnikoog, and by comparing one population in three succeeding years. This paper tries to develop a general strategy for investigating the degree of variability in the age-composition of field populations of carabid beetles. Two methods of investigation are discussed, i.e.: By means of experiments with separated individuals of the overwintered and young generation inside enclosures it is shown that the females of the two generations can be distinguished by differences in the developmental stages of the ovaries, especially indicated by the distinctness of the corpora lutea. As for the males, the colour of the aedeagus and of the border of the genital plate is used for this purpose. Also from these experiments the length of the period within the breeding season during which the two generations can be distinguished is estimated. The results of these experiments can be used with sampled field populations to estimate the frequency distribution of the different developmental stages of the reproductive organs and their changes in time. From the release of individually marked specimens of both generations inside enclosures and by their recapture and further release it is demonstrated that differences occur between the curves of cumulative recaptures for the two generations. In this way it has been shown also that the main period of reproductive activities of the two generations occurs in different parts of the breeding season. The paper also discusses to what extent the relative contribution of each generation to the sizes of populations can be estimated by comparing the curves representing the cumulative captures from field populations.