Sex ratio of Odonata at emergence

Abstract

Final-instar exuviae left at the emergence site by Odonata can provide information of high quality for measuring sex ratio, especially of Anisoptera. Criteria are listed according to which counts of such exuviae are acceptable for this purpose. Records of sex ratio of Odonata, published and unpublished, are critically reviewed, and 194 that meet the listed criteria are presented and analysed. Variability of sex ratio differs widely among taxa but is less in large (N >299) than in small (N = 100-199) collections. Large collections indicate that the proportion of males is greater in Zygoptera than in Anisoptera (respectively 65 and 21% of records featuring >50% males). Because the sex-determination mechanism in Odonata predicts a sex ratio of unity in the zygote, variability and imbalance of sex ratio detected at emergence can plausibly be attributed to differential mortality of eggs and/or larvae and therefore, probably, to differential predation on larvae. The effect, if any, of sex ratio at emergence on reproductive potential of the adult population is unlikely to be significant, except perhaps when, rarely, the number of females is unusually low, thus reducing the size of the next generation.

Keywords: Odonata, sex ratio, emergence, exuviae

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