Adult sex ratios and female reproductive potential in Calopteryx (Zygoptera: Calopterygidae)
Odonatologica , Volume 9 - Issue 3 p. 217- 230
Census data suggest that population sex ratios of adult C. maculata (P. de Beauv.) and C. dimidiata Burm. are 1:1 despite an excess of males (significant in C. maculata only) at the water. The adult sex ratios of both species, however, were strongly influenced by where censuses were made and the differences found appeared to reflect different activity patterns of males and females and not true distortions of the population sex ratios. Reproductive potential of C. maculala females was estimated from egg counts and sperm volume indices of dissected females. Those females collected along the stream prior to the daily (1300-1500 hrs) peak in reproductive activity and those collected immediately before or after copulation carried maximum numbers of eggs (averages = 267-272). There was a steady decrease in egg number per female during the day. Females collected after 10-15 min of oviposition and those collected in woods adjacent to the stream carried the fewest eggs (averages = 155 and 177). Sperm volume indices also decreased during the day in samples of females at the water. Maximum sperm volumes were found in postcopula females. However, females collected in tandem prior to copulation had 83% of the maximum and females collected after 10-15 min of oviposition had 90% of the maximum sperm index. Females collected in the woods still carried 65% of the maximum. The substantial sperm volumes carried by females prior to mating and after oviposition suggest that many matings may occur primarily to provide access to specific oviposition sites and guarding males. The variations found in female reproductive potential are consistent with predicted female activity patterns that would result in an excess of males at the water.
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