Patterns in Size,Sex Ratio and Time at Emergence in a South Swedish Population of Sympetrum sanguineum (Odonata)

Differences between sexes in life history patterns of Sympetrum sanguineum were studied in a small pond in southern Sweden by means of exuviae and adult sampling. Emergence occurred from 4 to 28 July, and mean emergence date was 10 July for both males and females. The sex ratio at emergence (53% females) did not differ from 1:1, but significantly more females emerged during the first 5 days of the emergence period. Size of emerging individuals (immatures) decreased as season progressed and males emerged at a larger size than females. While immature males were heavier than immature females, no such difference was found in mature individuals. We suggest that the sexual differences in size and emergence patterns observed are the result of different optimisation by males and females with respect to the growth-mortality risk trade-off in the larval and adult stages.