Emergence of the damselflies, Coenagrion mercuriale and Ceriagrion tenellum (Odonata: Coenagrionidae), at their northern range margins, in Britain

Emergence of Coenagrion mercuriale and Ceriagrion tenellum was examined in a mixed population at their northern range margin in Britain. Mortality at emergence was quantified in C. mercuriale. Consistent with their larval diapause characteristics, both species had an asynchronous emergence pattern, typical of "summer" species. Daily emergence of C. mercuriale was positively correlated with the duration of sunlight on the previous day (controlling for season) and its emergence period was found to be shorter than that observed in its core populations in Central and Mediterranean Europe. No differences were found between the patterns of emergence of the sexes in either species. Sex ratio at emergence differed significantly from 1: 1 (at 1.35 : 1 - mates : females) in C. mercuriale but not in Ceriagrion tenellum (at 1.04 : 1). Body size at emergence declined more steeply with time in females than in males of C. mercuriale because large size may confer a greater reproductive advantage in females (larger females may be more fecund) than males in non-territorial odonates. Percentage mortality of C mercuriale at emergence was low (4.9% including deformed individuals), the main cause of mortality being deformity.