Abstract
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 - males : 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.
Highlights
Odonates are almost exclusively aquatic in their larval stages and metamorphosis to the adult stage includes a niche shift from the aquatic to the terrestrial habitat (Cor bet, 1980)
The aim of this study was to investigate the effect of season and climatic factors on the form of the emergence curve in these two rare damselflies that reach their northern range margins in Britain (50-53°N, 1-2°W)
Numbers of C. mercuriale and C. tenellum emerging each day throughout the season (Cages 1-10) are shown in Fig. 1. 24th May is referred to as day of emergence (Day) 1 of the season for both species
Summary
Odonates are almost exclusively aquatic in their larval stages and metamorphosis to the adult stage includes a niche shift from the aquatic to the terrestrial habitat (Cor bet, 1980). The emergence period is restricted (Corbet, 1980) despite differences in development rate accumulated between individuals in the larval stage due to differential hatch dates (Johanson, 1991) or differences in the nutritional condition of indi viduals Harvey & Corbet, 1985) This restriction is achieved by diapause characteristics in the larvae, by the distinct responses to critical day length and temperature of different instars (Corbet, 1957a; Sternberg, 1994). Corbet (1954) distinguished between populations that overwinter (and diapause) in the final instar and have a closely synchronised and often early emergence (“spring” species) and those that overwinter in one or more earlier stadia, have less well synchronised emergence and emerge later (“summer species”)
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