Abstract
Voltinism and larval growth pattern were investigated in an edge-of-range population of Coenagrion mercuriale. Coenagrion mercuriale is semi-voltine in Britain and growth is inhibited in winter. The 2nd year group overwinters in a range of instars between the antepenultimate and final instar consistent with the early, asynchronous emergence pattern of this species. A facultative autumnal diapause in the penultimate instar is the most likely mode of seasonal regulation. The broad size distribution of larvae produced by this growth pattern was wider than that found in co-occurring populations of Pyrrhosoma nymphula, a spring species with synchronous emergence. The broad size distributions may lead to considerable intraspecific interference between C. mercuriale larvae. Sex ratio in the last three larval instars of C. mercuriale did not differ significantly from unity. A laboratory investigation of the effect of temperature and photoperiod on growth and diapause in C. mercuriale is recommended to determine whether high minimum temperature thresholds for development limit both the width of the temporal niche and microhabitat use by this species at its range margin.
Highlights
The significance of developmental polymorphism in conservation biology is that survival depends on the optimal survival of all developmental stages (Samways, 1994)
Voltinism and larval growth pattern in the aquatic stage of C. mercuriale in Britain are investigated and compared with the growth pattern found in mainland European populations
Since eggs hatch without diapause (Corbet, 1955), the bimodal length-percentage histograms indicate that two years were required for larval development in C. mercuriale. (Fig. 1. shows the head width-percentage histogram for October)
Summary
The significance of developmental polymorphism in conservation biology is that survival depends on the optimal survival of all developmental stages (Samways, 1994). For rare species of Odonata, with an aquatic larval stage and a terrestrial adult stage (Corbet, 1980), baseline information on all developmental stages is required for formulation of adequate management policy. Coenagrion mercuriale has a southwestern distribution in Europe and reaches its northern range margin in Britain. It has declined in countries along the northern and eastern boundaries of its distribution (Grand, 1996). In Britain, it is listed as rare (category 3) on the red list and is the subject of a UK Biodiversity Action Plan (HMSO, 1995). Voltinism and larval growth pattern in the aquatic stage of C. mercuriale in Britain are investigated and compared with the growth pattern found in mainland European populations
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