Emergence timing and fixation height in Pachydiplax longipennis (Odonata: Libellulidae) at varying substrate density and sunlight exposure


Emergence substrate and sunlight penetration inherently trade off in patchy vegetation. Given the importance of solar radiation at emergence, we expected greater sunlight availability in sparse vegetation to advance emergence timing and reduce the average height of emergence fixation. We used outdoor mesocosms stocked with varying cattail (Typha) densities and late-stage Pachydiplax longipennis (Odonata: Libellulidae) larvae. As predicted, emergence based on exuviae observations began significantly earlier (5 d) at lower cattail density and greater sunlight exposure, with over 60% of the emergence completed midway into the experiment period, compared to about 50% in the medium and higher density cattail. This finding suggests lag effects under relatively limited light availability in a temperate-centered lentic-breeding heliotherm. Contrary to our prediction, we found significantly greater emergence heights at lower cattail density ( 18.0 cm) than at medium ( 13.0 cm) and higher ( 10.0 cm) densities. We recommend further study of emergence heights using larval choice experiments in natural settings. Variation in emergence timing and fixation height under the substrate–sunlight tradeoff may be driven proximally by larval choices/development and ultimately by adult activity.

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