Temporal patterns of emergence of the riverine dragonfly Onychogomphus uncatus (Odonata: Gomphidae)

The emergence of Onychogomphus uncatus from three stretches at two adjacent canals in Southern France was compared in 1993. Whereas 50% of annual emergence from the two stretches at one canal had occurred after 25 and 36 days, respectively, it had occurred after 5 days at the second, where emergence began 14 days earlier. At the second canal most of the larvae of one generation had overwintered in the final instar but at the first canal only half did so. These differences correlate with greater annual fluctuations of temperature at the second canal which are responsible for high temporal synchronisation. Other factors that may cause differences in seasonal regulation, such as drying up and density, are discussed. Differences exist also in sex ratio and abundance of emerging individuals, particularly between the two collection sites at the first canal.