Larval development, emergence and seasonal regulation in Asiagomphus pryeri (Selys) (Odonata : Gomphidae)

Abstract

Larval development and seasonal regulation in a spring-emerging gomphid dragonfly, Asiagomphus pryeri (Selys), was investigated mainly by repeated sampling in the field and also by laboratory experiments. Eggs exhibited direct development. Larval duration was usually 3 or 4 years. Larvae in the penultimate instar (i.e. F-1) that entered the final instar (F-0) synchronously in their third autumn emerged in the following spring but F-1 of the same age-cohort that failed to enter F-0 in the autumn did not emerge in the following spring (i.e. cohort splitting); they had a smaller head width, underwent a supernumerary ecdysis and entered F-0 in the following autumn together with a cohort one year younger. Reduction of temporal variation in emergence, which lasted about 3 weeks, from late May to mid June, was achieved by synchronized entry to F-0 in the previous autumn. No additional synchronisation was detected in the overwintering F-0 population. Long-day photoperiod (LD 15:9; corresponding to the summer solstice) induced in F-1 intense diapause which was terminated by intermediate photoperiod (LD 13:11; the equinox). In nature, such photoperiodic responses apparently mediate the synchronous entry to F-0 in autumn. Discussion on the mechanisms of seasonal regulation is made.