Environmental Factors Influencing the Induction and Termination of Diapause in the Emperor Dragonfly, Anax Imperator Leach (Odonata: Aeshnidae)

Abstract

Preliminary experiments have been conducted to determine the environmental factors influencing the induction and termination of diapause in the Emperor Dragonfly, Anax imperator Leach. A facultative diapause is located in the final larval instar and takes the form of an interruption of morphogenesis such that, at about 230 C., metamorphosis does not occur until some 105 days after ecdysis. In non-diapause larvae this period is about 30 days. Feeding continues during diapause development. Diapause appears to be determined in the first few days of the final instar. It can be induced by decreasing photoperiods, irrespective of the lengths of day involved, and alsoby a regime of unchanging photoperiods of a length corresponding to 31 July. Diapause can be prevented by exposure to an increase in photoperiod immediately after ecdysis. This may take the form of a single large increase (of more than 20 min.), or of successive small increments such as occur naturally in March and April. It is possible that under unnaturally long photoperiods larvae do not respond normally to regular increments. Ecdysis to the final instar appears to be stimulated by photoperiods of slightly more than equinoctial length. This tends to localize the sensitive stage to seasons when changes in day length are most pronounced. A previous exposure to winter temperatures in the penultimate instar is not necessary for diapause to be averted in spring. It is likely that diapause development proceeds most rapidly at a temperature near io° C. Negligible diapause development had occured after exposure to 6° C. for 12 weeks, although variation in the period necessary for its completion was then reduced. Diapause development has already been completed in nature by February. After this, metamorphosis occurs as a simple response to adequate temperature and food. It cannot take place when the average temperature is below n° C. Metamorphosis is not prevented by a regime of decreasing photoperiods.