Growth, winter preparations and timing of emergence in temperate zone Odonata: control by a succession of larval response patterns

Abstract

As warm-adapted insects of tropical origin, Odonata cope with cold periods by seasonal regulation and diapause. A model for larval-overwintering species is proposed with three response patterns related to the timing of emergence, which can be predicted from seasonal cues during the last few stadia. For emergence during the present season, there is an often time-constrained preemergence development, accelerated by long days and higher temperatures. In regulatory development, emergence is postponed to the next season, and a complex of diapause-like delays controlled by photoperiod and temperature prevents premature emergence. Instead, development converges on a winter diapause in sizes suitable for emergence during the following year. Long days are particularly delaying, and thermal responses are variable, sometimes inverted. In early development, with rapid growth, emergence is usually not predicted to season, but short-day winter diapauses may occur, and precocious preparations for a penultimate winter may be predictive. Thermal responses are steep, extremely so if a short-day diapause is suppressed by higher temperatures. Other physiological and also behavioural properties may differ between response patterns. Changes in photoperiod and temperature control the timing of seasonal events, and the transition from regulatory to pre-emergence development follows the increase in temperature and photoperiod after winter, which is an important time-setter. Interactions of larval size, photoperiod, temperature and previous changes affect development rate, and long-term constant conditions often end in regulatory diapauses. Proximate mechanisms of cohort splitting and the implications of the model for design and interpretation of experiments are discussed.