Temperature-dependent growth increments and number of moults shape temperature-size responses during larval development in the dragonfly Sympetrum striolatum (Odonata: Anisoptera)

Abstract Temperature is a key driver of individual physiology and ontogeny in ectotherms, with important consequences for individual fitness and population dynamics. Freshwater ectotherms typically grow and develop faster but reach smaller adult sizes at higher temperatures, a pattern known as the “temperature-size rule” (TSR). However, temperature-size responses can vary across ontogeny. Some freshwater arthropods also adjust the number of larval moults, offering additional developmental flexibility that could modulate temperature-size responses. We investigated the temperature dependence of the larval development of Sympetrum striolatum , a widespread univoltine dragonfly with variable number of larval instars. Larvae were reared from egg to final larval instar at six constant temperatures (9, 13, 17, 21, 25, and 29 °C). We assessed number of moults, developmental rate and body size of selected instars, and measured the metabolic rate at the last larval instar (F0). No hatching occurred at 9 °C, and all larvae died early at 29 °C, indicating a narrow thermal window for development (< 20 °C range). Development rate increased with temperature between 13 °C and 25 °C. While early instars followed TSR expectations, with smaller size at higher temperatures, final body size increased with temperature due to more larval moults and larger size increments in later instars at higher temperatures. Metabolic rate at 21 °C was significantly lower in individuals reared at 25 °C, indicating thermal acclimation under thermal stress. These findings reveal complex, stage-specific responses to temperature and highlight the developmental plasticity of S. striolatum . Our results provide insights into how freshwater ectotherms with flexible development strategies may cope with climate change.