Maintenance and termination of the embryonic diapause in the univoltine damselfly Lestes sponsa Hansemann

Embryonic diapause and its termination are key to seasonal timing in the univoltine damselfly Lestes sponsa , ensuring that eggs overwinter and hatch in spring. Following summer oviposition - typically in plants above water - diapause begins after 2–3 weeks. We investigated diapause termination in eggs from southern Sweden (~ 55°N) using combinations of chilling in darkness and subsequent exposure to photoperiods at elevated temperatures. Diapause development - a physiological process underlying termination - was fastest at 10°C, slower at 5°C, and slowest (possibly stagnating) at 20–21°C. Longer chilling enhanced the terminating effects of long photoperiods and high temperatures, resulting in faster, more synchronous hatching and a shorter critical photoperiod. At immediate 21°C and solstice-like photoperiods (LD 19.5:4.5), hatching began 1–2 weeks after diapause initiation and continued for weeks, but synchrony improved with prior exposure to diapause-maintaining conditions (< LD 17:7). LD 18:6 was only weakly effective in terminating diapause without chilling. After 19–21 weeks at 5°C, virtually complete hatching occurred within 2–4 days independently of photoperiod. If diapause was not terminated immediately at 21°C after chilling, diapause development appeared to reverse, restoring pre-chill photoperiodic responses. In contrast, 10°C post-chill accelerated diapause development and reduced laggards, but some effects of photoperiod on hatching time still persisted after 19–21 weeks chilling. Post-diapause development was 3-3.5 times slower at 10°C and ~ 20 times slower at 5°C compared to 21°C. Hatching was successful at 5°C, and larvae survived two weeks near 0°C, suggesting potential for winter hatching under a warmer climate. Eggs from Poland (~ 54°N) and northern Sweden (~ 66°N) differed in critical photoperiods, with a weaker diapause at higher latitudes. This system, adapted to latitude, ensures early and synchronous spring hatching, with hatchlings resistant to cold spells and prevents premature hatching during untimely warm periods via short-day inhibition.