Warming accelerates development but alters synchrony in a season-dependent manner in a damselfly

This dataset contains larval development data from a laboratory experiment on the univoltine damselfly Lestes sponsa, designed to assess how temperature and photoperiod influence development time and synchrony. The data include individual- and family-level measurements of development time, treatment group assignments, and survival status. Larvae were reared under four seasonally fluctuating treatments combining early- or late-season photoperiods with ambient or elevated temperatures, simulating spring and summer environmental conditions. Development time is provided for all surviving individuals until the penultimate larval instar (F–1). The dataset supports analyses of thermal plasticity, photoperiodic responses, and developmental synchrony across ecological contexts. All data were collected under controlled laboratory conditions with no ethical restrictions, and the dataset can be reused for meta-analyses or comparative studies of insect life-history strategies, plasticity, or climate change responses.