Dissecting Oxidative Stress and Organismic Response to a Temperature Gradient in the Midge <scp> <i>Chironomus riparius</i> </scp>

ABSTRACT Oxidative stress, caused by reactive oxygen species (ROS), poses a major challenge for organisms facing temperature fluctuations. This study provides the first direct in vivo measurements of ROS production together with transcriptome analysis of oxidative stress genes across a broad range of ecologically relevant temperatures in insect larvae of the dipteran midge Chironomus riparius . We observed a U‐shaped pattern of oxidative stress, with minimal ROS levels within an optimal thermal window (12°C–18°C) and significantly elevated stress at both cold and warm extremes. Crucially, our findings reveal distinct underlying molecular mechanisms for ROS generation at these extremes: at low temperatures, predominantly ROS produced is of the superoxide group, linked to hypoxia‐induced hemoglobin autoxidation. Conversely, at high temperatures, the hydrogen peroxide group dominates, associated with increased metabolic rate and heat stress signaling pathways. Transcriptomic analysis shows that C. riparius 's antioxidant defense system adapts accordingly, selectively upregulating mechanisms to counteract the specific dominant ROS type at different temperatures. This mechanistically differentiated oxidative stress at different temperatures and the modulated organismic response reflect the ecological niche and evolution of C. riparius .