Which are the most heat-tolerant animals? Insights from a Mediterranean lepismatid under thermal stress in the context of climate change.

Measuring the thermal limits of insects is crucial for understanding how thermoregulatory behavior and thermal tolerance interact with the environment. A key question is the maximal temperature terrestrial animals can endure and how plastic these limits are under extreme conditions. Lepismatids are a basal group of insects with remarkable diversity in arid habitats, making them promising candidates for extreme thermoresistance, yet their thermal ecology remains largely unexplored. We present the first experimental evaluation of acclimation effects on voluntary (VTmax), critical (CTmax), and upper lethal thermal limits (UTL) in the Mediterranean silverfish Sceletolepisma guadianicum. Individuals were acclimated for 6 day at either 25°C (n = 32) or 35°C (n = 29) and gradually heated (∼0.5°C min-1), with VTmax, CTmax, and lethal limits recorded. S. guadianicum exhibited some of the highest thermal limits reported among terrestrial arthropods: VTmax averaged 50.92°C, CTmax 54.45°C, and UTL 55.85°C, suggesting that extreme thermal resistance among terrestrial arthropods may be more widespread across diverse ecosystems than previously recognized. Thermal limits were consistent between sexes, while VTmax showed slight (1.04°C) but significant plasticity, increasing with higher acclimation temperature and heating rate, whereas CTmax and UTL remained unchanged. This decoupling of behavioral and physiological thermal limits indicates that extreme heat tolerance in terrestrial ectotherms relies on constitutive physiology, with behavioral avoidance as the main flexible buffer. Our findings underscore the importance of integrating physiological and behavioral metrics when assessing animals' heat tolerance and we provide hypotheses explaining the co-occurrence of exceptional heat tolerance with limited plasticity in this and other highly heat-tolerant species.