Elytral thickness and hidden species-specific traits shape passive thermal responses in beetles.

Body temperature strongly constrains insect activity, performance and survival, yet the mechanisms governing heat gain under solar radiation remain incompletely understood. In ectothermic insects, passive thermal traits may modulate body heating independently of energetically costly behavioral or physiological thermoregulation. Passive thermal responses were experimentally quantified in 147 freshly dead beetle specimens belonging to 34 Coleoptera species exposed to simulated solar radiation. Three complementary descriptors of body warming were measured - initial heating rate (IHR), final heating rate (FHR) and time required to reach 40°C (T40) - capturing distinct phases of passive heat acquisition. Elytral thickness emerged as the strongest morphological predictor of heating dynamics, showing a consistent negative effect on both IHR and FHR and a positive effect on T40. Body size influenced only the initial heating phase and had no effect on thermal resistance during prolonged exposure, indicating a partial decoupling between body size and passive thermal performance, whereas elytral darkness appeared to have only a slight influence on FHR. Substantial interspecific variation persisted after controlling for morphology and air temperature, revealing species-specific passive thermal strategies likely driven by unmeasured structural or compositional properties of the exoskeleton. These findings identify elytral thickness as a key determinant of passive thermal resistance in beetles while demonstrating that passive heating responses cannot be explained by body size alone. The persistence of species-specific differences suggests that additional, currently unknown exoskeletal traits contribute to thermal performance, highlighting passive thermal architecture as an underappreciated axis of ecological differentiation and thermal adaptation in Coleoptera.