| Summary: | Heteroptera, or true bugs populate many climate zones, coping with different environmental conditions. The aim of this study was the evaluation of their thermal limits and derived traits, as well as climatological parameters which might influence their distribution. We assessed the thermal limits (critical thermal maxima, CT<sub>max</sub>, and minima, CT<sub>min</sub>) of eight seed bug species (Lygaeidae, Pyrrhocoridae) distributed over four Köppen−Geiger climate classification types (KCC), approximately 6° of latitude, and four European countries (Austria, Italy, Croatia, Bulgaria). In test tubes, a temperature ramp was driven down to −5 °C for CT<sub>min</sub> and up to 50 °C for CT<sub>max</sub> (0.25 °C/min) until the bugs’ voluntary, coordinated movement stopped. In contrast to CT<sub>min</sub>, CT<sub>max</sub> depended significantly on KCC, species, and body mass. CT<sub>max</sub> showed high correlation with bioclimatic parameters such as annual mean temperature and mean maximum temperature of warmest month (BIO5), as well as three parameters representing temperature variability. CT<sub>min</sub> correlated with mean annual temperature, mean minimum temperature of coldest month (BIO6), and two parameters representing variability. Although the derived trait cold tolerance (TC = BIO6 − CT<sub>min</sub>) depended on several bioclimatic variables, heat tolerance (TH = CT<sub>max</sub> − BIO5) showed no correlation. Seed bugs seem to have potential for further range shifts in the face of global warming.
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