Does size matter? – Thermoregulation of ‘heavyweight’ and ‘lightweight’ wasps (Vespa crabro and Vespula sp.)

In insect groups with the ability of endothermy, the thermoregulatory capacity has a direct relation to body mass. To verify this relationship in vespine wasps, we compared the thermoregulation of hornets (Vespa crabro), the largest species of wasps in Central Europe, with two smaller wasps (Vespula vulgaris and Vespula germanica) in the entire range of ambient temperature (T(a): ~0-40°C) where the insects exhibited foraging flights.Despite the great difference in body weight of Vespula (V. vulgaris: 84.1±19.0 mg, V. germanica: 74.1±9.6 mg) and Vespa (477.5±59.9 mg), they exhibited similarities in the dependence of thorax temperature on T(a) on their arrival (mean T(th) = 30-40°C) and departure (mean T(th) = 33-40°C) at the nest entrance. However, the hornets' thorax temperature was up to 2.5°C higher upon arrival and up to 3°C lower at departure. The thorax temperature excess (T(th)-T(a)) above ambient air of about 5-18°C indicates a high endothermic capacity in both hornets and wasps. Heat gain from solar radiation elevated the temperature excess by up to 1°C. Results show that hornets and wasps are able to regulate their body temperature quite well, even during flight. A comparison of flight temperature with literature reports on other vespine wasps revealed a dependence of the T(th) on the body mass in species weighing less than about 200 mg.