Non-shivering thermogenesis in asynchronous flight muscles of bumblebees? Comparative studies on males of Bombus terrestris, Xylocopa sulcatipes and Acherontia atropos

Abstract

1. 1. Time course of thorax surface temperature elevation and of muscle action potentials in the dorsoventral flight muscles of Bombus terrestris, Xylocopa sulcatipes and Acherontia atropos were recorded. For continuous long-time measurement the spike activity was integrated (discharge 100-37% was 3.5 sec) before being recorded by a personal computer. 2. 2. In all three insect species “warming-ups” were always coupled with muscle action potentials. While in the moth the wings clearly vibrate during these phases, no movement of thorax or wings could be observed in both Hymenoptera by direct visual observation. 3. 3. In addition, in all three insects individual action potentials and thorax vibrations were measured during such heating periods in parallel. In hawkmoths spike frequencies diverted were between 18 and 25 Hz, while the thorax vibrated synchronously. Similar spike frequencies were also found in carpenter bee night muscles during heating and, unexpectedly, a nearly synchronous mechanical response of the thorax was recorded. The average amplitudes of these movements were 15μm and in maximum they were 25 μm. 4. 4. In contrast, no movements of the thorax were detectable in bumblebees by the vibration monitoring system (disintegration limit: 0.1 μm) during intensive “warming-ups”, although action potentials with up to 20 Hz appeared in the flight muscles. 5. 5. That means, no stretch activation of Bombus asynchronous flight muscles occur and consequently the actomyosin-ATPase of these muscles is not activated during heating, excluding that heat generation by a shivering mechanism taking place.