Body temperature and flight performance of honey bees in a servo-mechanically controlled wind tunnel

Abstract

1. Workers ofApis mellifera ligustica were tethered to an aerodynamic balance in front of a laminar wind tunnel. They had control over the tunnels wind velocity. Lift, wind speed, wingbeat frequency, thorax and air temperature, and the action potentials in the fibrillar dorsoventral (DV) and dorsolongitudinal (DL) flight muscles were continuously recorded. The air temperature in the tunnel could be controlled. Correlations between all mentioned parameters and the thorax temperature were analyzed. 2. Animals that assumed flight position and flew for more than thirty seconds disregarded optical inputs. They did not respond to changes in speed or direction of an optical pattern, and flew even in complete darkness. 3. In flights that started with thorax temperatures at environmental values, temperature differences between thorax and environment reached 5.2 °C after 4. 5 min of flight (Fig. 1). In animals with body temperatures above 36 °C, active temperature regulation could be observed: A large drop of fluid appeared between the ventral side of the head and the thorax, and the front legs frequently rubbed the fluid over the ventral part of the thorax. 4. The average wingbeat frequency showed a linear increase from 136 Hz at a thorax temperature of 24 °C, to 194 Hz at thorax temperature of 33.5 °C. A further increase of the thorax temperature from 33.5 °C to 38 °C was correlated to an increase of the wingbeat frequency to 202 Hz. This increase had a much smaller slope (Fig. 2). 5. The average lift increased from 28.5 dynes at 24 °C to 78.5 dynes at 33 °C (body angle 0°). The lift stayed practically constant from 33 °C to 38 °C (Fig. 2). The lift at thorax temperatures between 33 °C and 38 °C would keep the empty body weight aloft. The flight velocity increased from 0.8 m/sec, thorax temperature at 24 °C, to 3.3 m/sec, thorax temperature at 38 °C (Fig. 2). 6. The average action potential frequencies (APF) in the fibrillar flight muscles rose from between 8 and 9 Hz at thorax temperatures of 24 °C to between 12 and 13 Hz at thorax temperatures of 38 °C. In the range of biggest change in lift and thrust, at thorax temperatures between 26 °C and 31 °C, the APF in the dorsoventral muscles stayed constant, while it rose only one Hz in the dorsolongitudinal muscles (Fig. 2). 7. The increase in lift showed a linear relationship to the increase in wingbeat frequencies between 122 Hz and 158 Hz. Between 158 Hz and 208 Hz the gain in lift was bigger. From 208 Hz on the lift stayed constant. The flight velocity rose linearily with wingbeat frequencies from 122 Hz to 208 Hz (with some irregularities near 140 Hz) and with a steeper increase at wingbeat frequencies over 208 Hz (Fig. 3). 8. It was concluded that the large increase in the power output of the flight motor that occurred with rising thorax temperatures was mostly caused by an augmentation of the efficiency of the flight system and not by an increase in neural activation.