Forewing movements and motor activity during roll manoeuvers in flying desert locusts

Abstract

Desert locusts, tethered on a roll torque meter and flying in a wind tunnel are surrounded by an artificial horizon (Fig. 1). Flight motor activity and movement of forewings are monitored continuously. Movements of the artificial horizon elicit roll manoeuvers of the animal with latencies of several seconds; concomitant changes in flight motor pattern and wing movement can be correlated with the animal's roll angle and roll torque. Flight sequences with constant torque and roll angle (steady state) have been analysed with the following results.

Wing Kinematics. A phase difference between the movements of the forewings on either side is correlated with roll angle (Fig. 3). Pronation of a forewing is always greater on the side to which the animal rolls, i.e. on the side that produces less lift (Fig. 5). In some experiments the slope of the wing tip path is also decreased (Fig. 5). In both cases, the aerodynamic angle of attack is decreased and the forewing on this side produces less lift. In most experiments, changes in pronation are less pronounced in the contralateral wing (Fig. 11). All factors contribute to a net roll torque that sustains the animal's roll angle. Other kinematic parameters of forewing movement, e.g. wing stroke amplitude, were not found to be correlated with roll angle and torque (Fig. 4).

Motor Pattern. Activity of several flight muscles (depressors M97, M98, M99, and M129; elevators M83, M84, and M90) was investigated for changes in burst length and temporal coordination in response to roll stimuli. Most flight muscles fired only once per wing beat cycle in our preparations. Thus, burst length was not found to be correlated with roll angle. Time intervals of firing between all muscle pairs investigated change in correlation with the torque and roll angle (Fig. 9).

All mesothoracic muscles are active earlier-relative to the ipsilateral metathoracic subalar muscle M129-during roll to the ipsilateral side than during roll to the contralateral side.

Correlations Between Motor and Movement Pattern. The phase of muscle firing within the wing beat cycle varies with roll angle (roll torque). The first basalar M97 and second tergosternal M84 muscles, when referenced e.g. to the upper (M97) or lower (M84) reversal point of the wing tip trajectory, are active earlier on the side the animal turns to (Fig. 10). The onset of the first basalar M97 relative to the beginning of downstroke is correlated with maximum pronation and roll angle (Fig. 11).

Mechanisms of Lift Control. Wing pronation, which is very important for lift production is controlled by the central nervous system by altering the phase of muscle activity within the wing beat cycle.