The motor pattern of locusts during visually induced rolling in long-term flight

Desert locusts (Schistocerca gregaria F.), mounted in a wind tunnel on a low-mechanical-impedance torque meter, flew for at least 30 min in the posture typical of long-term flight. As they flew, they were induced to rotate about their long axis (roll) by rotation of an artificial horizon. All maintained departures from the horizontal attitude were brought about actively, by the animal's own efforts. In the roll maneuver, the hindlegs and abdomen were bent toward the side ipsilateral to the direction of rotation. However, these rudderlike movements were not adequate to initiate and maintain a constant roll angle.During a roll, there was a change in the pattern of excitation of all the wing muscles that were monitored: the depressorsM81, 97, 99, 112, 127, and 129, and the elevatorsM83, 84, 89, 113, 118, 119 (numbering according to Snodgrass 1929). Hence all 12 muscles probably not only provide power for the flight but also steer it. Evidently, then, for these muscles a rigid distinction between power and steering muscles is not appropriate.The period of the contraction cycle changed in correlation with the roll angle, but was not a parameter for control of the roll maneuver, because the changes were the same in all muscles (Fig. 2).Even with constant burst length, the phase shifts between the muscles changed. These changes were the main control parameter for rolling (Figs. 3–9).There was a latency coupling between elevators and the following depressors (Fig. 3).The changes in phase shift were tonic or phasic (sometimes phasic-tonic) in different muscle pairs (Fig. 4).When a roll angle of ca. 15° was adopted, the phase shifts between depressor muscles in a given fore- or hindwing (e.g.,M127R vs.M129R) changed by about 5 ms, whereas the elevators changed by less than 1 ms (Fig. 6).The phase shifts between the anterior elevators and depressors of a given wing, as well as the posterior elevators and depressors, changed by ca. 5 ms (in some cases with different time courses) when the animal rolled to an angle of ca. 15° (Fig. 7).The changes in phase shift between muscles of the fore-and hindwing on one side of the body amounted, as a rule, to about 4 ms at ca. 15° roll (Fig. 8).Corresponding muscles on the two sides of the body change in phase with respect to one another by as much as 10 ms (Fig. 9). The phase shifts of all such contralateral muscle pairs except for the posterior basalar muscles,M127, have the same sign, such that the muscle ipsilateral to the direction of rotation becomes active sooner.