The effect of forewing depressor activity on wing movement during locust flight

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Locusts are passively yawed in the laminar air current of a wind tunnel (Fig. 1). In order to study the influence of depressor muscles of the forewing on its movement, electromyography is combined with true 3-dimensional inductive forewing movement recording. In quick response to the yaw stimulus, many kinematic parameters (e.g. shape of the wing tip path, amplitudes of wingstroke, ratios of downstroke to upstroke duration, time interval between beginning of downstroke and time of maximum pronation etc.) vary differently in both forewings (Figs. 3---5). Pronation changes in correlation to yawing reciprocally on both forewings with comparable differences of pronation angles (Fig. 5a). Maximum pronation is decreased on that side, to which the animal is-passively-yawed, whereas the slope of the wing tip paths remains almost constant. Therefore, decreasing pronation most probably indicates increasing thrust. The animal appears to perform a disturbance avoidance behaviour. Although the burst length of muscle firing is almost constant here, the onset of 8 depressor muscles (1 st basalar and subalar muscles of all 4 wings) varies in correlation to the stimulus (Figs. 6---8). The changing time intervals between the 1 st basalar muscle M97 and subalar muscle M99 are responsible for the alterations of forewing downstroke. Quantitative analysis of combined motor and movement pattern (Fig. 9) shows the following: (i) the maximum pronation and time interval between the onset of 1 st basalar muscle M97 as well as subalar muscle M99 and the beginning of downstroke are positively correlated (Figs. 10 and 12a and b). (ii) Maximum pronation is greatest, when muscles M97 and M99 act simultaneously (Fig. 12c). Thus, both muscles work synergistically, concerning pronation. Muscle M99 is of less importance than muscle M97. On failing activity of the depressor muscle M97, downstroke is greatly reduced. Some depressor as well as elevator muscles are switched on and off separately on each side (Fig. 11).