The Activity of A Steering Muscle in Flying Locusts

Abstract

ABSTRACT The pleuroaxillary muscle of a forewing (M85) or hindwing (M114) in the locust is supplied by two motor neurones. Each of the two motor neurones innervates a different part of the muscle. Single impulses in these motor neurones produce small twitches in the muscle which tetanize at about 30 Hz. At the wingbeat frequency they show considerable tonic tension upon which ripples are superimposed, 1:1 for each stimulus pulse. During sustained, straight, tethered flight, the motor neurones spike rhythmically, producing one (less often two) spike(s) per wingbeat in the first half of each downstroke. At the end of flight, when the wing is folded, a high-frequency, unpatterned burst of spikes occurs. During flight-like motor activity where rhythmic sensory feedback is reduced, the pleuroaxillary muscle of a hindwing spikes throughout the ‘wingbeat’ cycle, with little sign of rhythm. The forewing muscle, M85, responds to imposed rolling during flight by advancing the timing of its spike, increasing the number of spikes at each wingbeat, and recruiting a second motor unit on the side which is rotated downwards; converse changes occur on the side that is rolled upwards. The magnitude of the time-shift response in M85 depends on the angular position of the locust about the roll axis. The hindwing muscle shows similar changes in the number of spikes and in recruitment. Motor neurones of both the forewing and hindwing muscles can spike in response to imposed rolling in locusts that are not flying. Excitation increases on the side that is rolled down. The response to angular movement about the roll axis is primarily phasic and is dependent on visual cues. It is concluded that these muscles take part in steering behaviour during corrective reactions. Activation is increased on the side where more lift must be produced. Similar changes of activity in these muscles may play a role in active steering manoeuvres.