The motor output during turning flight in a hawkmoth, Manduca sexta

Abstract

Electrical activity was recorded extracellularly from mesothoracic flight muscles and the position of the wings was observed during attempted turning by Manduca sexta in fixed flight. The results show that changes in the phase relationships among the motor neurons to the flight muscles are an important part of the neural code by which this hawkmoth controls its flight direction. The third acillary muscle, previously presumed to function in folding the wing at the end of flight, was phasically active during flight. During turns toward the side from which the recordings were made, the wing was remoted, and the third axillary muscle was excited by one to several impulses just before the main depressor muscle (dorsal longitudinal, DLm). During a turn in the opposite direction, the third axillary muscle fired shortly before or with the elevator muscles. The subalar muscle was excited shortly before the DLm during remotion; during promotion it was excited in phase with the DLm or with both the DLm and the elevator muscles. Large changes in phase relationships also occurred in the basalar muscle. In the third axillary and basalar muscles, transition from one phase relationship to another usually involved firing at twice the wingbeat frequency. This observation suggests that the motor neurons to the direct muscles can be loosely coupled to the neurons supplying either the DLm or the elevator muscles. Phase changes during flight also occurred in decapitated animals stimulated with a touch on one side of the abdomen. Therefore alteration of phase relationships and production of motor patterns which result in a turn do not require signals from the brain.