Forewing movements and intracellular motoneurone stimulation in tethered flying locusts

A new optoelectronic method was used for the measurement of wing movements in tethered flying locusts. The method is based on laser light coupled into a highly flexible optical fibre fastened to a forewing. A dual-axis position-sensing photodiode, aligned to the wing hinge, revealed the flapping, i.e. up-down movement, and lagging, i.e. forward-backward movement, of the wingtip as indicated by the emitted light. Measurements were combined with electromyographic recordings from flight muscles and with intracellular recording and stimulation of flight motoneurones. Compared with muscle recordings, intracellular recordings showed an increase in the variability of motoneurone activity. Stimulation of flight motoneurones reliably caused distinct effects on wing movements. Inhibition of elevator (MN83, MN89) activity led to a decrease in the amplitude of the upstroke. Inhibition of depressor (MN97) activity reduced the amplitude of the downstroke and sometimes stopped flight behaviour. An increase in MN97 activity caused a reduction in the extent of the upward movement and prolonged the flight cycle.