1. Four flight muscles, one depressor (M 99) and three elevators (M 90, M 91 and M 120) of the wings of the locust Schistocerca gregaria have been investigated using extracellular and intracellular recording techniques. The innervation and anatomy of these muscles have also been studied histologically.
2. Every fibre in each of these muscles is innervated by ‘fast’ motoneurones. M 99 contains two anatomically distinct ‘fast’ motor units. M 90 contains three ‘fast’ motor units.
3. M 91 and M 120 are innervated by at least one ‘slow’ excitatory and one inhibitory neurone as well as by a ‘fast’ excitatory neurone. Sometimes the inhibitory responses recorded from fibres of these muscles appeared as depolarizing IPSPs.
4. The roles of these muscles in the behaviour of the locust, especially during flight performance, are discussed.