The direct flight muscles of Drosophila melanogaster are innervated by the anterior dorsal mesothoracic (ADM) nerve and the mesothoracic accessory (MAC) nerve. Each of the four conspicuously large axons in the ADM nerve serves one of the muscles designated pal, pa3, pa4 and pa5. Muscle pa4 is additionally innervated by a very small neurosecretory axon. Muscle pa6, also innervated by the ADM nerve, receives at least one small nerve fibre but no large axon. Muscle pa2 is innervated by a large axon from the MAC nerve. Large motor axons, identified by serial section tracing from their respective muscles, are consistent among different individuals in both relative positions and relative diameters within the ADM nerve.
King, David G. and Tanouye, Mark A. "Anatomy of Motor Axons to Direct Flight Muscles in Drosophila." (Jul 1983).