1. The sounds produced during courtship in Drosophila melanogaster are similar to a component of the flight tone.
2. The normal flight wing beat has a rapid upstroke and a slower downstroke. By analysis of the form of the wing beat and its velocity it is shown that the sound that is heard from behind the fly is produced by a rapid increase of thrust in the early part of the upstroke followed by decrease in thrust, or even thrust reversal, during the period which the wing is rotating at the top of the stroke. The downstroke is relatively quiet.
3. The courtship wing vibration consists of a series of single wing beats of about 7.5 msec, duration. Courtship sounds consist of single cycles of pressure change of 3 msec, duration produced at 34 msec, intervals at 25° C. These figures are compatible with the suggestion that courtship sound pulses are produced by one half of an asymmetric wing beat.
4. Drosophila persimilis produces short trains of pure notes of 1.9 msec, period. Here, the wing vibrates at about 250 beats per second and the wing beat is symmetrical.
5. The differences between these two species is discussed in terms of inertial loading of the thorax and the tension in the muscle activating the click mechanism.
6. The courtship wing beat occurs when the extra-coxal depressor of the trochanter of either the mesothoracic or the metathoracic legs is cut. This suggests that this muscle is not responsible for the initiation of flight, but instead, acts as a transmitter of thoracic movement causing the starting jump.
7. Cutting one of the dorso-ventral indirect flight muscles stops courtship vibration, the initiation of flight and jumping, and it is suggested that this muscle is more likely to be involved in flight-starting and courtship vibration.