1. Electrical activity from the indirect, myogenic muscles of calliphorid flies was recorded during flight. The animals were suspended from an aerodynamic balance in the laminar air-stream from a wind-tunnel. Muscle action potentials, recorded with 25µ wire, were 5-7 msec. in duration, up to 10mV. in amplitude and positive in sign. Frequencies were mostly under 20/sec.

2. Frequencies in all the indirect muscles were similar, but these varied together with changes in aerodynamic power.

3. Frequencies in the indirect muscles of the two sides varied by no more than ± 10% during extreme turns to right or left (only left or only right wing beating).

4. Electrical records from the non-myogenic direct muscles were made during tethered flight. The potentials were 2-4 msec. in duration, up to 2 mV. positive and had frequencies up to 180/sec.

5. A nearly linear positive correlation exists between impulse frequency in the musculus latus (pleurosternal muscle), the inward movement of the pleural wall, and the wingbeat frequency, suggesting that this muscle is the basic frequency determiner.

6. Strong turning behaviour is associated with opposed frequency changes in the pairs of antagonistic adductor and abductor muscles of the wings on the two sides of the body.

7. The musculus dorsoventralis IV (tergo-trochanteral) is activated by a short impulse burst at the beginning of flight. It apparently acts as an oscillation starter.

8. Flight initiation normally requires 30-60 msec. Usually activity begins in the musculus latus, which stiffens the thorax. Then simultaneously the myogenic muscles are activated and the ‘starter’ muscle causes a jump and the beginning of oscillation of the thorax. Then the wings are drawn gradually forward and full wingbeat amplitude develops within the first six wingbeats. Flight begins with maximal lift and wingbeat frequency and a nearly synchronous burst discharge in all the indirect muscles.

9. Power production and the transmission and distribution of power are under separate control. The myogenic indirect motor varies only in total power output, this being influenced by its own state of excitation and by a muscle-controlling wingbeat frequency. Steering is accomplished by non-myogenic direct muscles which are capable of differentially engaging the two wings with the motor.

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