MOVEMENTS OF THE THORAX AND POTENTIAL CHANGES IN THE THORACIC MUSCLES OF INSECTS DURING FLIGHT

¹ Department of Biology, Tufts College, Medford, Massachusetts

1. Simultaneous records were made of thoracic movements and potential changes within the thorax of a number of insects during stationary flight.

2. The American roach and a moth with a wingbeat frequency in the range 20-40 per second showed complete synchrony between spikes and movements, and little change or a decrease in the frequency of both phenomena when the wings were amputated.

3. Several flies and a wasp with a wingbeat frequency above 100 per second showed no synchrony between spikes and movements, the former appearing once in every 5-20 wingbeats. Amputation caused either little change or a decrease in spike frequency, while the wingbeat frequency increased by nearly 100 per cent.

4. While the muscles of the former group retain their excitability to indirect electrical stimulation after dissection, the flight muscles of the latter are inexcitable following hemisection of the thorax. However, if the thorax is intact the indirect flight muscles of flies can be driven at the natural flight frequency by any stimulus frequency above 3 per second.

5. It is concluded that in the flies and wasp, the level of resting tension determines a degree of excitability in the flight muscles. This excitability is augmented by the accumulation of a neural factor brought about by the arrival of motor nerve impulses (spikes). Upon threshold being reached in one set of indirect flight muscles, they contract, and by increasing the tension factor in the antagonists, bring them rapidly to threshold. Flight continues as a myogenic oscillation at a natural frequency determined by muscle tension, elasticity of thorax, and wing load, provided the neural factor is maintained at a certain level to offset damping.

6. In the roach and moth, the resting tension does not appear to be sufficient to give the system resonance or to contribute greatly to the muscle excitability. Therefore, the arrival of a motor impulse is the determining factor in contraction of the flight muscles.

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