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

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Abstract
Thoracic movements and changes of potential in the thoracic muscles were recorded simultaneously during stationary flight in a number of insects. Periplaneta and a moth, Agrotis. had a wingbeat frequency of 20-40/sec. Movements and potentials were completely synchronized and underwent little change in frequency when the wings were amputated. Several spp. of Diptera and a wasp, Vespa. had a wingbeat frequency exceeding 100/sec. In these insects potentials showed no synchrony with wing movements, appearing once in every 5-20 wing beats. Amputation of the wings caused either no change or a decrease in the frequency of the potentials, while the wingbeat frequency increased nearly 100%. The 2 groups of insects differed also in the fact that in the roach and moth the muscles retained normal excitability to indirect stimulation after exposure and dissection, while in the others the flight muscles became completely inexcitable after hemisection of the thorax. It is concluded that in the flies and wasp the flight muscles depend upon the resting tension for a certain level of excitability. At the onset of flight tension excitability is augmented by the accumulation of a neural factor developed by the arrival of a sequence of motor nerve impulses. When this combined effect reaches the threshold of one set of flight muscles, they contract, and by increasing the tension in the antagonists, bring them in turn to threshold. Flight continues as a myogenic oscillation at a natural frequency determined by the physical characteristics of the system, provided nerve impulses maintain excitability at a certain level to offset damping. In the roach and moth, on the other hand, resting tension does not appear to be a major factor, and contraction is determined solely by the arrival of a motor nerve impulse.