The physiology of insect fibrillar muscle - II Mechanical properties of a beetle flight muscle

Abstract When an insect fibrillar muscle is stimulated, it may under certain circumstances perform a rhythmical series of contractions at a frequency different from that of the stimuli. This phenomenon has been studied in the basalar muscle of lamellicorn beetles, using specially designed electrical apparatus. The resting muscle is exceedingly stiff. When stimulated, the isometric tension increase is relatively small; active tension/length curves taken under isometric and isotonic conditions are not significantly different. When an inertial load is applied to the muscle, stimulation gives rise to an oscillatory contraction at the mechanical resonant frequency of the system. Although the oscillation frequency is independent of the stimulus frequency, the amplitude of oscillation decreases if the stimulus frequency is reduced below a certain value; there is also a reduction in mean tension. Evidence exists, however, that contraction and oscillatory behaviour are not always so closely associated; different processes may be involved in these two manifestations of muscle activity. Power can be extracted from the muscle by viscous damping of the load, there being an optimum value of damping at which maximum power is obtained. The power output is also dependent on mean muscle tension, reaching a maximum at about the tension which exists in the intact insect. The mechanical properties of the beetle basalar muscle can be correlated with its role in the flight of the insect.