Twitch Contractions of Isolated Flight Muscle of Locusts

Summary. A new type of myograph (Fig. 1) was used to study the mechanical changes during twitch contractions of the striated, longitudinal flight muscle of the desert locust ( Schistocerca gregaria. ) The changes in length and in force could be recorded simultaneously or in quick succession. The results are interpreted by comparison mainly with striated muscle of the frog. The flight muscle is specialized in three ways: (1) the passive elastic system in parallel with the contractile system is so strong and adjusts itself so quickly to alterations in muscle length that passive elastic forces are important even at the length in the body and may become dominating when the muscle is stretched by only five or ten per cent above this length. Only a minor fraction of these passive forces are due to the sarcolemma. (2) Although the “active state” curve is of the same shape as that of the frog muscle, the duration of the activation lasts for such a short time that there is little time for shortening to develop. When the load on the contractile component was optimal for work production (0.3 P 0 ; Fig. 7B), the shortening only amounted to 3 or 4 per cent of the muscle length (11° C; Fig. 6), as compared with about 25 per cent in frog muscle at 0° C. (3) In the flying locust the wing stroke frequency (= twitch frequency) is equal to or little less than the frequency at which mechanical summation begins at the thoracic temperature for flight (30° to 42° C); since the oxygen debt is insignificant, the metabolic recovery must be complete by the end of the mechanical change. It may be added that contrary to frog muscle, the twitch force of flight muscle increased with rising temperature. If parallel elastic forces are allowed for, the mechanical properties of the contractile or “active” components are similar to those in skeletal muscle of vertebrates. This applies both to the force parameter a in H ill 's equation and to the velocity parameter b as well as to the relationship between “active” work and load. The shortening velocity reached a maximum at the beginning of the shortening; in contrast to findings in frog muscle it declined linearly with time from then on so that no phase of constant velocity exists. The series elastic element appeared to be of the same nature as in frog muscle but may be twice as stiff. The existence of a strong, passive elasticity in parallel with the contractile system, even at the length in the body, must be taken into consideration for an understanding of the mechanics of flight muscle in addition to the active state curve, the series elastic element, and the force‐velocity relation. We thank Dr. J. W. R. P ringle , F. R. S., for his comments upon the manuscript.