Contraction Kinetics of the Fast Muscles used in Singing by a Katydid

Abstract

1. The sound-pulse frequency, and by inference the wing-stroke frequency, in the song of the katydid, Euconocephalus nasutus, is 160 Hz. The thoracic temperature during singing is 36 °C, which represents a gradient between the insect's thorax and its environment of 12 °C. 2. Contraction kinetics were measured as a function of temperature for a singing muscle (first tergocoxal forewing muscle of males) and homologous, purely flight muscles (forewing muscles of females, hindwing muscles of males). Because the performance of the singing muscle was found to be easily degraded, the measurements were made from nearly intact animals. 3. At 35 °C the contraction rise time for an isometric twitch of a singing muscle is 7·7 msec. The twitch duration, measured from onset to 90% relaxation, is 12·2 msec. Purely flight muscles are about half as fast, but develop more tension per crosssectional area, both in twitches and tetani. The temporal characteristics of twitch contractions, including the delay between the onset of the muscle action potential and the onset of contraction, have Q10 values between 1·5 and 2. 4. At 35 °C a singing muscle contracts in an unfused tetanus when stimulated at 150 Hz, relaxing about half-way between tension peaks. This result suggests that this is the way the muscle operates during singing. At 25 °C the muscle contracts in a smooth tetanus when stimulated at 150 Hz. A warm thorax is not only a consequence of the intense activity during singing but also necessary for the performance achieved. 5. Isometric twitch tension is maximum at muscle lengths more than one-third greater than the in situ length. Twitch contraction time is nearly independent of muscle length but relaxation time increases as the muscle is stretched.