Contraction Dynamics of Flight and Stridulatory Muscles of Tettigoniid Insects

Abstract

Isometric contraction kinetics and force-velocity relations were examined in wing muscles of two tettigoniid insects, Neoconocephalus robustus and N. triops. The muscles were first tergocoxal muscles of the mesothoracic and metathoracic segments. The metathoracic muscle is a flight muscle. The mesothoracic muscle is used in flight and in stridulation. In the field, the wing stroke frequency during stridulation by N. triops is about 100 Hz ; the thoracic temperature during singing is about 30 C ; and the temperature gradient between the thorax and surround is about 15 C. Published data for N. robustus give the wing-stroke frequency during stridulation as about 200 Hz at a thoracic temperature of 35–40 C. The wing-stroke frequency during flight by both species is approximately 20 Hz at 25 C. The twitch time course is similar in equivalent muscles of the two species. At 35 C the twitch duration (onset to 50 % relaxation) is 5·5–6·5 ms for mesothoracic muscles and 11–13 ms for metathoracic ones. Twitch and tetanic tension per unit area are about twice as great in the metathoracic muscles as in the faster, mesothoracic ones. Despite the differences in isometric contraction kinetics, the maximum shortening velocity (Vmax) is similar in mesothoracic and metathoracic wing muscles. Vmax values (lengths per second, 35 C), determined by extrapolation of force-velocity curves, were 10·1 (mesothoracic) and 11·1 (metathoracic) for N. robustus; 12·2 (mesothoracic) and 16·1 (metathoracic) for N. triops. With N. triops, Vmax was also determined from the time taken to re-develop tension following quick release. The values obtained were somewhat higher than from extrapolation of force-velocity curves, but again similar for mesothoracic and metathoracic muscles. Twitch time course becomes more rapid and Vmax increases with increasing temperature. Neither twitch nor tetanic tension is greatly affected by temperature change in the range 25 – 35 C. As for many other fast muscles, force-velocity plots for these muscles have little curvature. It is suggested that the relative straightness of these plots is a consequence of internal viscosity.