tg k

25 30 35 40

Thoracic temperature (oC)

Figure 6.3 Effect of temperature on performance of the basalar muscle of Cotinus mutabilis (Scarabaeidae). (a) The effects of changing cycle frequency on mechanical power output at different temperatures. (b) The effects of temperature on the optimal frequency for power output and the power output at the optimal frequency. Both increase linearly with muscle temperature, with Q10 values of 1.4 and 1.6, respectively.

Source : Josephson et al. (2000b).

(Stevenson and Josephson 1990), suggesting an evolutionary advantage to asynchronous operation. High-frequency operation is achieved in asynchronous muscle without high rates of Ca2 + cycling and without large investment in sarcoplasmic reticulum, so more volume can be allocated to myofibrils. This leads to greater efficiency and greater power output. Mechanical properties of synchronous muscle of Schistocerca americana and asynchronous muscle of C. mutabilis were compared by Josephson et al. (2000a). The efficiency of muscle contraction can be estimated by comparing mechanical work with metabolic rates measured during flight, and values are low for both synchronous and asynchronous muscle (Dudley 2000). These low efficiencies (5-10 per cent) lead to high rates of heat production, especially in large insects.

For the synchronous muscle preparation of M. sexta, maximal mechanical power output is produced by temperatures of 35-40°C and cycle frequencies of 28-32 Hz (Stevenson and Josephson 1990). This is in excellent agreement with the Tth (35-42°C) and wing beat frequency (24-32 Hz) observed during hovering flight. It can also be calculated that muscle temperatures must be at least 30-35°C to reach the minimum power threshold for flight in this moth (Stevenson and Josephson 1990).

There are few studies of the relationship between temperature and power output in small or weak fliers, with muscles that operate at low temperatures. Marden (1995a) compared muscle performance in M. sexta with that of 12-mg males of a winter-flying ectothermic moth (Operophtera bruceata) (Geometridae), which fly over a very broad range of temperatures and can have flight muscle temperatures near zero. Thermal sensitivity of flight muscle of the two moth species (shortening velocity, tetanic tension, and the resulting instantaneous power output) is illustrated in Fig. 6.4. Muscle of O. bruceata contracts more slowly but generates more force than that of M. sexta. As a result, the maximum instantaneous power output of O. bruceata at 15-20°C is as high as that of M. sexta up to about 35°C. Note that maximum instantaneous power output obtained from such force-velocity data is about twice that achieved during normal contraction cycles (Stevenson and Josephson 1990). Muscle tension is generally less sensitive to temperature than twitch duration (Josephson 1981). In addition to possessing cold-adapted muscle, males of O. bruceata (females are wingless) have the morphological advantages of very low wing loading and a high ratio of flight muscle mass to total body mass (Marden 1995a). Geometrid moths, in general, are capable of immediate flight, regardless of Ta, and do not require preflight warm-up (Casey and Joos 1983).

The higher wingbeat frequencies associated with asynchronous flight muscle result in greater force production, and this permits reduction in wing

tg c

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