Temperature effects on digestion and growth rates were considered in Section 2.6. Here, we are concerned with the thermal sensitivity of muscle performance, especially in flight. Short reaction times are vital for prey capture and predator evasion. The speed of terrestrial and aerial locomotion is strongly dependent on temperature, even in caterpillars (Joos 1992). Thermal biology also has important consequences for mate acquisition tactics in ectotherms, often because of improved aerial performance (Willmer 1991a). For example, male Hybomitra arpadi (Diptera, Tabanidae; about 85 mg) hovering at mating sites in Canada maintain a mean Tth of 40oC, fuelled by highly concentrated crop contents (Smith et al. 1994). Oviposition is also governed by temperature, often via its effect on flight (Watt 1968; Toolson 1998). Stridulation of katydids employs the flight muscles (Stevens and Josephson 1977) and singing of cicadas uses tymbal muscles (Josephson and Young 1979): both are high-performance muscle systems and strongly temperature-dependent.
Thermoregulation physiologists are naturally most interested in the thorax of insects, which is packed with muscle and the warmest region of the body (Schmaranzer 2000). Flight requires a substantial investment in flight muscle—up to 60 per cent of body mass in dragonflies (Marden 1989), and a minimum of 12-16 per cent—which is expensive to build and maintain, and can vary considerably in size within individuals and within species, with resulting effects on flight performance (reviewed by Marden 2000). Flight polymorphisms, in which female insects invest in either flight muscle or fecundity, have already been discussed in Chapter 2, and trade-offs between mobility and egg mass are important constraints in butterfly design (Marden and Chai 1991).
Coleoptera, Diptera, and Hymenoptera (three of the four largest insect orders, therefore, the majority of insect fliers) have asynchronous flight muscle in which multiple contractions originate from a single nervous impulse. This muscle type, also known as myogenic or fibrillar, is stretch-activated and operates at high contraction frequency by mechanical deformation of the metathorax ( Josephson et al. 2000a). The frequency at which asynchronous flight muscle operates most efficiently is determined by temperature and the resonant frequency of the wing-thorax system. Asynchronous muscles of both beetles and bumblebees have been studied using work-loop techniques (see Josephson et al. 2000a), but the basalar muscles of the cetoniine beetle Cotinus mutabilis (Scarabaeidae), which depress the hindwing and comprise one-third of its flight muscle mass, make a better preparation (Josephson et al. 2000b). Mechanical power output in flight muscle is the product of the work per cycle and the cycle frequency. The optimal cycle frequency of the beetle basalar muscle and the power output at that frequency increase with increasing muscle temperature (Fig. 6.3). Average measurements for mechanical power output at the wingbeat frequency (94 Hz) and temperature (35oC) of free flight were 127 Wkg_ 1 muscle (Josephson et al. 2000b). This value is about twice that measured for synchronous dorsoventral muscle of the sphingid moth Manduca sexta under similar conditions tg
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