Figure 6.4 Flight muscle physiology of moths: a comparison of ectothermic Operophtera bruceata (Geometridae) and endothermic Manduca sexta (Sphingidae). (a) Maximum shortening velocity Vmax, (b) tetanic tension, and (c) instantaneous power output are plotted as a function of flight muscle temperature.
Source: Marden (1995a).
area relative to body mass, as in the conversion of one pair of wings either to elytra or to halteres (Dudley 2000). Multiple independent origins of asynchronous flight muscle are most evident in the major endopterygote orders of Coleoptera, Diptera, and Hymenoptera, which all show strong evolutionary trends towards miniaturization (Dudley 2000). Josephson (1981) stressed that most insect behaviour involves muscles operating more or less at ambient temperature (due to small size). Wingbeat frequency is typically inversely related to body size, varying from 5 Hz in large butterflies to 1000 Hz in minute ceratopogonid flies, with enormous variation between species of the same body mass but different morphology (Dudley
2000). Temperature sensitivity was evident in only some of the beetle species studied by Oertli (1989) and was attributed to limited changes in the resonant properties of the flight system. Power output is varied by changing both the frequency and amplitude of muscle contraction, but it was long assumed that insects do not modify heat production by varying wingbeat frequency (e.g. Joos et al. 1991). Recently, however, several insects have been shown to decrease wingbeat frequency and metabolic heat production (MHP) at high Ta (Section 6.5.2).
Muscle physiology of dragonflies Dragonflies are superb flying machines, but adults emerge with immature flight muscles which increase 2.5-fold in mass during maturation, resulting in equally dramatic changes in aerial competitive ability. Marden (1989) attached small weight belts to territorial male dragonflies, decreasing the ratio of flight muscle mass to total mass, and measured declines in their mating success. Vertical force production during brief flight attempts provides a measure of changes in flight performance with ontogeny (Marden 1995b). The optimal thoracic temperature, corresponding to peak performance, increases from 35 to 44°C during maturation in Libellula pulchella (Libellulidae), while the thermal sensitivity curve narrows (i.e. the thermal breadth decreases) (Fig. 6.5). Field data show an increase in time spent flying from 2-32 per cent, and the Tth of 29-40°C measured during the flight of tenerals increases to 38-44°C during routine territorial patrolling flight and to 40-45° C during high-speed chases (Marden et al. 1996). Age, Ta, and level of exertion all have significant effects on Tth (Fig. 6.6). The weak flight and relatively sedentary existence of tenerals contribute to their lower Tth, but also conserve energy. Horizontal bars in Fig. 6.5 show the close correspondence between field-active Tth and laboratory-measured optimal temperature in each age class; for mature dragonflies the match improves if field Tth for high-speed flights is used. Maturational changes are also evident at the ultrastructural level as muscle cells increase in diameter and the fractional cross-sectional area of mitochondria increases threefold (Marden 1989). This work on the ontogeny
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