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Figure 3.16 Schematic lateral view of the thorax of Manduca sexta (Lepidoptera, Sphingidae) during flight showing the deformation of the air sacs and the direction of airflow during the downstroke and upstroke.

Note: Thoracic segments I-III are labelled as are spiracles one (Sp I) and two (Sp II). Source: Redrawn from Wasserthal (2001).

Figure 3.16 Schematic lateral view of the thorax of Manduca sexta (Lepidoptera, Sphingidae) during flight showing the deformation of the air sacs and the direction of airflow during the downstroke and upstroke.

Note: Thoracic segments I-III are labelled as are spiracles one (Sp I) and two (Sp II). Source: Redrawn from Wasserthal (2001).

hypoxia it is clear that abdominal pumping rate is elevated to meet the demands associated maintenance of a constant metabolic rate under hypoxia (Joos et al. 1997).

Power production, like many features of muscle physiology, is temperature dependent, and tends to be greatest at the temperature characteristic of free flight of an insect (Dudley 2000). In many small insects this temperature is reached either very rapidly or is equivalent to that of ambient air temperature, while in larger species several minutes of preflight warm-up are required before flight can take place (Chapter 6). From an energetic perspective, flight at or marginally above ambient temperatures, such as that found in small species is least expensive. In general, metabolic rate varies positively with body size such that the smallest species have the lowest metabolic rates (e.g. Bartholomew and Casey 1978) (Fig. 3.17). However, there is considerable variation about this relationship, the slope of which also appears to vary between taxa (from c. 0.80 to 1.06) (Harrison and Roberts 2000). Wingstroke frequency and wing loading are also of considerable importance, explaining much of the variation in metabolic rate both within and between higher taxa (Casey et al. 1985; Casey 1989; Harrison and Roberts 2000). In general, metabolic costs of flight are lowest in species with low wingstroke frequencies and low wing loading, and lower cost may be traded off against reduced performance. For example, the gypsy moth is a large species (0.1 g) with a large wing area (5.39 cm2), and low wingstroke frequency. By contrast, the tent caterpillar moth is only marginally lighter (0.09 g), but has a much smaller wing surface area (2.36 cm2). In consequence, the latter species has both a higher wingstroke frequency (27 s—1 versus 58 s_1) and a higher metabolic rate (22.6 versus 60.0 mW) (Casey 1981). In the winter-flying geometrid moth, Operophtera bruceata, induced power output required for flight of an 11.7 mg male is approximately 27.9 mW, whereas in Manduca sexta males weighing approximately 1473 mg, induced power output required for flight is in the region of 12960 mW. Coupled with a low wing loading, and compensation of muscle performance for low temperatures, this low induced power output enables the geometrid males to remain active at temperatures between —3 and 25°C, whereas preflight warm-up, and consequently considerable energy expenditure, is required for flight at thoracic temperatures exceeding 30°C in M. sexta (Marden 1995a).

In endothermic species (or more correctly heterothermic species given that at least some part of the day is spent at ambient temperatures), both preflight warm-up and flight are energetically demanding. Endothermic warm-up results in an

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