Gas exchange and metabolic rate during activity

Insects show tremendous scope for the increase of metabolic rates above resting levels. This is due mainly to the energetic demands of flight, which can be responsible for 100-fold increases in metabolic rate. Although many insects, such as ants and honeybees, might never experience levels of metabolism as low as SMRs measured under solitary conditions (sometimes including anaesthesia), and therefore have factorial aerobic scopes more in keeping with those of vertebrates (4-10) (Harrison and Fewell 2002), large moths such as those examined by Bartholomew and Casey (1978) are often quiescent. Therefore, factorial aerobic scopes of more than 100 (as high as 170) are likely to be accurate reflections of the increases in aerobic metabolism that are possible in insects. Such high aerobic scopes are of considerable interest not only because insects often make a rapid transition from alert rest to flight, but also because flight metabolic rate might also constrain resting levels of metabolism, and in so doing affect both abundance and fecundity (Marden 1995a; Reinhold 1999).

Flight is not the only reason for elevated metabolic rates. Other forms of locomotion, including both pedestrian locomotion and swimming are responsible for increases in metabolic rate, as are calling (Lighton 1987; Reinhold 1999), sexual activity (Giesel et al. 1989; Woods and Stevenson 1996), thermoregulation (both for flight and in the absence of flight), colony homeostasis (Heinrich 1993), and feeding. For example, Megasoma elephas (Coleoptera, Scarabaeidae) responds to reduced ambient temperatures by endothermic heat production, in the absence of any form of activity (Fig. 3.15) (Morgan and Bartholomew 1982), while in another group of scarabaeids, the winter active rain beetles, males maintain elevated thoracic temperatures and metabolic rates while searching for females (Morgan 1987). In the leaf-cutting ant, Atta sexdens, leaf-cutting activity results in high metabolic rates with factorial aerobic scopes of

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