As is the case in other animals, metabolic rate increases during and/or shortly after feeding in
insects. This metabolic cost of food processing is known as specific dynamic action, and it has been demonstrated in several species, including lepid-opteran caterpillars (Aidley 1976; McEvoy 1984; Bennett et al. 1999), tsetse flies (Taylor 1977), ants (Lighton 1989), locusts (Gouveia et al. 2000), white-flies (Salvucci and Crafts-Brandner 2000), beetles (Duncan et al. 2002b), and bugs (Bradley et al. 2003). In general, metabolic rates increase between two and fourfold (but can be as high as 15-fold—Bradley et al. 2003), and the time course of the increase and subsequent decline in metabolic rate varies substantially between species. The increase and the decline in metabolic rate can be either rapid or prolonged (Fig. 3.21), and, as might be expected, in those species showing discontinuous gas exchange at rest, the elevation in metabolic rate necessitates a change to a more continuous form of gas exchange (Duncan et al. 2002b).
The costs of feeding are associated with a fixed cost of food processing (Taylor 1977; Willmer et al. 2000), behavioural arousal (Gouveia et al. 2000), and perhaps, in a few species, such as leafcutter ants (Roces and Lighton 1995), the costs of chewing. Failure to take the costs of digestion into account sometimes gives rise to energy budgets that will not balance (Schmidt and Reese 1986), and also has a marked influence on estimates of SMR and the scaling thereof. The latter problem is not easily resolved, because in some species of insects, such as ants (Lighton 1989) and Rhodnius prolixus assassin bugs (Bradley et al. 2003), there is active downregulation of metabolic rate in response to starvation, making the term 'post-absorptive' difficult to apply (see also Chapter 2).
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