Body size is a major factor affecting efficiency of energy use. Larger organisms have greater energy requirements than do smaller organisms. However, smaller organisms with larger surface area-to-volume ratios are more vulnerable to heat loss than are larger organisms. Accordingly, maintenance energy expenditure per unit body mass decreases with increasing body size (Phillipson 1981). In addition, larger organisms tend to use energy more efficiently during movement and resource acquisition, have a competitive advantage in cases of direct aggression, and have greater immunity from predators (Ernsting and van der Werf 1988, Heinrich 1979, Phillipson 1981, Streams 1994), reducing relative energy expenditures for these activities.
Physiological condition, including the general vigor of the insect as affected by parasites, also influences food requirements and assimilation efficiency. For example, hunger may induce increased effort to gain resources that would be ignored by less desperate individuals (Ernsting and van der Werf 1988, Holling 1965, Iwasaki 1990,1991, Richter 1990, Streams 1994). Slansky (1978) reported that cabbage white butterfly larvae parasitized by Apanteles glomeratus (Hymenoptera) increased food consumption, growth rate, and nitrogen assimilation efficiency. Schowalter and Crossley (1982) found that Madagascar hissing cockroaches, Gromphadorhina portentosa, with associated mites, Gromphadorholaelaps schaeferi, had a significantly greater egestion rate than did cockroaches with mites excluded, although assimilation efficiency did not differ significantly between mite-infested and mite-free cockroaches (Fig. 4.13).
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