1. Despite their short life span, insect herbivores such as grasshoppers can manipulate their gut in response to changes in food quality and body size, as previously documented in mammals and birds. 2. Diet dilution greatly influenced the survival, development, weight gain and the proportional allocation to gut dry weight relative to body dry weight (G/B) of Melanoplus differentialis (Orthoptera: Acrididae). 3. When diet quality increased from 1% N (total nitrogen) to 5% N, M. differentialis survived better, developed faster and gained more weight. However, increased gut size contributed to the ability to compensate for reduced food quality so that M. differentialis was able to survive equally well and develop equally fast on 3% N diets compared with individuals fed on 5% N diets. Full compensation was not attainable on 1% N diets and grasshoppeers suffered significantly lower survival rate, slower development and less weight gain. 4. As body size increased, the relative gut size decreased both intraspecifically in M. differentialis and interspecifically among 29 species of grasshoppers representing a range of body sizes and life styles. Compared with 3-day-old sixth-instar M. differentialis nymphs, 3-day-old adults weighed twice as much but allocated 20% less to their digestive tract. Among species, gut dry weight in relation to body dry weight exhibited a significant quadratic relationship on a log-log scale. 5. Despite larger body sizes, females exhibited a larger gut per unit of body weight than was observed in smaller males. 6. A previously proposed isometric gut-body size relationship was not supported by these results.
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