Predictions from an insect-herbivore digestion model (Yang 1993) were empirically evaluated in this study using the nondiapausing strain of a generalist-feeding grasshopper, Melanoplus differentialis (Thomas) (Orthoptera: Acrididae). Food residence time (FRT) within the digestive tract was manipulated by M. differentialis as a means of digestive compensation. Food residence time decreased linearly when diet was diluted from 5% total nitrogen (N) to 1% N. Food residence time was 16% shorter in sixth-instar nymphs than in adults and was reduced 24% when ambient temperature increased from 30 degrees to 35 degrees C. Approximate digestibility (AD) decreased linearly when diet was diluted from 5% N to 1% N. Adults digested food 43% move efficiently than sixth-instar nymphs. Approximate digestibility was positively related to FRT. Digestibility rate (AD per unit of FRT) was not affected by body size, sex, or temperature but increased linearly with diet quality. Diet dilution did not induce gut size changes when grasshoppers were confined on poor-quality,food for only 3 d. However, the allocation of total dry matter to digestive tissue varied greatly between developmental stages as well as between sexes. The proportion of gut dry weight to body dry weight (G/B) was 30% higher in smaller-bodied sixth-instar nymphs than in larger adults and was 6% higher in females than in males. Wet weight gain increased with diet quality. Sixth-instar nymphs gained more weight per gram of body weight than adults at each level of N content. But the nymphs were affected more by poor food quality than adults. Temperature had no effect on weight gain at either 3% or 5% N levels. But low food quality (1% N) combined with high temperature resulted in greatly reduced weight gain, Correct interpretation of differences in RD requires knowledge of the underlying whole-organism digestive responses. The relative stability of digestibility rate exhibited by M. differentialis in response to changes in food quality, developmental stage, and temperature suggest that, within certain physiological limits, insects may regulate digestive means to maintain an optimal or near-optimal digestibility rate. Digestibility rate, indicating the amount of limiting dietary component extracted per unit of time, should be a good index of an animal's digestive capability.
