Fig.3. Effect of selection treatment and time on normal food on urea loss. After rearing on 300mmoll^-1 urea food, larvae were switched to normal food or urea food. The zero time point represents larvae switched to urea food as a control for the effect of handling. Hemolymph urea concentrations decrease with increasing time on normal food. The rate of urea loss is significantly greater in the controls than in the selected larvae (10.4mmoll^-1h^-1 versus 7.0mmoll^-1h^-1, respectively; values are means ± 1 S.D., N=5, ANCOVA, P<0.001). Note that under a diffusion model, the higher rate of urea loss in the controls can be explained by their higher urea concentration gradient (see text).