An experimental component analysis approach is used to define the physiological interactions of carbohydrate nutrition and the manner in which it regulates the crop-emptying rate. The basic components of the crop-emptying rate are frequency of valve opening and the volume of the slug of fluid which passes through the valve. This volume is solely a function of crop volume, but the frequency of opening is a complex function determined by the crop volume, and by gut activity and blood osmotic pressure, both of which are related to nutritional parameters. To guide the formulation of the relationship, a model of the mechanism of valve function is proposed, based in the phenomenon of muscle creep.
The osmotic pressure is estimated by calculating the change in blood trehalose level estimated from observed rates of disappearance of sugar from the crop. Sugar emptying from the crop diffuses from the midgut to the blood, where it is partitioned between trehalose and glycogen according to established biochemical relationships. The model developed fits a wide variety of published data on crop-emptying rates of flies fed different volumes and concentrations of sugar, and may be applied to active, inactive, and flying insects.