Effect of diet viscosity on the operation of the pharyngeal pump in the blood-feeding bug Rhodnius prolixus.

1. The rate of pumping and total number of pump strokes was recorded whilst Rhodnius prolixus fed on measured amounts of artificial diets. 2. Increasing the viscosity of the diet caused a decline in both the frequency with which the pharyngeal pump operates and the average stroke volume of the pump. 3. In 5th-instar Rhodnius, the stroke volume tends toward a maximum value of about 60 nl at viscosities lower than about 3 cP. 4. Feeding rates (volume per unit time) agree with Poiseuille's law at high viscosities, but are less than predicted at low viscosities. 5. The maximum power output of the pump occurs at a viscosity between 1 and 3 cP, which is probably the range of the effective viscosity of blood in tubes with the dimensions of Rhodnius stylets. 6. These results are inconsistent with a hypothesis of a central nervous system 'oscillator' controlling the pump muscle, independent of feedback, but are consistent with a model involving peripheral feedback from stretch receptors, with an appropriate delay.