1. The object of this investigation was to examine the mode of operation of the pharyngeal pump of Rhodnius and its possible role in determining rate of feeding and maximum size of meal.
2. The rate of pulsation of the pump remains constant throughout the meal, falling only in the last few seconds.
3. The pump is filled by muscular withdrawal of the piston and empties by elastic return of the piston.
4. The properties of the pump, which is very small, were studied on a working model. The model displayed marked ‘click’ properties, the force behind the emptying stroke increasing as the stroke progressed; thus the emptying stroke, once initiated, was always completed.
5. The pressure volume relations of the isolated abdomen were explored as a function of time.
6. It is concluded that the pump is stopped at a critical, abdominal pressure of 2.5 cm. Hg., which prevents the initiation of the emptying stroke, that the rate of feeding is mainly limited by the rate of filling of the pump and that the size of the meal taken depends upon the ease with which the cuticle can be stretched.
7. Confirmatory evidence is adduced and discussed.