1. The initial hydrostatic pressure developed in the crop on feeding was independent of the osmotic concentration of the ingested glucose solution. It is calculated that the pressure increase was not the result of a simple compression of the air contained in the crop, but that the feeding process must have involved a significant release of air from this structure.

2. The pressure pulses which developed after feeding appeared to be dependent on the same nervous pathway as that determining the frequency of opening of the proventricular valve. They also decayed at the same rate as the frequency of opening of the valve. Such a linkage between the pressure pulses and the opening of the valve might ensure a maximum pressure gradient between the lumen of the crop and the midgut.

3. An appreciable negative hydrostatic pressure was demonstrated in the haemolymph, which, if transmitted to the lumen of the midgut, would facilitate the maintenance of the hydrostatic pressure gradient across the proventricular valve.

4. Both the general and transient pressures within the crop decayed exponentially, there being no significant difference when solutions of different osmotic pressure were used.

5. With a viscous solution (containing 4% methyl cellulose) the pressure changes were of the same order as with normal solutions, except that the transient pressure pulses tended to remain constant after an initial relatively rapid fall.

6. A quantitative analysis, based on a modified Poiseuille relation, showed that the effective dimensions of the proventricular valve increased with time, the initial value of the ‘valve parameter’ being smaller and the rate of increase more rapid with the more concentrated solution. It is concluded that variations in the effective dimensions of the valve contribute to the demonstrated control of crop-emptying at different osmotic concentrations.

7. The possible nervous mechanism capable of controlling the release of fluid from the crop is considered.

Research Fellow of Gonville and Caius College.
 A.R.C. Unit of Insect Physiology