1. The proventriculus of the worker honeybee is an organ which effects a highly efficient separation of pollen grains from the medium in which they are suspended.
2. The pollen grains are packed tightly together by the proventriculus and are passed as a bolus down to the ventriculus.
3. The boluses pass quite quickly towards the posterior end of the ventriculus (5-20 min.), depending on the concentration and amount of pollen suspension which is fed.
4. The peritrophic membranes do not move down the ventriculus with these packages but pass down at a slower rate.
5. At the posterior end of the ventriculus the pollen may be held up for a considerable time. In the brood-rearing bee it stays there for many hours (up to 12 or more). In the forager it begins to pass down the hindgut to the rectum after 3 hr. or less.
6. The volume of fluid within the honey stomach, the size of particles in suspension and their concentration have significant effects on the rate and efficiency of filtration by the proventriculus.
7. Some reflexions are made on the possible physiological significance of the mechanism of the proventriculus to the worker honeybee.