Relatively little is known about the mechanism of pollen digestion in bees. 
Bees manipulate pollen masses, mixed with nectar, with their mandibles. 
Some authors infer this to be a grinding function to crack the hard coats of 
the pollen grains. On the other hand, WHITCOMB and WILSON (1929) never found
cracked pollen grains in the honey stomach; only 1% of the grains in the intestine
showed signs of mechanical cracking. The true function of the mouth parts in
relation to pollen feeding seems to be passive and practically the same as in the
intake of liquids.

Material is mechanically carried from the mouth to the honey stomach by a
slight peristaltic action along the esophagus. Pollen seldom stays in the honey
stomach longer than 20 minutes; from there it is rapidly transferred by the honey
stopper to the mid-intestine.

Pollen grains undergo physical changes during passage through the alimentary
canal. The cellulose, pectic and sporopollenin coat is generally indigestible by
insects. Therefore, extraction of nutrients must take place through the cell walls or
through ruptures in those walls. Crushing of the grains is not necessary for their
complete digestion. In the honey stomach few changes occur in the pollen, but
there is a tendency toward a germination-like swelling of the apical caps in the
pollen pore area. Digestion begins minutes after the pollen reaches the ventriculus.

These observations all confirm the concept that bees lack enzymes to break
down pollen wall components. The digestion of proteins, lipids, and carbohydrates
takes place through the pollen germination pores. MAURIZIO (Pers. communication)
indicated that in many cases the external structure of the grains, especially the
exine, remains unchanged.

Stanley, R. G., & Linskens, H. F. (1974). "Pollen." Springer Berlin Heidelberg.

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