An Ultrastructural Study of Pollen Grains Consumed by Larvae of Osmia Bees (Hymenoptera,Megachilidae)

Pollen samples from provisions and faeces found in the nests of four Osmia species were analyzed by SEM and TEM. The pollen grains studied were Cistus, Sonchus type oleraceus. Primus type dulcis, and Quercus type ilex. The apertures of Cistus and Sonchus pollen stored in the provisions were slightly expanded, and the cytoplasm protruded through them. Conversely, Prunus and Quercus pollen grains showed hardly any signs of such apertural protrusions. Further, the cytoplasm of Cistus and Sonchus pollen (which have thin intines) was almost entirely lacking in the pollen grains recovered from faeces, while in the faecal pollen grains of Prunus and Quercus (with thick intines) the cytoplasm was much less modified. These preliminary results indicate that both the protrusion of the cytoplasm in the provisions and the thickness of the intine may play an important role in the digestion of pollen grains by Osmia bee larvae.