The rectum of Oxaea flavescens (Andrenidae) has a specialized structure among bees

The rectum of the bee Oxaea flavescens contains six hollow rectal papillae, in contrast to rectal pads found in others Hymenoptera. The rectal epithelium is formed by cubical cells with structural specializations for fluid transport, such as apical plasma membrane invaginations, endocytic vesicles, scalariform junctions, wide extracellular space and double-membrane vesicles. Globular-shaped cells form the rectal papillae with surface plasma membrane invaginations that vary from small depressions to very large sinuses. The cell-cell contact is narrow, exhibiting extensive regions with scalariform junction, while cytoplasmic free ribosome-like material is prominent. These ultrastructural features are discussed in relation to their role in excretion in this bee.