Fig. 1. Life cycle of Nosema apis, causing bee dysentery. 1Infectious spore (4–6 × 2–4 μm) containing a typically dinucleate sporoplasm; bees become infected by swallowing spores from the feces of parasitized members of the colony. 2, 3 Inside the intestine the tubular polar filament becomes extruded, penetrates the peritrophic membrane (not drawn) and enters an intestinal cell. The sporoplasm (SP) is injected into the epithelial cell through the tubular lumen of the polar filament. 4–12 The sporoplasm (9) grows and asexually divides via quadrinucleate stages in its host cell (merogony). Finally, encystation (i.e. spore formation) is initiated from a diplokaryon stage (10) via a final division (sporogony). 13 When mature spores are present, host cells are disrupted and release the infectious spores into the lumen, which are voided with the feces (or infect neighboring cells). At the end of summer, development of Nosema apis may become reduced (hypobiosis) and starts again in spring. As well as the intestine, all organs of bees become parasitized. CW, cyst wall; EN, encystation; HC, host cell; N, nucleus; NH, nucleus of host cell; PP, polaroplast; SP, sporoplasm; TI, tubule-injected; TU, tubule (polar filament)