Eugregarines inhabiting the midgut of zygopteran imagines usually appear as sublethal parasites; when present in large numbers, they enfeeble the host by damaging its gut wall. After the dragonfly adult season the gregarines survive in the habitat as extrahost oocysts. To become infective, they presumably have to pass the winter in the habitat; the infection occurs through ingestion. — In western Norway, Pyrrhosoma nymphula and Enallagma cyathigerum appear either in large, compact colonies. based on small, solitary tarns or ponds, or as marginal populations of low density, associated with streams and lakes. The intermingling of imagines from neighbouring breeding sites is insignificant. The dense colonies of small waters appear most prone to overinfection; the marginal populations harbour low numbers of gregarines and many are gregarine-free. — There is a gregarine-induced susceptibility to predation. The dense dragonfly colonies accumulate infective gregarine germs, maintaining a high infection potential for successive generations. In such populations the gregarine infection is endemic, and represents one of the selection agents.