The nematodes establish various and characteristic relationships with the insects; in particular the EPNs belonging to the orders Mermithida, Rhabditida, Aphelenchida and Tylenchida, show interesting association with exapods and some species are used in controlling insect pests. In particular the Steinernematidae and the Heterorhabditidae that are widely widespread in nuinerous habitats in almost all continents, harbor in their gut sym- biotic microorganisms pathogenic for many arthropod species.