The Diplogastrina include about 290 species of free living nematodes. Traditional classifications of this taxon are not based upon hypotheses of phylogenetic relationships. The highly variable structures of the buccal cavity were examined in 21 species using light microscopy and SEM. The function of the stomatal structures was studied with the aid of video recordings of living worms. The morphological data were used to reconstruct a first outline of the phylogenetic relationships of the Dipolgastrina. A rhabditoid gymnostomatal tube which is longer than wide, a short stegostom and a small dorsal tooth as in Pseudodiplogasteroides belong to the stem species pattern of Diplogastrina. Diplogastrina with a ‘Rhabditis’‐like gymnostomatal tube feed on bacteria and small fungal spores. A short and broad gymnostom as well as a right subventral tooth which forms a functional unit with the dorsal tooth were acquired step by step in the ancestral line leading to Mononchoides and Tylopharynx. The cuticularized cheilostom was divided into six plates connected by pliable regions twice independently within the Diplogastrina. The teeth‐bearing posterior part of the buccal capsule can move forewards by pushing apart the plates of the cheilostom so that the teeth can get in contact with food items that are too big to be sucked into the buccal cavity. Diplogastrina with a divided cheilostom can feed not only on bacteria, but also on larger fungal spores, ciliates or other nematodes. Tylopharynx is specialized to rip apart the cell wall of fungal hyphae with the movements of a dorsal and a subventral tooth in order to suck out the contents of the fungus. This shows that the transformation of the buccal cavity in Diplogastrina is linked with an expansion of ecological niches.

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A new hermaphroditic species belonging to the diplogastrid taxon Koerneria is described. Although Koerneria sudhausi n. sp. displays all known apomorphies of Koerneria, it shares apomorphic characters with Mononchoides colobocercus (originally described as Diplenteron colobocercus). I assume that the apomorphic features of Koerneria exist in M. colobocercus too and were overlooked in the original description. Consequently M. colobocercus is transferred from Mononchoides to Koerneria, becoming Koerneria colobocerca n. comb. Koerneria sudhausi n. sp. and its sister species, K. colobocerca, together form the Diplenteron species group within Koerneria. Characteristic features of K. sudhausi n. sp. are the species group-specific prerectum in combination with a short but pointed tail in both sexes. In K. sudhausi n. sp. a stoma dimorphism between stenostomatous and eurystomatous morphs occurs. Eurystomatous K. sudhausi n. sp. hermaphrodites lack two denticles in the armature of the left subventral stegostomatal sector. All adults possess pore-like amphids. Although K. sudhausi n. sp. is hermaphroditic, a considerable frequency of males (up to 16.6%) occurs. In cultures of K. sudhausi n. sp. males are functional. In K. sudhausi n. sp. males, the phasmid is situated between caudal papillae v6 and v7. The formation of a secondary egg shell by a uterine shell gland is described. Koerneria sudhausi n. sp. possesses an exceptionally high number of intestine cells, an inexplicable dimorphism in egg length, and a dauer juvenile that lacks the oily substance usually found in diplogastrid dauers.

A new species of the diplogastrid genus Oigolaimella is described in colonies of termites belonging to the genus Reticulitermes from Corsica (France) and USA. Oigolaimella attenuata n. sp. males can be recognised by the conspicuous length of the ventral unkeeled part of the otherwise keeled gubernaculum and, in contrast to the other members of the genus, the fact that the lateral field of both adult stages is marked by a single line. A diagnostic key for the five species of Oigolaimella is presented. Life cycle and development, including spermiogenesis, sperm transfer, sperm competition and fertilisation are described in detail. Some interesting aberrations of the reproductive system are documented. The new species uses the preoral cavities of the termites for internal phoresis and is associated with non-pathogenic gut-inhabiting flagellates of the taxon Kinetoplastida. The heads of 76 of 117 examined termites were infested with dauer juveniles of O. attenuata n. sp. with an average of 6.4 nematodes per termite. Six additional nematode species were isolated from the bodies of the termites, particularly a species of Pristionchus and, for the first time, Halicephalobus sp., Mesorhabditis spiculigera and Rhabditella axei. Rhabpanus ossiculum was isolated from termite-inhabited wood from Corsica, the first such detection in Europe. In the course of our discussion of the literature on termite-associated nematodes, we propose the new combination Pristionchus formosianus (Poinar, Meikle & Mercadier, 2006) n. comb. (= Chroniodiplogaster formosiana).