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Gastromermis cordobensis n. sp (Nematoda: Mermithidae) a parasite of larvae of the blackfly Simulium lahillei Paterson & Shannon (Diptera: Simuliidae) in Argentina, is described. Diagnostic characters of this species include a mouth ventrlly shifted; six cephalic papillae; eigh hypodermal chords; small and pear shaped amphids; a long and S-shaped vagina; a singl spicule, which is long, has non-uniform walls, and a tip with sculpture; three rows of genital papillae, the middle one with 18 pre-anal and 10 post-anal papillae, the lateral rows have 36 papillae each; oval eggs; and post-parasitic juveniles with long thin tails. Pre-parasitic and parasitic juveniles are included in the description.

Two new species of mermithids (Nematoda: Mermithidae), Gastromermis doloresi n. sp. a parasite of Simulium wolffhuegeli Roubaud and Hydromermis doloresi n. sp. a parasite of S. jujuyense (Paterson & Shannon) from Córdoba, Argentina, are described and illustrated. G. doloresi n. sp. is characterized by having medium sized, pear-shaped and oval amphids, eight hypodermal chords all around the body, cylindrical S-shaped vagina, single long spicule 528 [mi]m, whith teh sculpture tip, three rows of genital papillae in which the middle row contains 17 pre-anal and 9 post-anal papillae, and lateral rows contain 18 papillae. Hydromermis doloresi n. sp. can be distinghished by the following: medium sized, pear-shaped amphids, eight hypodermal chords, a cylindrical, S-shaped vagina, one short curved spicule with sculpture and genital papillae in three rows. The medium ventral row of papillae has 4 double and 9 single pre-anal papillae and 3 double and 6 single port-anal papillae, lateral rows have 18 single papillae each.

Two new species of the Mermithidae (Nematoda) are described from the Japanese blackfly Simulium japonicum Matsumura. They are the first representatives of Gastromermis and Isomermis from Japan. Gastromermis mesostoma n. sp. is characterized by a terminal or near terminal mouth, 6 hypodermal cords and the absence of cloacal or vulvar lips. Isomermis bipapillatus n. sp. is characterized by the small size of the spicules, their curved shape and by a pair of terminal hypodermal protuberances in the tail.

During the life cycle of the mermithid nematode Gastromermis boophthorae the main components of the body wall, namely the cuticle, hypodermis and somatic musculature, undergo radical changes in structure and relative proportions. In the non-feeding, free-living stages the cuticle is a well-defined, multilayered structure which lies upon a generally thin hypodermis; the somatic musculature has the extensive and highly organised appearance typical of motile nematodes. In parasitic juveniles the body wall has a completely different structure which is adapted for the suggested purpose of nutrient uptake from the host insect's haemocoelomic fluid; the cuticle is characteristically thin and rather diffuse, the underlying hypodermis becomes very extensive and metabolically active, and there is a corresponding reduction in the proportion of somatic musculature. The sub-cuticular surface of the enlarged hypodermis has a microvillous configuration and has associated alkaline phosphatase activity.

An examination of the trophosome of a mermithid nematode (Gastromermis sp.) revealed large aggregates of dense tubular rods in the cytoplasm. These rods were 18nm in diameter and variable in length. They were apparently formed in the trophosome and may represent a unique form of nutrient reserve. Other aspects of trophosome morphology are described.

From material collected by R. Le Berre in 1972 and by J. J. Petersen and B. Mondet in 1975, two new species of Mermithidae are described from blackfly larvae, including Simulium damnosum s.l., for the first time in West Africa. These species, Gastromermis philipponi n.sp. and Gastromermis leberrei n.sp. possess a rudimentary excretory gland and the preparasitic larvae of the former contain a characteristic caudal appendage which is lost during penetration into the simuliid larvae. Therefore, these two species are the only ones of Gastromermis known to have six longitudinal cords.

Twenty-four described mermithids are listed as known members of the Canadian fauna. Five of these are redescribed, and eight new species are described; namely Capitomermis micropeos, Gastromermis plethelos, Gastromermis leptopeos, Gastromermis pachopeos, Hydromermis macrospiculatus, Neomesomermis leptoderos, Neomesomermis welchi, and Neomesomermis yukonensis. A taxonomic key is provided to separate the 10 genera to which the 24 species belong. Keys identify the species of the genera Gastromermis, Hydromermis, and Neomesomermis. Morphology of the male head and genitalia are critically analysed and consistent male characteristics incorporated into four keys.

Gastromermis metae sp.n., a parasite of Simulium equinum and S. ornatum (Diptera: Simuliidae) is described and illustrated. The morphological variation in this species led to an evaluation of diagnostic characters currently used in mermithid taxonomy and an assessment of species in the genus Gastromermis. Ninety-five out of 99 published descriptions were examined: 25 were adequate, the remaining 70 are species inquirendae.

One female of Eusimulium rendalense Golini and three of E. fallisi Golini, among nearly 470 simuliids captured from Anas boschas L. in Norway, were found naturally infected with mermithid nematodes Two of these infected flies had fully fed on avian blood. The nematodes may be a species of the Gastromermis boophthorae complex Welch and Rubzov.

Different methods of killing, fixing and processing to mounting medium for adult males of Romanomermis culicivorax and Gastromermis sp. were tested and compared. Qualitative characters used in species diagnosis, particularly head shape and cuticle thickness, and quantitative characters were differentially altered by the methods tested. No method produced specimens with all the characters of live individuals. The best results were obtained after killing by immersion in water at 65° for 3s, fixing in TAF for 1 week and processing to glycerol by Seinhorst's method. Romanomermis culicivorax is currently distinguished from R. iyengari by fixation artifacts and is a species inquirenda. Interbreeding experiments are required to determine the taxonomic status of R. culiciaorax.

The ultrastructure of the trophosome of Gastromermis boophthorae (Welch & Rubzov, 1965) is described at selected points in the life cycle of this mermithid nematode. The trophosome is grossly modified for the assimilation and storage of nutrients, and comparison is made with a generalised nematode intestine, with which it may be homologous. At its maximum development the trophosome is a solid syncytial cylinder which almost fills the body cavity of the nematode. Internally it is packed with nutrient reserves, whilst the outer cytoplasmic layer exhibits features indicative of transport functions. The trophosome is linked to the hypodermis of the body wall by radially-orientated cytoplasmic bridges, and the possible significance of this arrangement is discussed in relation to the nutritional function of the trophosome.

>Gastromermis boophthorae is distinguished from related species by its longer spicule, its characteristic lateral cord, numerous and small cells in the head region, and longer proximal reflex of the vagina. This species is divided into six varieties that are based mainly on differences occurring in the free-living postparasitic stages. It is recognized that the term “variety” is seldom used in nematology and is unprotected nomenclatorially, but its use here seems justified to designate infraspecific populations that appear at present to be sympatric and anatomically discontinuous, and that can only be recognized as immature nematodes. Six species of blackflies were found to be hosts and infection is reported from May until August in the Leningrad region. Percent infection ranged from 1 to 10 but reached 100% in one sample. Synchronization of host and parasite development is probably important in determining incidence of parasitism.

Caistromermis viridis and Isomermis wisconsinensis are described from larvae of Simulium vittatum Zett. in Wisconsin, and Mesomermis flumenalis from larvae of Simutlium vennstum Say in Ontario. No species of Gastromermis or Isomermis have previously been known from North America. The free-living and parasitic juveniles of all three species, as well as the adults, are described and illustrated.

The life cycles of three mermithid species were synchronized with those of the chironomid hosts. Hydromermis palustris overwintered as an advanced parasite in Cladotanytarsus sp., emerged in early May, and infected new hosts throughout the summer. Gastromermis deltensis overwintered as an early parasite, mainly in Harnischia sp., emerged in late May, and also infected new hosts throughout the summer. Hydromermis conopophaga probably overwintered as an egg, infected new hosts in spring, and emerged from Polypedilum simulans and Harnischia sp. in late June. It had only one generation per year. The chironomids overwintered as third instars. Cladotanytarsus sp. was the first to molt to the fourth instar in the spring, Harnischia sp. was second, and P. simulans last. Both Cladotanytarsus sp. and Harnischia sp. emerged throughout the summer; P. simulans had a limited period of emergence in the summer. The sex of the mermithids was correlated with their number per host. Intersexes in parasitized adult Harnischia sp. are described and illustrated.

The amphids of adult Gastromermis boophthorae exhibit an organization unlike any previously reported. Each amphid consists of a distal cuticular channel which opens to the exterior, and a proximal amphidial gland. The cuticular channel is double and 15–18 cilia are held in a tight bundle by the inner channel. The amphidial gland contains an extensive reticulum, and the nerve axons which give rise to the amphidial cilia are arranged around the outside of the gland; anteriorly these axons are characteristically flattened. A granular secretion is produced by the amphidial gland and passes via the reticulum to the double cuticular channel, thus bathing the ciliary bundle. The ultrastructure of the amphids is discussed in relation to functional considerations.

Gastromermis deltensis n. sp., Hydromermis conopophaga Poinar, 1968, and Hydromermis palustris n. sp., parasites of chironomids (Diptera) in the Delta Marsh, Manitoba, Canada, are described and illustrated. The three species showed patterns of morphological variation, the most common being variation in ventral displacement of the mouth. This led to a reconsideration of the genera Gastromermis and Hydromermis. Spicule length is proposed as the differential characteristic between the two genera. Species placed to date in Gastromermis are reassessed on this basis.