Forty-one sites located on 14 New Caledonian streams were surveyed four times between October 1996 and October 1997 in order to examine the spatial and temporal changes in the structure of the benthic macroinvertebrate communities. About 250 000 invertebrates representing 167 taxa were collected in the streams. Seventy-five percent of identified taxa and 67% of individuals were insects. Major spatial and temporal changes in the composition of the fauna were detected by multivariate analyses (ordination and classification). Overall, the number of individuals was significantly higher in the dry season (October) than in the wetter seasons (January and June). However, a low temporal variability was detected in the structure of benthic communities during the sampling period. A cluster analysis based on taxonomic composition separated five groups of sites in relation with rock type, land use, and geographic characteristics. Several metrics (total invertebrate density, taxon richness, relative abundance of major invertebrate groups, diversity indices) were used to characterize each group of sites. Forested streams, where the highest specific diversity occurred, represented the most speciose habitat for benthic fauna. A less rich and abundant fauna occurred in streams draining ultramafic rocks probably because of their low content in food resources and organic matter. Key-words : benthic macroinvertebrates, New Caledonia, tropical streams, biodiversity, spatial and temporal variability. VARIABILITÉ SPATIO-TEMPORELLE DE LA STRUCTURE DES COMMUNAUTÉS DE MACROINVERTÉBRÉS BENTHIQUES DANS LES RIVIÈRES DE LA NOUVELLE-CALÉDONIE. RÉSUMÉQuarante et une stations situées sur 14 cours d'eau répartis sur l'ensemble de la Nouvelle-Calédonie furent périodiquement échantillonnées entre octobre 1996 et octobre 1997. Des analyses multivariées (ordination et classification) permirent d'étudier la variabilité spatio-temporelle de la structure des communautés benthiques. Environ 250 000 invertébrés représentant 167 taxons furent prélevés durant l'étude. Soixante quinze pour cent des taxons identifiés et 67 % des individus prélevés furent des insectes.

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A checklist of identified oribatid mite taxa from riverine freshwater environments from six islands in Polynesia (New Caledonia, Tahiti, Moorea, Rurutu, Tubuai, Raiatea) is presented; 18 species, 16 genera and eight families were recorded. Trhypochthoniellus longisetus (Berlese, 1904) and Trimalaconothrus albulus Hammer, 1972 prevailed on distribution. Fortuynia smiti sp. n. (Fortuyniidae) is described from New Caledonia. The new speciesis morphologically most similar to Fortuynia marina Hammen, 1960 from New Guinea, but it differs from the latter by the longer notogastral setae dm, lm, c2, p1, epimeral setae 3b and adanal setae ad1 and the presence of prodorsal lateral ridges.

Helicopsyche trispina sp. n. is described from Grande Terre, New Caledonia, based on pharate males, larvae and pupae. The species appears to belong to the monophyletic New Caledonian Helicopsyche clade.

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