Morphology, ultrastructure and formation of resting cysts of E. agilis H.J. Carter and E. anabaena Mainx are described. TheSEM micrographs of cysts of E. anabaena and E. agilis are presented for the first time. Information on the neuston film formedby these species is given. An ecological database and list of accompanying species is also included.

Le cyclopoïde Mesocyclops ogunnus Onabamiro, 1957 a été identifié dans la retenue de Sidi Salem (Tunisie). C'est la premièreobservation d'une espèce du genre Mesocyclops en Tunisie. Déjà signalé en Algérie, cette nouvelle observation permet deconfirmer le lien entre les aires de répartition africaine intertropicale et euro-asiatique de M. ogunnus (Israël, Ouzbékistan,Bangladesh, Sri Lanka, Laos).

Streams and rivers on the Vanuatu islands of Espiritu Santo, Pentecost, Éfate and Tanna were sampled for gastropods and thephysical and chemical characteristics of the water were investigated between 25/11/95 and 13/12/95. Twenty three species of neritidand forteen of thiarid prosobranch gastropods were found, together with the pulmonate Physastra nasuta and an opisthobranchspecies Strubellia. Six thiarid, two Clithon and one Neritina species, which are present in the Solomon Islands and SouthEast Asia, but which are absent further south on New Caledonia and the Fiji islands, were found on the Vanuatu islands. Onthe other hand, Septaria bougainvillei and Clithon pritchardi, which are present on the Fiji islands and New Caledonia, werefound no further north than the southern Vanuatu island of Tanna. This suggests that Vanuatu is a transition zone for both northernand southern freshwater snails. Éfate had many more species (26) than the other islands (16-19-. This was partly explainedby its central geographical position and the higher ion content of its streams.

The 138 species and subspecies of Ephemeroptera now known to occur in Mexico, of which 35, approximately 25 %, areendemic, are listed along with distributional annotations. New Mexican records are included for Americabaetis lugoi Waltz &McCafferty, Baetodes velmae Cohen & Allen, Drunella coloradensis (Dodds), Farrodes otiesa Lugo-Ortiz & McCafferty,Plauditus virilis (McDunnough), Tricorythodes fictus Traver, T. texanus Traver, and Varipes lasiobrachius Lugo-Ortiz &McCafferty, the latter of which also constitutes a new continental record for the genus Varipes Lugo-Ortiz & McCafferty inNorth America. Traverella sallei (Navás), n. comb., is transferred from the genus Thraulus.

We investigated the abundance, diversity and community structure of mayflies (Ephemeroptera), stoneflies (Plecoptera) andblackflies (Diptera : Simuliidae) in the Sébaou basin (Algeria). Fifty-five species were collected (i.e. 22 mayflies, 14 stonefliesand 19 blackflies species) in 18 sampling sites located in both main streams and tributaries of the Sébaou river (Great Kabylie,North Algeria). Analysis of species diversity of the three considered taxa showed an altitudinal gradient with maximal diversityin the piedmont and low altitude mountain areas (from 200 to 500 m), which provided the most heterogeneous habitats to thebenthic fauna. In contrast, the low diversity of the lowland sites was due to high water temperatures, reduced summer dischargeand anthropogenic disturbances. In the same way, the low diversity of the high mountain areas can be attributed either to lowwater temperatures or high fluctuation in daily temperature. Three species assemblage groups are revealed, the first two are relatedto an upstream-downstream gradient, with headwater sites characterised by stoneflies species and middle stream sites by ahigh mayfly and blackfy diversity and abundance. The third group was identified as spring sites characterised by a low abundanceand occurrence of stonefly species.Both altitudinal species diversity gradient and species assemblage of the Sébaou wadi were different from European and temperateareas, revealing that the accentuated North African Mediterranean climate could induce changes to both species and communitystructure. These results provide insights to the influence of some environmental features on aquatic insect species distributionand community structure in North African streams.

Cette note se propose de confronter l'aptitude de deux indices biotiques à la détermination de la qualité de l'environnement du cours inférieur du Tibre et d'évaluer si et comment la méthode d'échantillonnage peut influencer la valeur finale de ces indices. Une des méthodes met en œuvre un filet troubleau utilisé pendant 3', tandis que les autres sont basées sur l'utilisation de substrats artificiels immergés à chaque station pendant toute la durée de l'étude. Nos résultats semblent démontrer une sensibilité différente pour les deux indices biotiques dans la description des changements de la qualité de l'environnement et une influence significative de la technique d'échantillonnage sur la détermination de la qualité des eaux obtenue avec les indices biotiques.