PROC. ENTOMOL. SOC. WASH. 85(3), 1983, pp. 472-484 BIOLOGY AND IMMATURE STAGES OF NOSTIMA APPROXIMATA (DIPTERA: EPHYDRIDAE), A GRAZER OF THE BLUE-GREEN ALGAL GENUS OSCILLATORIA' B. A. FooTE Department of Biological Sciences, Kent State University, Kent, Ohio 44242. Abstract. — In^orrmXion is presented on the life cycle and larval feeding habits of Nostima approximata Sturtevant and Wheeler, a relatively rare species of the ephydrid subfamily Notiphilinae and tribe Philygrini. The larvae are highly spe-cialized trophically and are restricted to feeding on th? algal genus Oscillatoria and certain other related genera of blue-green algae. In northeastern Ohio, the life cycle can be completed in ca. 25 days, and there are at least five generations a year. The egg, mature larva, and puparium are described and illustrated. A key is given that separates the eggs, mature larvae, and puparia of .V. approximata from those of Lytogaster excavata (Sturtevant and Wheeler) and Pelina truncatula Loew, two other ephydrid species whose larvae are known to feed on soil-inhab-iting blue-green algae. The life history of this species is contrasted and compared with those other species of Ephydridae having larvae that feed on blue-green aigae. The larval feeding habits are also compared with those of other species of Notiphilinae. It is suggested that the genus Nostima and tribe Philygrini would be better placed in the subfamily Parydrinae rather than in the Notiphilinae. Containing more than 1 200 species, the Ephydridae constitute one of tue largest families within the acalyptrate Diptera (Rohdendorf, 1974). Over 400 sptcies are recorded from America north of Mexico (Deonier, 1979), and equally rich faunas occur in the other major biogeographic regions. The family obviously has under-gone considerable adaptive radiation and has been referred to as a "family ol flies in the full flower of its evolution" (Oldroyd, 1964). The larval feeding habits ire remarkably diverse, although a majority of the species probably are microphagots on autotrophic and heterotrophic microorganisms. Larvae of many species art known to utilize algae, bacteria, and yeasts. However, other larvae are scavengers on decaying plant and animal remains; are phytophagous in the leaves and stems of higher plants; or are predators of spider eggs, frog eggs, and midge larvae. Perhaps the most interesting trophic story involves the use of algae as a larval food. Species of Cirrula, Dimecoenia, Ephydra, and Setacera occur in floating algal mats (Aldrich, 1912; Oliveira, 1954, 1958; Wirth, 1971; Simpson, 1979; Research supported by a research grant (DEB-79 12242) from the National Science Foundation.