PROC. ENTOMOL. SOC. WASH. 84(4), 1982, pp. 828-844 BIOLOGY AND IMMATURE STAGES OF SETACERA ATROVIRENS, A GRAZER OF FLOATING ALGAL MATS (DIPTERA: EPHYDRIDAE)' B. A. FooTE Department of Biological Sciences, Kent State University, Kent, Ohio 44242. Abstract. — Information is presented on the life history and larval feeding habits of Setacera atrovirens (Loew), a common and widely distributed shore fly species in lentic freshwater habitats. Adults and larvae are asso-ciated with floating algal mats where the larvae feed rather non-selectively on algal cells, including those of various blue-green genera. The egg, mature larva, and puparium of S. atrovirens are described and illustrated. Miscellaneous observations on the habitat distribution, life cycle, and larval feeding habits are given also for S. diirani (Cresson) and S. pacifica (Cresson). The family Ephydridae, shore flies, consists of at least 1200 species in the world (Rohdendorf, 1974), with over 400 species in 68 genera being recorded from America north of Mexico (Deonier, 1979). This vast fauna is reflective of the large amount of adaptive radiation that has occurred within this family of wetland-inhabiting flies. Among the radiations into different habitats has been the invasion of floating algal mats by at least three Nearctic genera of the tribe Ephydrini of the subfamily Ephydrinae. Adults of species of Cir-rula, Ephydra, and Setacera typically are found on the surface of the shal-low water of lake margins, ponds, and marshes. Larvae of several of these species have been collected repeatedly from floating or partially submerged mats of filamentous algae, although they may be actually feeding micropha-gously on the interstitial unicellular algae growing within the filamentous matrix (Brock et al., 1969; Collins, 1975; Simpson, 1973). The biology, life history, and larval feeding preferences of certain of the algal mat-inhabiting species have been elucidated previously. Aldrich ( 1912), Beyer (1939), Brock et al. (1969), Collins (1975, 1977, 1980a, 1980b), Hennig Research supported by NSF grant DEB-7912242.