Cresson 1916 : 152 Coenia spinosa Loew Sturtevant and Wheeler 1954 : 166 Wirth and Stone 1956 : 472 Wirth 1965 : 755 Steyskal 1970 : 462 Mathis and Simpson 1981 : 29 Mathis and Zatwarnicki 1995 : 238 Revision of Ephydrini Zetterstedt (Diptera: Ephydridae) from the Americas south of the United States Mathis, Wayne N. Marinoni, Luciane Zootaxa 2016 4116 1 1 110 45JR Cresson Cresson [238,491,1526,1552] Insecta Ephydridae Dimecoenia Animalia Diptera 82 83 Arthropoda genus     Dimecoenia  Cresson 1916: 152[ typespecies:  Coenia spinosaLoew, by original designation].—  Sturtevant and Wheeler 1954: 166[review, in part].—  Wirth and Stone 1956: 472[review in part, species of California].—  Wirth 1965: 755[Nearctic catalog].—  Steyskal 1970: 462-465 [review in part, Figs. of male and female terminalia].—  Mathis and Simpson 1981: 29[revision of North American species, natural history].—  Mathis and Zatwarnicki 1995: 238-240 [world catalog].   Diagnosis.  Dimecoeniais distinguished from other genera of the tribe Ephydriniby the following combination of characters: Moderately large to large shore flies, body length 4.25–6.25 mm; mostly dull, olivaceous brown to grayish brown, dorsum with some subshiny to shiny areas dorsally.  Head:Cruciate interfrontal setae 1 pair, well developed; lateroclinate fronto-orbital setae 2, slightly divergent. Basal flagellomere simple, lacking secondary seta inserted laterally just below arista; arista tapered gradually from thickened base to style-like apex, approximately basal 2/3 with dorsal rays, thereafter bare, aristal rays nearly as long as width of pedicel. Facial hump poorly developed, little evident; ventral margin of antennal grooves nearly horizontal, not sloping ventrally at conspicuous angle.  Thorax:Postpronotum bare of setae; dorsocentral setae 5 (1+4). Dorsal costagial seta subequal in length to anteroventral costagial seta; costa with numerous, conspicuous, spine-like setulae; legs of both sexes similar.  Abdomen: Maleterminalia: Aedeagus a simple tube or bifurcate. Female ventral receptacle variously shaped.  Larva:larvae lacking well-developed prolegs on segments other than 12.   Distribution. Ascharacterized here, Dimecoenianow includes just two species,  D. fuscifemurSteyskaland  D. spinosa(Loew). The monophyly of the genus is established by the following apotypic characters: 1. The anterior margin bears conspicuous, spine-like setae; 2. Margin of ventral margin of antennal groves rounded, nearly horizontal and not steeply angled. Larvae of Dimecoeniarepresent an apparent reversal in the generalized adaptive scheme of Ephydriniby inhabiting mud substrates of salt marshes. This has apparently resulted in the atrophy of the prominent, ventral prolegs, including the crochets, which are functionally adapted to movement within algal mats.