Dimecoenia Cresson

Mathis, Wayne N. & Marinoni, Luciane, 2016, Revision of Ephydrini Zetterstedt (Diptera: Ephydridae) from the Americas south of the United States, Zootaxa 4116 (1), pp. 1-110 : 83-84

Genus Dimecoenia Cresson

Dimecoenia Cresson 1916: 152 [type species: Coenia spinosa Loew , 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. Dimecoenia is distinguished from other genera of the tribe Ephydrini by 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: Male terminalia: Aedeagus a simple tube or bifurcate. Female ventral receptacle variously shaped.

Larva: larvae lacking well-developed prolegs on segments other than 12.

Distribution. As characterized here, Dimecoenia now includes just two species, D. fuscifemur Steyskal and 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 Dimecoenia represent an apparent reversal in the generalized adaptive scheme of Ephydrini by 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.