Dimecoenia Cresson, 1916

Mathis, Wayne N. & Sepúlveda, Luciane Marinoni and Tatiana A., 2025, Phylogeny and taxonomy of the shore-fly tribe Ephydrini with comments on related tribes in Ephydrinae (Diptera: Ephydridae), Zoologia (e 24044) 42, pp. 1-42 : 17-18

Dimecoenia Cresson, 1916

Figs 27–30

Dimecoenia Cresson 1916: 152 (feminine; 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, figures 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 4.25–6.15 mm; mostly dull, olivaceous brown to grayish brown, dorsum with some subshiny to shiny areas dorsally.

Head: Frons with shiny, submetallic mesofrons, parafrons dark but not shiny. one pair of well-developed cruciate interfrontal setae; two well-developed lateroclinate fronto-orbital setae, slightly divergent; paravertical setae small; both medial and lateral vertical setae well developed. 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, hair-like rays, thereafter bare, aristal rays nearly as long as width of pedicel. Facial hump poorly developed, shallowly projected, dorsal portion of hump with some shiny metallic coloration; ventral margin of antennal grooves nearly horizontal, not sloping ventrally at conspicuous angle; facial setae best developed along lateral and oral margins; one large genal seta. Gena relatively short, height about equal to height of basal flagellomere.

Thorax: Chaetotaxy: Acrostichal area with setulae only, no well-developed prescutellar pair; 5 (1+4) dorsocentral setae, posterior seta displaced laterally; 1 presutural supra-alar seta; 1 supra-alar seta; postpronotum with 1 large seta and a smaller seta; 1 postalar seta; 2 lateral scutellar setae; prosternal setulae sparse on at least posterior portion; anepisternum with 1 long seta; katepisternum with 1 long, dorsoclinate seta. Wing generally hyaline; dorsal costagial seta subequal in length to anteroventral costagial seta; costa with numerous, conspicuous, spine-like setulae; legs of both sexes similar; costal vein ratio 0.24–0.26; M 1 vein ratio 0.81–0.85. Tarsal claws long and nearly straight; pulvilli greatly reduced or essentially absent.

Abdomen: Generally subshiny; anterior portion of each tergite dark brown posterior portion lighter, grayish green; male tergite 5 as long as wide, longer than tergite 4. Male terminalia: Epandrium more or less oval in posterior view, anteroventral margin evenly rounded; surstyli with large medial flange and posterolateral, slender processes; gonite 4X longer than wide, anteroventral margin broadly and shallowly U-shaped; phallapodeme with posteromedial broad keel; aedeagus a simple tube, mostly parallel-sided. Female terminalia: Female ventral receptacle with operculum much smaller than extended process, trapezoidal; extended process broadly curved, widest medially.

Third-instar larva: Larvae lacking well-developed prolegs on segments other than segment 12, only a short, bump-like process evident.

Natural history: Larvae of Dimecoenia represent an apparent reversal in the generalized adaptive scheme of Ephydrini by inhabiting mud substrates associated with 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.

Distribution. The composite distribution for the two species now included in Dimecoenia , D. fuscifemur Steyskal and D. spinosa (Loew) , is temperate North America with specimens of D. spinosa occurring southward to Costa Rica and on some islands of the West Indies.

Remarks. As characterized here, Dimecoenia now includes just two species, and these were most recently revised by Mathis and Simpson (1981). The monophyly of this clade is confirmed by the following synapomorphies: 1. The anterior margin bearing conspicuous, spine-like setae; 2. Ventral margin of antennal facial groves rounded, nearly horizontal and not steeply angled.

Figures

Figures 24–30.Coenia and Dimecoenia.Coenia palustris,(24) head anterior; (25) head lateral; (26) thorax dorsal. Dimecoenia spinosa, (27) head anterior; (28) head lateral; (29) thorax dorsal; (30) hind leg anterior.