Orthacheta Becker, 1894: 101. Gender: feminine. Type species: Cordylura pilosa Zetterstedt, 1838, by original designation. Orthochaeta: emend. (Aldrich, 1905: 567; Hendel, 1930: 8; Hackman, 1956: 47).
Orthacheta Rondani: error (wrong author given by Vockeroth, 1995: 733).
Orthacheta species are slender, medium-sized flies (about 4–8 mm long). Head. Frons yellow to black; face, parafacial and gena pale yellow. Postcranium black, grey dusted, covered with yellow and black hairs and with black postocular setulae. Chaetotaxy: 3 orbital, 2–4 frontal, 1 ocellar, 1 inner vertical, 1 outer vertical, 1 postocellar; 1 pair of strong vibrissae and 1–2 pairs of short subvibrissae present. Antenna black. Postpedicel with acutely angled upper apical corner, 2–3 times as long as wide, usually in male longer than in female. Arista black, pubescent throughout its length (Fig. 177). Palpus slender, yellow.
Thorax black, grey dusted. Scutum with following setae: acrostichals short in two rows, prescutellar pair not differentiated or only slightly longer than the other acrostichals, dorsocentrals 3 + 3, intra-alars 1 + 2, supra-alars 1 + 2, postpronotals 2, notopleurals 2, postalars 2. Proepisternum with hairs anteriorly, with 2 setae near lower margin. Proepimeron with 1 seta. Anepisternum covered with hairs in posterior part and with 2–4 black setae along posterior margin. Katepisternum covered with long setae in ventral corner and 3 strong katepisternal setae. Anepimeron bare. Scutellum black, with pair of strong basal scutellar and pair of strong apical scutellar setae (Fig. 8).
Legs yellow, only tarsi darkened. Fore femur usually with rows of anterodorsal (anterior), posterodorsal, posterior and ventral setae. Fore tibia with 1 posterodorsal, 2 dorsal (anterodorsal) and 1 posterior setae at middle, and usually with apicals: dorsal, anteroventral and posteroventral. Mid femur with row of anterior setae, with 1 anteroventral on apical third, 2–4 ventral (posteroventral) on basal half, 1 preapical posterior, and 1 preapical posterodorsal setae. Mid tibia with 2 posterodorsal, 2 anterodorsal, 1 posterior, 1 anterior (anteroventral), and 0–1 ventral at middle setae, and ring of apicals. Hind femur with row of anterior setae, with 2 preapical anterodorsal and 1 preapical posterior (posterodorsal) setae, with 1 anteroventral seta on apical third, and 2–4 ventral (posteroventral) setae on basal half. Hind tibia with 3 anterodorsal, 3 posterodorsal, 1–2 anteroventral, 1 preapical dorsal setae, and ring of apicals, apical posteroventral setae absent, if present then two times shorter than anteroventral apical seta.
Wing tinged with brownish or distinctly brownish; veins blackish; vein R 1 setulose on apical third of dorsal surface. Calypters, margins of calypters, and halteres yellowish.
Abdomen cylindrical, black, greyish dusted, covered with hairs. Tergites 2–6 each with row of marginal setae. Male sternite 4 simple, approximately 2 times as long as wide. Male sternite 5 with black or yellow triangularshaped lobes (Fig. 166, 182). Surstyli simple, cerci fused (Figs 167, 168, 175, 176). Ovipositor long, cylindrical (Fig 171–174). Apex of female sternite 7 flattened laterally (Fig. 173, 174). Female tergite 8 of V-like form, sternite 8 as two small sclerites (Fig. 171).
Larvae of Nearctic species Orthacheta hirtipes Johnson, 1927 are predators of Cordilura spp. larvae (Neff & Wallace, 1969). Nothing is known about the biology of Palaearctic species.