Vockeroth (1969) provided an excellent diagnosis of this genus.
Medium to large, moderately slender to robust species with very clear wings, oblique and often lunulate abdominal maculae, and usually with strongly swollen frons in male. Eye densely haired although as much as posterior half sometimes almost bare in female. Dorsal 2/3 of eye of male with an extensive area of almost uniformly enlarged facets, this area anteriorly reaching the margin of the eye above and progressively farther from it below, posteriorly narrowly separated from margin throughout; facets much less enlarged in occidentalisShannon than in other species. In all species but occidentalisfront of male greatly broadened, strongly swollen, and with abundant erect black hairs, eyes meeting anteriorly at an angle of 135°-145°. In occidentalisfront of male only slightly broadened and swollen but with moderately conspicuous black hair, eyes meeting anteriorly at an angle of 120°. Front of female only slightly swollen but very broad, at vertex from 0.28 to 0.32 head width. Face in both sexes moderately to strongly swollen, sometimes more strongly produced below, with moderately large and sometimes slightly compressed tubercle; face at its widest point from 0.50 to 0.66 times as broad as head. Face bright to pale yellow, usually with narrow brown to black median vitta which always ends just above the tubercle. Basoflagelloemere from 1.4 to 1.9 times as long as broad. Scutum shining black, always pale pilose, usually very obscurely yellowish laterally, rarely with well-defined, entire, pale yellow, lateral margin. Scutellum with the disc or rarely the entire surface translucent, dull yellow to dark yellow-brown usually with a narrow to broad posterior margin opaque and slightly paler. Pleura black, densely pale pilose, subshining to moderately pollinose, sometimes with posterodorsal corner of anepisternum distinctly yellowish. Dorsal and ventral katepisternal pile patches narrowly to broadly joined posteriorly and rarely narrowly joined anteriorly, otherwise narrowly separated; the ventral patch extending much farther dorsad than usual, anteriorly extending broadly anterodorsad to dorsoanterior corner of katepisternum. Metasternum bare. Vein R4+5 usually very broadly and shallowly dipped into cell R4+5 more rarely nearly straight. Microtrichia greatly reduced, absent from at least basal half of wing and usually from almost entire wing; usually at least a few short, weak, erect, scattered microtrichia present near posterior wing margin, rarely such microtrichia sparsely present on outer half of wing. Abdomen narrowly to broadly oval, flattened, strongly margined from just beyond base of tergite 2 to apex of tergite 5. Tergites 2 to 5 each with a pair of slightly to strongly oblique, narrow to broad, whitish-yellow to bright yellow maculae which usually are clearly separated from the lateral margin; maculae of tergites 3 to 4 at least slightly, and usually strongly, lunulate. Sternites very variable in colour; usually yellow, with or without transverse black median bands or longitudinal lateral and median stripes, rarely black with broad yellowish-white incisures. In melanostoma (Macq.) only, the yellow maculae of the tergites, and the entire surface of the sternites, when viewed obliquely from in front, show transverse striae of very fine pale pollen (Vockeroth 1969).