Mayfly Species Rhithrogena robusta

A large dark brown species, allied to Rhithrogena brunneotincta; dark mid-ventral ganglionic marks.

Head and thorax deep black-brown. A paler brown area anterior to wing roots; intersegmental areas of pleura pale brown. Fore leg dark brown; middle and hind legs slightly paler brown. No distinct dark streaks or dots on femora. Apices of femora, claws, tarsal joinings of middle and hind legs darker brown. Wings hyaline; stigmatic area distinctly brown-tinged, mainly in costal space, venation deep brown; longitudinal veins heavier than cross veins. Humeral cross veins light red-brown, pale at costal margin. Basal costal and subcostal cross veins quite pale and faint. Stigmatic cross veins of costa anastomosed as usual.

Abdominal segments 2-7 semi-hyaline; lateral and posterior margins of segments, and intersegmental areas, pale yellowish brown. Remainder of each tergite dark red-brown; of each sternite paler red-brown. Narrow paler mid-dorsal line and pale hyaline wedge-shaped submedian streaks near center of each tergite. Ganglionic areas of apical sternites blackish; these areas outlined in dark brown on basal and middle sternites. Segments 8-10 opaque, darker; almost, black-brown. Genitalia dark red-brown. Penes widely divergent apically, tips roundly truncate. Three spines on apical portion; two on inner margin, of which the smaller is directed upward and the larger, its tip slightly curved, is directed inward; the one nearer the outer margin is short and stout. A long spine-like lateral process, its tip truncate, slightly curved outward, on outer margin of each side, above base (see fig. 101).

The brown-tinted wings of R. brunneotincta, as well as structural details of the penes, distinguish that species from R. robusta, to which it is closely allied.