Mayfly Species Heterocloeon frivolum

Abdominal tergites 2-6 of male imago deep brown; genitalia of the modified moffati type (now a synonym of Baetis tricaudatus), the fourth joint of the forceps very long; no costal projection on hind wing, which has two veins only.

Turbinate eyes very large, deep blackish brown, the edges paler. Head and thorax shiny blackish. Light brown markings on the postero-lateral edges of the prothorax, the antero-lateral edges of the mesothorax, the pleural sutures (these are tinged with reddish), the lateral edges and side projections of the mesosternum, and the posterolateral margins of the mesonotum near the scutellum. Fore legs smoky brown; middle and hind legs pale yellowish. Wings hyaline, venation pale. Cross veins in the stigmatic area of the fore wing 6-7 in number, anastomosed. First intercalary in the first interspace rudimentary or lacking; all others well developed. Hind wing long and narrow, third vein entirely lacking. Costal projection entirely wanting.

Abdominal tergites deep brown, opaque. Sternites dull yellowish, becoming whitish posteriorly. Traces of a broken black spiracular line. Second joint of the forceps cylindrical, the fourth unusually long, being fully three times as long as wide, its tip knob-shaped. Between the bases of the forceps is a small raised plate.

This species, related, by reason of the complete lack of costal projection of the hind wing, to propinquus (now a synonym of Labiobaetis propinquus), dardanus (now a synonym of Labiobaetis dardanus), insignificans (now a synonym of Acentrella insignificans) and amplus (now a synonym of Heterocloeon amplum), may be distinguished from each of these by the deep brown color of the abdomen.