Mayfly Species Procloeon rivulare (Tiny Sulphur Duns)

Abdominal tergites 2-6 of male imago semi-hyaline, yellowish; anterior margins very narrowly brown.

Head reddish brown. Turbinate eyes quite large, rather kidney-shaped, bright orange margined with dark brown (alcoholic specimen). Thorax deep reddish to blackish brown; pleura and areas around the wing bases paler, yellowish brown. Legs yellowish or yellowish white, the fore leg rather more deeply tinged with yellow. Wings hyaline, venation pale. Stigmatic area of fore wing faintly opaque whitish; cross veins simple, slightly aslant, few in number; usually 5 to 6 only. Hind wing relatively long and narrow, much as in C. ozburni (now a synonym of Procloeon ozburni); costal projection strongly curved (see fig. 165).

Abdominal tergites 2-6 semi-hyaline, yellowish or yellowish white, often shaded faintly with olive brown; anterior margins very narrowly dark brown. Traces of pale brown submedian streaks on each; a pale brown dot anteriorly above the pleural fold; a short black oblique line at each spiracle; often a broken blackish spiracular line. Sternites 2-6 paler, more whitish than the tergites; unmarked. Segments 7-10 opaque, both tergites and sternites russet or chestnut brown, the latter only slightly paler than the tergites; anterior margins very narrowly darker. Forceps and tails whitish. Tubercle at inner margin of second forceps joint very poorly developed. ‘Penis-cover’ quite large, more or less truncate on the apical margin; distal joint of forceps short (see fig. 166).

This species seems closely allied to C. caliginosum (now a synonym of Procloeon caliginosum) and C. bellum (now a synonym of Procloeon bellum); the tubercle on the inner margin of the second forceps joint is less well developed, however, than in either of those two species. The reddish brown of the posterior sternites separates it from caliginosum, in which tergites 7-10 are chocolate brown, the sternites paler. From bellum it may be distinguished by the total lack of reddish markings on the basal abdominal tergites.

Head of female flesh-colored; a dark brown median streak on vertex, extending forward between the ocelli. Mesothorax pale reddish brown; sutures narrowly darker, intersegmental areas pale. Legs yellowish white, tarsi often tinged with smoky brown. Venation pale yellowish brown. Abdomen pale reddish dorsally, venter yellowish. Broken black spiracular line present.

The nymph has small dorsal flaps on the basal pairs of gills.

The nymph resembles bellum (now a synonym of Procloeon bellum) and ozburni (now a synonym of Procloeon ozburni) in its abdominal color pattern, but has paler tails and darker tarsi than either of those species; the sternites are pale, not marked with brown as in ozburni.