Mayfly Species Heterocloeon curiosum

Head: Pale with brown stripes between eyes, clypeus and area below eyes pale: antenna pale, rather long, extending back beyond posterior extremity of thorax.

Thorax: Contrasting light and brown areas; a brown triangular median area seated on anterior border of mesonotum, two brown submedian strokes on scutellum, a brown area near each antero-lateral corner; wing-pads pale; in female individuals, particularly, the thorax may be more uniformly brown; legs pale with dark longitudinal stroke in basal half of femur extended laterally at distal end; tip of femur, tibia and tarsus brownish; tarsal claws brown and pectinate.

Abdomen: Segments 4, 5, 9, and 10 largely pale, others darker though in many specimens the dorsum is more obscurely pigmented lacking such contrast on the segments; on the anterior three segments there is a median dark spot about the middle of each; on the following segments is a pair of dark dots, one on each side of the middle line and in the darker segments the area immediately behind each dark dot is somewhat paler; a pale irregular area along the lateral border becoming more extensive on the anterior segments; lateral caudal filaments long and unbanded, median filament very short projecting less than the width of the base of a lateral filament; gills sometimes with prominent branching tracheae and usually with an extensive grey granular area in the middle. Female nymphs with dorsum of abdomen obscurely marked brown and usually with two faint submedian darker lines and a median brown patch in segments 2 and 3.

Abdominal tergites 1-5 of male imago pale hyaline, shaded slightly with greenish yellow.

Head black. Turbinate eyes large, lengthily oval; sessile portion of stalk black, upper portion pale yellow; eyes deep brown. Thorax shiny black; pleura with brown shading below the wings. Legs whitish; fore femur blackish; coxae and claws brown. Wings hyaline, venation pale. 6-8 cross veins in the stigmatic area of the fore wing; somewhat aslant, some incomplete toward subcosta, and with short horizontal veins between several of them. Marginal intercalaries quite well developed except in the first interspace, where they may be wholly wanting, and between the anal veins. A reddish brown spot at base of fore wing. Hind wing a mere thread, much narrower than in Centroptilum; traces of a single vein may be present.

Abdominal segments 1-5 hyaline; tergites slightly shaded with pale greenish yellow, sometimes also tinged with light sepia brown. A small round red-brown spot is present on tergites 2 and 3, on the median line near the anterior margin; traces of similar spots may be present also on tergites 4 and 5. Tergites 6-10 light sepia brown; sternites shaded with opaque white. Spiracular area often marked with black. Forceps white. Tails white, the basal joinings red-brown. Between the basal joints of the forceps are two slight projections (penis-covers?) separated rather widely from one another. On the inner margin of the basal joint, near the base, is a slight protuberance. Second joint conical, tapering apically; third joint strongly bowed, a spinous projection at its inner apical margin; distal joint slightly bowed, about 2 1/2 times as long as wide (see fig. 167).