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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Dorsal view of a Neoleptophlebia (Leptophlebiidae) Mayfly Nymph from the Yakima River in Washington
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Mayfly Species Isonychia sicca (Slate Drakes)

Where & when

Time of year : June and July

In 16 records from GBIF, adults of this species have been collected during July (44%), June (31%), August (13%), April (6%), and March (6%).

In 1 record from GBIF, this species has been collected at elevation of 5135 ft.

Species Range

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Body length: 8.5-17 mm
Wing length: 9-15.5 mm

This species has short fore legs and dark venation; the abdomen is paler and redder ventrally.

Head light reddish. Antennae pale. Head of female light yellowish, shaded with light brown; a sub-triangular reddish patch between the eye and the lateral ocellus; a small blackish spot in the corners of the occiput. Thorax reddish black, the pleura reddish. Fore leg shorter than in other allied species, the fore tarsus about 3/4 the length of the tibia (in some specimens almost as long as tibia). Fore leg dark reddish to reddish black; tip of tibia black; tarsus smoky, the basal joint whitish except in the apical third. Middle and hind legs clear light yellow, sometimes tinged with reddish; tips of tarsi faintly smoky. Wings hyaline, with fine veins distinctly dark-tinged.

Abdomen reddish black dorsally, slightly paler and redder ventrally. Pleural fold also paler and redder than the tergites. The posterior margins are narrowly darker, and longitudinal dark dashes are usually present in the spiracular area. Faint traces of a paler mid-dorsal line and of smoky submedian streaks may be present. Tails whitish or greenish white, the basal joinings definitely reddish; joinings in remainder of tail faintly darker. Forceps base and basal joint of forceps reddish; forceps smoky; forceps base deeply excavated apically. Penes short and quite distinctly truncate apically (see fig. 126).

This species may be distinguished from I. manca (now a synonym of Isonychia sicca) by the shorter fore leg and the somewhat paler venation (brownish in sicca, blackish in manca.) The deep reddish black of the thorax separates it from Isonychia campestris, in which the thorax is light brown.

Described as I. manca

Body length 11 mm, wing length 11 mm

A reddish species having dark venation. Head reddish brown; antenna brown. A double median brown stripe is usually present on the vertex and occiput; a blackish triangular patch between the eye and the lateral ocellus; the corners of the occiput have a small dark spot. Thorax reddish brown, the scutella and the sternum almost blackish, the notum more reddish. Fore leg longer than in I. sicca, the fore tarsus as long as the tibia. Femur and tibia of fore leg reddish black; tibia black at the apex, the femur somewhat paler basally; tarsus brownish or smoky, the first joint pale in its basal two-thirds. Middle and hind legs yellow, femora sometimes shaded with reddish; last tarsal joint rather smoky. Wings hyaline, venation dark. The cross veins of the fore wing are distinctly heavier and darker than in sicca, except in the stigmatic area, where the cross veins are pale in male specimens, as in sicca. Abdomen reddish to reddish brown, distinctly redder than in sicca, and not much paler ventrally. Posterior margins of all segments, and the pleural fold, darker brownish to black. Faint traces of a paler median line, and of darker submedian streaks margining it. Tails pale, tinged with ruddy at the base, where the joinings are red. Other joinings not marked. Forceps base reddish, forceps smoky brown; the former deeply excavated apically. Penes of the sicca type, but rather broader and longer than in I. sicca, the upper margin less distinctly truncate.

Described as I. similis

Body length 12 mm, wing length 12.5 mm

Very similar in appearance to Isonychia serrata, but differing in the detailed structure of the penes. Head yellowish brown. Median carina and frontal margin purplish red. Black bar below frontal margin. Head of female yellowish white, with faint shading of brown in the corners of the occiput, and a very narrow dark line beside the eye near the lateral ocellus. Thorax dark reddish brown; margins of mesonotum, and a spot just anterior to the scutellum, pale yellowish. Pleura pale with lavender markings. Sternum very dark brown. Fore leg dark purple, femur somewhat lighter purple at base. Middle and hind legs yellow; claws and tarsal joinings purplish grey. Wings hyaline, venation dark purple. Stigmatic area faintly opaque, its cross veins tending to fork and anastomose.

Dorsum of abdomen bronze; the posterior margins and postero-lateral angles widely dark purplish brown, also the submedian lines bordering the paler mid-dorsal line, and a lateral line extending from the dark margin. A bronze triangle is thus left in the antero-lateral angle, a larger one nearer the median line, and the pale median streak. The anterior half of each sternite bronzy, its posterior half dark purplish brown. Four paler median dots on sternites 2-5. A dark purple spot in each anterolateral angle near the pleural fold, and purplish shading posterior to it. Tails purplish rose, darker at base. Joinings dark purplish brown. Forceps and forceps base yellowish brown. Forceps base only slightly excavated apically. Penes of the Isonychia arida type; the incurved portion bears 2 large and 2 small teeth (see fig. 125).

Female Spinner

Body length: 11-17 mm
Wing length: 11-15.5 mm

Subanal plate of female rather deeply excavated on its apical margin, its outer angles acute.

Described as I. manca

Body length 11 mm, wing length 11 mm

Head of female yellowish, shaded with brown. Subanal plate of female rather deeply excavated apically, its outer angles acute.

Described as I. similis

Body length 12 mm, wing length 12.5 mm

Head of female yellowish white, with faint shading of brown in the corners of the occiput, and a very narrow dark line beside the eye near the lateral ocellus. Subanal plate of female slightly excavated on its apical margin, rather than almost straight as in Isonychia serrata. Outer angles rounded.

Described as I. aurea (now a synonym for I. similis)(see above description)

Body length 16-17 mm, wing length 13-15.5 mm

A large yellowish orange species, known from the nymph and female imago only; venation dark brown.

Head greenish yellow, vertex washed with orange. Keel of median carina purplish red. Antenna light grey basally, filament darker grey. Posterior border of occiput, and its corners, purplish. Thorax greenish yellow, the scutella dark reddish brown. Purplish grey markings on pleura anterior to the wing roots and above the hind leg. Sternum light yellow; brown spot near center of mesosternum; three whitish transverse bands between legs. Fore femur very light brown at base, the distal half dark red-brown. Tibia and tarsus very dark purplish brown. Middle and hind legs greenish yellow; claws and three distal joints of tarsus dark grey; first joint washed with grey at apex. Venation dark purplish brown; many stigmatic cross veins are anastomosed.

Abdomen light orange, brighter dorsally. Posterior margins and postero-lateral angles of tergites purplish brown. Brown crescentic submedian marks on 3-9. Ventrally, a deep orange triangle, based on the posterior margin, attains the anterior border; located medially. Tails very dark reddish brown basally, slightly paler brown beyond base. Joinings slightly darker. Subanal plate shallowly excavated apically, its outer angles rounded.

Start a Discussion of Isonychia sicca


Mayfly Species Isonychia sicca (Slate Drakes)

Species Range
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