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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 Amphizoa (Amphizoidae) Beetle Larva from Sears Creek in Washington
This is the first of it's family I've seen, collected from a tiny, fishless stream in the Cascades. The three species of this genus all live in the Northwest and are predators that primarily eat stonefly nymphs Merritt R.W., Cummins, K.W., and Berg, M.B. (2019).
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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Isonychia bicolor (Mahogany Duns)

This is by far the most important species of Isonychia. Many angling books once split its credit with the species Isonychia sadleri and Isonychia harperi, but entomologists have since discovered that those are just variations of this abundant species.

See the main Isonychia page for more about these intriguing mayflies.

Where & when

Time of year : Late May through October

This species has two distinct emergence peaks, once in late spring or early summer and again in the fall. It may be found on the water in lighter numbers at any time in between.

The first peak begins in the freestone mountain streams of Pennsylvania in early to mid-June. It reaches the Catskills in mid-June and continues through early July. The Upper Midwest and the northern ranges of the Appalachians peak from late June through mid-July.

The second peak usually comes in September, and those flies are more likely to emerge at midday than their Summer brethren.

Isonychia bicolor is only abundant on certain rivers, even in ideal regions. One river can be fantastic while the next drainage over shows no sign of Isonychia.

In 23 records from GBIF, adults of this species have mostly been collected during June (43%), July (22%), August (17%), and September (9%).

In 2 records from GBIF, this species has been collected at elevations of 600 and 761 ft.

Species Range

Hatching behavior

Time of day : Usually concentrated in evening with a peak near dusk, especially in Summer; may be sporadic all day

Habitat: Medium to fast water; usually riffles

Water temperature: Below 70°F

The duns are known for emerging by crawling out onto shore, and one of my streamside pictures shows the nymphal shucks they left behind on a rock. But they may also emerge on the surface in standard mayfly style. Many books attribute this to high water submerging the convenient places to crawl out of the water, and others attribute it to the size of the stream, but I think it is more likely a combination of these things, current speed, and geographic region. My home river in Wisconsin seems to have entirely midstream emergence, and some Catskill rivers display both.

When the duns emerge midstream, they're clumsy and make excellent targets for trout. Their heavy bodies ride low in the water, and they often make several failed attempts at flight before succeeding. Low-riding dry flies are good imitations of the placid duns, and heavily hackled patterns may be twitched and skittered to imitate the takeoff commotion.

I've had some of my best dry-fly fishing during sporadic hatches of Isonychia bicolor duns. Trout have an unusually ferocious appetite for them compared to other mayflies of similar size and profile, and they will often smash an imitation when no real duns are on the water.

The duns have pale off-white fore tarsi, which they often hold up on the air when they're resting. This feature has given them both the scientific name bicolor and one of their common names, the "White-Gloved Howdy."

Spinner behavior

See the main Isonychia page for spinner details.

Nymph biology

Isonychia has some of the most interesting mayfly nymphs; see the main page for the genus for details, which do not really differ between species.

Some books have made a big deal over the strength of the stripe running down the back of Isonychia bicolor nymphs, because it was a confusing identification characteristic used to separate them from Isonychia sadleri. Because the two species are now synonyms, the confusion is no longer needed--they're all Isonychia bicolor. However, anglers tying nymph imitations should collect some local specimens before putting bright white stripes down the backs of all their flies. In some populations the stripes are only a little bit lighter than the rest of the body and don't run its whole length. In others, the stripes are indeed vivid enough to warrant the prettier imitation.

Isonychia bicolor Fly Fishing Tips

The size and color of Isonychia bicolor adults may vary throughout the season on a single river, so one should catch specimens from time to time and check them against their imitations.

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

Described in Needham et al (1935) as Isonychia christina
Body length: 10-16 mm
Wing length: 10-14 mm

A bright red species, with pale smoky fore tarsi and very pale brownish venation.

Head reddish. Vertex of female cream-colored, largely suffused with reddish. Wide reddish areas between the ocelli, the eye and the lateral ocellus, and a wide median geminate red stripe; corners of occiput purplish black, the posterior margin reddish. Thorax very dark red, pleura somewhat paler. Deep rose markings on the pleura and mesonotum, and a pale area anterior to the scutellum. Femur and tibia of fore leg deep reddish black, each paler basally. Tarsus pale smoky brown, faintly paler at the joinings and the base of the first joint. Claws marked with deep rose. Tarsus fully as long as the tibia. Middle and hind legs yellowish white; claws and last tarsal joints tinged with reddish. Coxae deep rose colored. Wings hyaline, the stigmatic area opaque white. Venation very pale brownish; each longitudinal vein outlined by two narrow dark lines.

Abdomen intense red dorsally and ventrally, apical segments brighter. Posterior margins of all segments black. Pleural fold margined by an interrupted black line, parallel to which dorsally is another black line. Paler markings on tergites almost obsolescent. Ganglionic areas of the sternites paler, margined by dark oblique submedian streaks. Tails white, washed with brownish red at the base; 4 to 6 of the basal joinings are usually red, all others colorless. Genitalia reddish, forceps smoky brown, the terminal joint pale. Forceps base and subanal plate of female deeply excavated apically. Penes of the albomanicata type (now a synonym of I. bicolor).

Described as I. circe

Body length 9.5-10.5 mm, 9-10 mm

A rather pale reddish species, with smoky fore tarsi and pale venation.

Head yellowish. A black spot between the eye and the base of the antenna. Antenna pale, tinged with red, its filament dusky. Eyes purplish. Head of female flesh-colored, with indications of a faint dusky pattern near the middle of the vertex, from which extensions may be traced forward and backward. A reddish line along the inner margin of the eye; space between eye and lateral ocellus reddish; corners of occiput black. Thorax reddish, tinged with yellow on the sides of the scutella, the pleura, and a spot anterior to the mesonotal scutellum. Scutellum, sternum and a small area in the middle of the anterior portion of the mesonotum dark reddish brown. Fore leg somewhat shorter than the body; tarsus usually equal to tibia; tibia relatively short, not much longer than the femur. Femur dark red, paler at base; tibia dark reddish black. Tarsus smoky brown; basal two-thirds of the first joint, basal half of the second, and a narrow margin at base of third, pale yellowish. Middle and hind legs clear pale yellow; claws faintly smoky at tip; coxae marked with rose. Wings hyaline. Stigmatic area somewhat opaque, whitish. Venation very pale yellowish.

Abdomen pale red, slightly duller and paler ventrally. Basal segments darker red; apical segments brighter and paler, often tinged with yellow and with powdery white areas; opaque. Middle segments semi-translucent. Posterior margins of all segments distinctly purplish black, these bands wider on the tergites. Pleural fold pale in the basal half of each segment, smoky apically; a small black mark at the stigma. Mid-dorsal line obscurely pale; narrow at base, wider apically. Opaque powdery white areas may margin it, on the apical tergites. Tails white, a few basal joints faintly yellowish. Two or three basal joinings bright red; all others white. Genitalia amber, sometimes tinged with rose. Forceps base rather deeply excavated apically. Penes of the I. albomanicata type (now a synonym of I. bicolor); rather broad, and turning-outward apically. Subanal plate of female with a moderately deep apical excavation; its outer angles subacute.

Described as I. fattigi

Body length 10-11.5 mm, wing length 10.5-11.5 mm

A dark red species with smoky fore tarsi and pale venation.

Front of head translucent; carina, frontal margin and bases of antennae marked with red. A black spot between the eye and the antenna; blackish shading between ocelli. Head of female whitish. A geminate rose streak on the middle of the vertex; a large red spot between the eye and the lateral ocellus; corners of occiput black, and black shading along the entire posterior margin. Thorax reddish brown; mesonotal scutellum, metanotum and sternum of male blackish. Pleura paler, with rose markings, which are more extensive on the female. Femur and tibia of fore leg reddish black; tarsus smoky brown, somewhat darker at apex of each joint. Fore tarsus of female pale at base of each joint, reddish apically. Middle and hind legs yellowish white; coxae rose colored; distal joint and joinings of tarsus reddish. Wings hyaline; venation of male pale, of female very pale brown.

Abdomen dark wine red. Posterior and lateral margins of segments rather widely blackish. Obscure paler mid-dorsal line, and traces of obscure submedian and lateral streaks. Not paler ventrally, but sternites rather translucent at base. Apical segments brighter red. Tails white; at base tinged with red, joinings reddish, elsewhere pale. Forceps base reddish brown; deeply excavated on its apical margin. Forceps reddish brown, terminal joints pale. Penes of the albomanicata type (now a synonym of I. bicolor). Subanal plate of female rather deeply excavated apically, its outer angles subacute. Lateral extension of the 9th segment translucent.

Described as I. harperi

Body length 12 mm, wing length 12 mm

A rather dark red species; fore tarsus reddish brown, venation pale.

Head reddish. Head of female yellowish; a large red spot between the eye and the lateral ocellus; corners of occiput black, posterior margin and a geminate median streak blackish. Notum and sternum of thorax reddish black, pleura paler, rather sharply distinct from the dark notum. Scutella, the mesosternum posteriorly, and a small area on anterior of the mesonotum, very deep reddish black. A rose spot anterior to the mesonotal scutellum. Fore tarsus slightly longer than the tibia. Femur dark red, paler at base; tibia deep reddish black. Tarsus reddish brown, the distal joints and the basal third of the first joint somewhat paler. Middle and hind legs greenish white, claws reddish. Wings hyaline, iridescent. Venation very pale yellowish; each longitudinal vein outlined by two narrow dark lines. Stigmatic area opaque whitish.

Abdomen dark wine red; all segments slightly paler basally, so that abdomen appears annulate. Apical segments brighter red, opaque; middle segments semi-translucent. Posterior margins of all segments darker; wider and darker on the tergites. Two dusky lines mark the pleural fold, one on the margin and the other dorsal to this; often a pale streak lies next to the pleural fold ventrally. A short black dash marks each stigma. Pale dorsal markings usually obscolesent. Tails deep reddish brown at the base, white in middle and at tips. All joinings colorless. Forceps base and penes reddish brown; forceps deep smoky brown, the terminal joint pale. Forceps base and subanal plate of female deeply excavated apically. Penes of the albomanicata type (now a synonym of I. bicolor).

Described as I. matilda

Body length 12 mm, wing length 12 mm

A dark red species; fore tarsus and tails dark reddish brown, venation dark brown.

Head dark reddish black. Frontal portion of head of female pale brown, darker at bases of antennae and ocelli. Vertex and occiput greyish; dark reddish brown between bases of ocelli; a narrow purplish brown line next to the inner margin of the eye; the corners of the occiput purplish brown, also the posterior margin. Notum and sternum of thorax deep reddish black; pleura reddish brown. Creamy white areas suffused with deep rose are on the pleura above each leg, and before the wing root. Fore tibia deep reddish black; femur very dark red, paler basally; tarsus entirely dark reddish brown. Tarsus slightly longer than the tibia; entire fore leg 5/6 as long as the body. Middle and hind legs yellowish white, tinged with pale reddish, the femora with indistinct dusky pencilings; claws and last tarsal joint dusky. Wings hyaline, iridescent. Venation dark reddish brown; stigmatic area, and sometimes the apical third of the fore wing, tinged faintly with brownish. Cross veins of the stigma numerous, often forked and anastomosing.

Abdomen deep red dorsally and ventrally, the apical segments brighter red. Basal and middle segments with a narrow pale translucent area anteriorly; remainder of each segment somewhat semi-translucent. Apical segments opaque. A double black line margins the pleural fold, and is paralleled dorsally by a third dark penciling. Only faint traces of a paler mid-dorsal line and of submedian streaks are present dorsally, but on the venter the pale mid-ventral streak and submedian marks are quite evident. Narrow black lateral streaks occur on the middle tergites. Tails dark reddish brown, paler brown apically; joinings slightly paler. Genitalia dark reddish brown. The combined terminal joints of the forceps are approximately equal to the long second joint. Forceps base deeply excavated apically, as is the subanal plate of the female. Penes of the albomanicata (now a synonym of I. bicolor) type.

Described as I. pacoleta

Body length 11-12 mm, wing length 10-11 mm

A yellowish orange species with pale venation. Head yellowish brown. Frontal margin narrowly blackish; a black spot at lower corner of each eye, and another below the frontal margin. Antenna greyish white. Pronota and metanota yellowish brown; mesonotum yellow, a reddish brown area near anterior margin. Scutella dark reddish brown. Pleura cream yellow with lavender markings. Sternum reddish brown. In female, corners of occiput purplish; no dark mark between eye and lateral ocellus. Fore femur and tibia purplish brown; the femur pale at base and in center along outer margin almost to the apex, the tibia narrowly pale at the extreme base. Tarsus white, faintly marked with lavender. Middle and hind legs white with yellow tinge; tips of claws and tarsal joinings faintly purplish grey. Wings subhyaline, veins entirely pale; stigmatic area opaque, some cross veins forked.

Abdomen light orange to bronze, the first 3 or 4 segments washed with rose; posterior margins of all segments purplish brown, bands on the tergites wider. Very narrow pale mid-dorsal line, bordered on posterior tergites with faint and diffuse submedian streaks. Pleural fold greyish white with a faint lavender streak. On sternites 2-7, indistinct light transverse dots may be evident. Tails white, very faintly flushed with flesh color or pale lavender at base; no darker joinings. Forceps base and forceps yellowish. Forceps base deeply excavated at its apical margin, as is also the subanal plate of the female; outer angles of the latter acute. Penes resembling those of I. albomanicata (now a synonym of I. bicolor), but considerably broader (see fig. 125).

Described as I. sadleri

Body length 14-16 mm, wing length 13-14 mm

A large red species; fore tarsi smoky brown, venation pale.

Head pale reddish. A black spot between eye and base of antenna. Median carina, frontal margin and base of antenna reddish; antennal filament pale. Ocelli ringed at base with dark red-brown. Eyes purplish, transverse band pale. Head of female yellowish, with median reddish geminate streak; red areas between eyes and lateral ocelli; occipital corners black. Thorax very dark reddish brown; scutella, median area of mesonotum and mesosternum, dark reddish black. Pleura rather paler brown, with deep rose markings. Narrow rose markings on sides of mesonotum. Fore leg shorter than body; tarsus fully as long as tibia. Femur very dark red-brown, paler basally. Tibia deep red-black. Tarsus dark smoky brown; fourth joint and bases of first and fifth joints tend to be paler. Middle and hind legs yellowish white; claws, distal tarsal joint and tarsal joinings reddish; coxae rose-colored. Wings hyaline, venation very pale yellowish brown. Longitudinal veins each outlined by two parallel brownish lines. Stigmatic area opaque whitish.

Abdomen intense dark red, brighter apically. All segments with rather wide black posterior margins, those of tergites widest. Traces of a paler mid-dorsal line are most evident on apical segments; reddish submedian streaks may margin it apically, becoming smoky brown on middle and basal tergites. Pleural fold margined with black, a narrow paler area at each end of each tergite. Tails white, tinged with reddish brown basally. 6 or 8 basal joinings red, all others colorless. Genitalia reddish brown. Forceps base deeply excavated apically; its lateral arms rather wider than in I. albomanicata (now a synononym of I. bicolor). Penes of the albomanicata type. Subanal plate of female rather deeply excavated apically, its outer angles subacute.

Female Dun

Body length: 11 mm
Wing length: 14 mm

Vertex of head, and thoracic notum, yellowish brown. Thorax ventrally with a black spot on each side. Fore femur dark reddish brown; tibia rather less red, more blackish; tarsus very light yellowish white. Middle and hind legs yellow to pale amber, the tibiae and tarsi whitish; claws pitch brown. Wings as in all subimagos of this genus. Abdomen brownish, paler beneath. Tails light yellowish brown.

Specimens of the Mayfly Species Isonychia bicolor

5 Female Duns
1 Male Spinner
1 Female Spinner
12 Nymphs

5 Streamside Pictures of Isonychia bicolor Mayflies:

1 Video of Isonychia bicolor Mayflies:

Isonychia nymph swimming around

These nymphs may be the best swimmers of all North American mayflies.

Discussions of Isonychia bicolor

1 replies
Posted by JMV on Sep 21, 2006
Last reply on Sep 21, 2006 by Troutnut
Great site, I'm an Iso. fanatic... JM

Start a Discussion of Isonychia bicolor


Mayfly Species Isonychia bicolor (Mahogany Duns)

Species Range
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