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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.

Ventral view of a Hydropsyche (Hydropsychidae) (Spotted Sedge) Caddisfly Larva from the Yakima River in Washington
With a bit of help from the microscope, this specimen keys clearly and unsurprisingly to Hydropsyche.
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 Baetis tricaudatus (Blue-Winged Olives)

Baetis tricaudatus is undeniably the most widespread and abundant baetid on the continent and arguably the most important mayfly species to trout and anglers alike. Eastern anglers used to know these important mayflies by the storied name of Baetis vagans. Conversely, the usually much larger and late Fall hatching brood of Baetis tricaudatus was considered an important Western species with its own tradition. But, entomologists recently determined that they are both in fact the same species. The nomenclature conventions guiding entomologists do not account for a name's regional fame among fishermen, and new or obscure species names may replace their old favorites. Sometimes taxa with disparate traditions are combined. Baetis vagans is one such casualty. Fortunately, trout think like Shakespeare: A rose by any other name would smell as sweet. The rose that was vagans has lost none of its charm. This species is multibrooded with the hatches of Spring being larger flies. As the weather warms the following broods are composed of progressively smaller flies. In the East, they range in size from 16 to 20. In the West, they may run a size larger.

Where & when

Time of year : Three broods: Late March to April, late June, and September

Altitude: Any elevation, including high-altitude Western creeks

Rivers and streams with good populations usually produce three broods with distinct peaks, but each may last long enough to overlap with the next, meaning that this mayfly can be found at almost any time of the season. Out West, tricaudatus is the largest of the "Baetis" group with specimens often reaching as large as size 14 (9 mm). Their hatches tend to be heaviest at the "bookends" of the season. Together with the similar appearing but smaller Diphetor hageni (prev. B. parvus), they make up the principal superhatches of Western Olive and Iron Quills.

In 58 records from GBIF, adults of this species have mostly been collected during June (28%), April (24%), May (12%), August (10%), July (10%), September (5%), and March (5%).

In 143 records from GBIF, this species has been collected at elevations ranging from 49 to 10522 ft, with an average (median) of 5118 ft.

Species Range

Hatching behavior

Time of day : Warmest part of the afternoon (at least for the early brood)

Habitat: Slow water

Water temperature: 40°F and up

Soft overcast days with intermittent drizzle can stimulate some of the finest hatch activity of the season. This species exhibits the baetid trait of hatching in "waves" intermixed with spinner falls. This has significant impact on the success or failure of angling methods or flies employed, and the astute angler should take note because the fish sure do. As the hatch begins with the first wave, the majority of the drift will be made up of nymphs followed by emergers with duns finishing as the most numerous form. During the short lull that follows (which can last a few minutes or as long as an hour or more), a spinner fall may occur. Then the cycle repeats itself. Under the right conditions this may happen a half-dozen times or more.

Spinner behavior

Time of day: Variable

Habitat: Same habitat as the emergence

Many baetids, including tricaudatus share the curious trait of landing on objects jutting out of the water and crawling down to deposit their eggs submerged. Even more curious is that the males have been reported to engage in the same behavior. One of their favorite locales is often the legs of stationary wading anglers. There is some evidence of their swimming ability as well. Gravid females and males can also be observed floating placidly on the water at times, especially in sections where weeds just barely meet the surface, perhaps to float up against their hard to see tips for crawling down to oviposit.

Nymph biology

Current speed: Any

Substrate: Gravel, vegetation

These nymphs are very active swimmers and are a substantial part of the drift at almost any time of the year.

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: 4.5-7 mm
Wing length: 5-8 mm

No color characters given. Genitalia of the moffati type (now a synonym of Baetis tricaudatus). Hind wing with three veins, the third running close to the hind margin to beyond the middle. Veins 1 and 2 converge distinctly toward the tip. Upper margin of hind wing almost straight, beyond the costal projection, which is quite well developed. Between veins 2 and 3 are two intercalaries, the second of which extends up from the hind margin toward the 2nd vein. The inner margin of the second (cylindrical) joint distinctly irregular and of a wrinkled appearance.

Described as B. incertans

Body length 4.5 mm, wing length 5 mm

Abdominal tergites 2-6 of male imago olive brown, the posterior margins deeper ruddy brown; genitalia of the moffati type (now a synonym of Baetis tricaudatus).

Turbinate eyes moderate in size; red-brown. Head and thorax deep olive brown. Scutellum and posterior margins of the mesonotum only slightly paler; anterior projection of the metanotum whitish; pleura with ruddy brown tinges. Femora pale yellowish; tibiae and tarsi dirty whitish, a smoky apical spot on the fore tibia. Wings hyaline, venation pale. About 6 cross veins in the stigmatic area of the fore wing, with only slight granulations between them; veins not anastomosed. First pair of intercalaries obsolescent, second and third pairs rather short, subequal. Hind wing broad, more than twice as long as its width; third vein rather weak, running close to the hind margin. Between the 2nd and 3rd vein are two strong intercalaries.

Abdominal tergites olive brown; 2-6 semi-translucent and narrowly deeper ruddy brown on the posterior margins; 7-10 opaque. Sternites pale yellowish white, the apical ones opaque. Tails and forceps white. The smaller size, smaller turbinate eyes, and abdomen more evenly brown in color, separate this species from vagans (now a synonym of Baetis tricaudatus).

Described as B. moffati

Body length 7 mm, wing length 7.5 mm

Abdominal tergites 2-6 of male imago brownish; genitalia of the moffati type (now a synonym of Baetis tricaudatus), see fig. 164, (second forceps joint cylindrical, no tubercle on the inner apical margin of the first joint).

Turbinate eyes somewhat shriveled, but appear to be of moderate size; reddish orange in color. Antennae pale reddish. Head and thorax deep red-brown, the mesonota and metanota much darker red-brown, almost blackish on the margins of the mesonotal scutellum. Postero-lateral and lateral margins of the pronotum darker than central area. Antero-lateral margin of mesonotum, and several sutures of pleura and sternum, blackish. Fore leg reddish brown, tip of tarsus rather paler. Apical margins of coxa, trochanter and femur, and base of tibia, deep reddish black. Only stubs of middle and hind legs remain; these are paler reddish brown. Wings hyaline, venation pale yellowish brown in costal half of fore wing, elsewhere pale. Hind wing rather large; three veins, the third rather faint and running close to the hind margin to a little beyond the middle. Costal projection well developed. No intercalaries (see fig. 163).

Abdominal segments 2-6 light reddish brown, in part semi-translucent; tergites only slightly darker than sternites; intersegmental areas pale hyaline, giving the abdomen an annulate appearance. Segments 7-10 opaque, darker reddish brown, especially on the dorsum. Genitalia of the moffati type; basal joint of forceps large, with no tubercle on the inner apical margin; second joint swollen and cylindrical in shape, usually considerably shorter than the third more slender joint; fourth joint much shorter than the third, about as long as the second.

Described as B. parallelus

Body length ? mm, wing length 8 mm

Head and thorax pale brown; margin of mesonotum hardly more yellow; remainder of mesonotum dark shining brown, sternum partly brown. Fore legs pale brownish, middle and hind legs pale. Wings hyaline, venation pale. Intercalaries of the fore wing moderately long. Hind wing elongate, the sides nearly parallel. Third vein quite well developed. One rather long intercalary between the 2nd and 3rd veins. Costal projection sharp.

Abdominal tergites 2-6 brownish; 7-10 darker brown. Sternites pale. Tails pale, darker at the joinings.

Described as B. vagans

Body length 6 mm, wing length 6 mm

Abdominal tergites 2-6 of male imago dull olive brown; genitalia of the moffati type (now a synonym of Baetis tricaudata).

Turbinate eyes very large, deep red-brown. Head and thorax deep pitch brown; a pale yellowish spot is usually present on each side of the anterior tubercle of the mesonotum; scutellum pale brown; anterior to the scutellum, and on the postero-lateral margins, are light ruddy brown shadings. Paler brown shading on the pleural sutures.

Anterior margin of metanotum, and its anterior projection, distinctly whitish. Legs dull yellowish white; fore femur distinctly tinged with brown, and a small dark patch near the apex of the fore tibia. Wings hyaline, venation pale. Cross veins in the stigmatic area of the fore wing strongly anastomosed, forming an irregular network. First pair of intercalaries usually not well developed, may be broken; second and third pairs moderately long, subequal. Hind wing twice as long as its width. Third vein well developed, running to the middle of the hind margin. One or two intercalaries between veins 2 and 3, and traces of one or two intercalaries between veins 1 and 2 (see fig. 163).

Abdominal tergites 2-6 dull olive brown, paler on the anterior margins, semi-hyaline; a dark ruddy brown line marks the posterior margin, but does not extend along the lateral area. Tergites 7-10 opaque, ruddy or chocolate brown. Sternites 2-8 pale smoky semi-translucent; 7-10 pale whitish brown. Tails white. Tracheae in the spiracular area more or less marked with black. It is larger than the related species B. incertans (now a synonym of B. tricaudata), and with larger turbinate eyes.

Specimens of the Mayfly Species Baetis tricaudatus

2 Male Duns
3 Female Duns
2 Male Spinners
2 Female Spinners
4 Nymphs

Start a Discussion of Baetis tricaudatus


Mayfly Species Baetis tricaudatus (Blue-Winged Olives)

Species Range
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