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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 Drunella cornutella (Small Blue-Winged Olives)

Few anglers can claim to have fished to hatches of this little olive mayfly, even though the species has been mentioned in popular angling entomologies like Caucci and Nastasi's Hatches or Knopp and Cormier's Mayflies. On many of the larger Eastern freestones, their emergence happens after the waters have warmed and dedicated anglers have turned their attention to streams that remain cold: headwaters, spring creeks, or tailwaters. Because good populations are usually not found in tiny streams, it is on some the Eastern tailwaters, like the branches of the Delaware, that fly fishers take notice of this species.

Drunella cornutella looks like a Mini-Me version of Drunella cornuta. In streams where the two species cohabit, size is the only easy way to tell them apart. Although there is some slight overlap between the largest cornutella and the smallest cornuta, the average difference is usually pretty obvious. Both share very similar coloration and morphology in all stages, even down to the little curved horns coming out of the frontal shelves of the nymph's heads.

Where & when

Time of year : Late June through August, sometimes into September on cold tailwaters

Preferred waters: Relatively fast riffles and runs

They usually follow cornuta by about 3-4 weeks, often when water temperatures approach their seasonal peak.

In 25 records from GBIF, adults of this species have mostly been collected during July (72%), August (12%), and April (8%).

In 8 records from GBIF, this species has been collected at elevations ranging from 649 to 2221 ft, with an average (median) of 1519 ft.

Species Range

Hatching behavior

Time of day : Usually early morning, though tailwater timing can differ

Habitat: Similar to cornuta, though not extending as far upstream

Water temperature: Emergence often commences as temperature approaches about 60-62 degrees.

On freestone streams, emergence often happens in the hour or so after dawn. On tailwaters with cold (hypolimnetic) releases in the summer, emergence can sometimes shift to the evening.

Spinner behavior

Time of day: Evenings

Habitat: Riffled areas

Like cornuta, the spinners are usually not very important. On larger, warmer freestones, the spinner fall is a dusk-into-dark affair, and fish may not feed until the water cools. On tailwaters, the spinners may compete with larger or more numerous species.

Juvenile biology

Current speed: Fast to moderate

Substrate: Rocks and gravel

The nymphs are virtually identical to Drunella cornuta, but smaller (6-8 mm at maturity). Slight differences in tibia length and other morphological niceties can be used to distinguish between cornutella and cornuta, but differences in size and emergence timing are more useful distinctions in the field. Range within a watershed also differs: cornutella tends to favor medium-to-large streams, and populations thin out in the headwaters; cornuta populations usually are found throughout the watershed, remaining strong into the headwaters.


To determine whether a specimen of Drunella belongs to Drunella cornutella, use the Key to Species of Drunella Nymphs.

Drunella cornutella Fly Fishing Tips

Larger "BWO" imitations used for Drunella cornuta need only be downsized, or "BWO" imitations used for Baetidae can upsized. Nymph imitations fish well during the period before or around emergence, even when opportunities to fish duns or spinners are slight.

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 Ephemerella cornuttela
Body length: 7 mm
Wing length: 7 mm

A species of the fuscata group (now a synonym of Drunella walkeri); very close to E. cornuta (now a synonym of Drunella cornuta), but considerably smaller.

Head and thorax deep blackish brown. Pleura tinged with reddish below the bases of the wings. Legs light brown, tinged with ruddy basally. Wings hyaline, venation pale; cross veins almost invisible. Abdominal tergites smoky brown; posterior margins darker brown; dark brown oval patches, rather indistinct, situated laterally. Sternites light olive brown; posterior sternites reddish brown, opaque. Forceps dull brownish, the third joint relatively shorter than in cornuta. Penes as in fig. 153. Tails dull whitish, the joinings faintly reddish brown.


Described in Needham et al (1935) as Ephemerella cornuttela

Nymph very much resembles a small cornuta (now a synonym of Drunella cornuta), but is darker in color, varying from dark reddish brown to brownish black. The frontal horn is considerably more strongly incurved. The tibial ‘thumb’ of the fore leg is short and blunt, not curving outward as in cornuta. Other structural characters as in cornuta. A dark basal spot is present on the tibia, also a dark median band; tarsus dark basally. Tails yellowish brown.

Start a Discussion of Drunella cornutella


Mayfly Species Drunella cornutella (Small Blue-Winged Olives)

Species Range
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