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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 Glossosoma (Glossosomatidae) (Little Brown Short-horned Sedge) Caddisfly Larva from the Yakima River in Washington
I caught this tiny larva without a case, but it seems to key pretty clearly to to Glossosomatidae. From there, the lack of sclerites on the mesonotum points to either Glossosoma or Anagapetus. Although it's difficult to see in a 2D image from the microscope, it's pretty clear in the live 3D view that the pronotum is only excised about 1/3 of its length to accommodate the forecoxa, not 2/3, which points to Glossosoma at Couplet 5 of the Key to Genera of Glossosomatidae Larvae.
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 Baetisca obesa (Armored Mayflies)

I have no experience with important fishing to this species, but it is probably the Baetisca species that Ernest Schwiebert discussed in Nymphs as locally abundant on one stretch of a Catskill river.

See the Baetisca genus page for more details.

Where & when

This is the most common species of Baetisca in the East. Its official range extends into the Midwest, but populations there are overshadowed by the more prolific Baetisca laurentina.

In 4 records from GBIF, adults of this species have been collected during May (50%), March (25%), and June (25%).

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: 7-8 mm
Wing length: 9-10 mm

Abdominal tergites of male imago pale reddish brown; venation yellowish on costal margin, otherwise pale.

Head ferruginous; antenna similar in color, the filament generally pale. Thorax ferruginous; sternum "paler behind, especially the space between the posterior coxae" (Walsh). Legs pale greenish yellow; in the fore leg, the apex of the femur, tips and joinings of the tarsi, are slightly fuscous. Middle and hind legs paler, only the tips of tarsi cloudy. Wings hyaline; longitudinal veins of the costal margin yellowish, radius piceous at the extreme base, "a few of the principal veins slightly tinged with fuscous” (Walsh); all other veins pale. Cross veins almost invisible. Abdominal tergites pale reddish brown, the posterior margins whitish; sternites somewhat paler. Tails whitish, the joinings fuscous, especially in the basal portion. Forceps pale, sometimes reddish toward the tip. Genitalia as in fig. 148.


The nymph has both dorsal and lateral spines on the mesonotal shield; the genae are somewhat produced into spine-like processes; the frontal projections are very prominent and well developed. The paler venation separates this species from the allied Baetisca laurentina.

Specimens of the Mayfly Species Baetisca obesa

1 Female Dun
2 Nymphs

Discussions of Baetisca obesa

Similiar found in Minnesota
1 replies
Posted by PRohlfsen on May 11, 2007
Last reply on May 11, 2007 by Troutnut
I was fishing the Vermillion River in Minnesota south of the metro area and found a nymph similar to the Armored Mayfly Nymph. It did appear to be much more green in color. It was moving through a really muddy part of the stream, which I found interesting.

Start a Discussion of Baetisca obesa


Mayfly Species Baetisca obesa (Armored Mayflies)

Species Range
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