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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
Baetis

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 Sweltsa (Chloroperlidae) (Sallfly) Stonefly Nymph from the Yakima River in Washington
This species was fairly abundant in a February sample of the upper Yakima.
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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Dark Cahills

Like most common names,"Dark Cahill" can refer to more than one taxon. They're previewed below, along with 9 specimens. For more detail click through to the scientific names.

Mayfly Species Stenonema ithaca

These are sometimes called Dark Cahills.
Lateral view of a Female Stenonema ithaca (Heptageniidae) (Light Cahill) Mayfly Dun from the Little Juniata River in Pennsylvania
This female looks very much like a male I collected a few hundred miles away a few days later, so I'm guessing it's the same species, which I believe is Maccaffertium mediopunctatum.
Lateral view of a Stenonema ithaca (Heptageniidae) (Light Cahill) Mayfly Nymph from Paradise Creek in Pennsylvania
This specimen seems to be of the same species as a dun I photographed which emerged from another nymph in the same sample.

Mayfly Species Stenonema vicarium

These are sometimes called Dark Cahills.
In the East and Midwest this is one of the most important hatches of the Spring. They are large flies which emerge sporadically, making for long days of good fishing.

This species contains the two classic Eastern hatches formerly known as Stenonema vicarium and Stenonema fuscum, the "March Brown" and "Gray Fox." Entomologists have discovered that these mayflies belong to the same species, but they still display differences in appearance which the trout notice easily. Anglers should be prepared to imitate both types.
Artistic view of a Male Stenonema vicarium (Heptageniidae) (March Brown) Mayfly Dun from the Namekagon River in Wisconsin
I collected this mayfly on the same trip as a female of the same species. After these photos it molted into a spinner. This is the form of Stenonema vicarium which anglers call the "Gray Fox."
Lateral view of a Female Stenonema vicarium (Heptageniidae) (March Brown) Mayfly Spinner from the Namekagon River in Wisconsin
I collected this mayfly on the same trip as a male of the same species. They are Maccaffertium vicarium mayflies of the type formerly known as Stenonema fuscom, the "Gray Fox."
Dorsal view of a Stenonema vicarium (Heptageniidae) (March Brown) Mayfly Nymph from the Beaverkill River in New York

Mayfly Species Stenonema femoratum

These are very rarely called Dark Cahills.
This species is only one of many minor Light Cahills and the only species remaining in the Stenonema genus after the classic superhatches were reclassified into Maccaffertium and Stenacron. Its habitat and behavior is not different from those species, except as noted below, and you should consult the pages for those genera if you need angling information for this hatch.
Dorsal view of a Stenonema femoratum (Heptageniidae) (Cream Cahill) Mayfly Nymph from Mongaup Creek in New York

Mayfly Species Cinygmula reticulata

These are very rarely called Dark Cahills.
Cinygmula reticulata is probably the second most important species of Cinygmula behind Cinygmula ramaleyi, perhaps because the waters where it can be found in good numbers are often more remote. They have been reported as abundant in many high country streams of the Southern Rockies as well as the High Sierra's Eastern slope. An obvious difference in their coloration may be the easiest way to tell them apart. Cinygmula ramaleyi is more somber with a brownish body and dark gray wings and is often confused with the similar sized and colored Ephemerella tibialis, in spite of the difference in tail counts. Cinygmula reticulata on the other hand is a bright cinnamon dorsally with pale creamy legs and pale wings that are often a brilliant canary yellow. This is one of North America's most beautiful mayflies.
Female Cinygmula reticulata (Heptageniidae) (Western Ginger Quill) Mayfly Dun from the Touchet River in Washington
Lateral view of a Male Cinygmula reticulata (Heptageniidae) (Western Ginger Quill) Mayfly Spinner from Mystery Creek #237 in Montana
The lengths of the wing and body, measured with a caliper, are both 8 mm.

Keys in Needham's 1935 Biology of Mayflies point to either Cinygmula reticulata or Cinygmula gartrelli. It seems to have crossveins in costal half of forewing only, slightly margined with brown and wings tinged with amber at base and along costal margin of both wing (gartrelli) as opposed to all crossveins of both wings faintly but broadly margined with pale smoky and wings entirely amber-tinged (although there is a slight amber tinge throughout, just more pronounced in places) as in reticulata. However, wing length reported for reticulata (9 mm) is closer to this specimen than gartrelli (10 mm). Ventral median marks are supposed to be "traces" for reticulata and "present" for gartrelli. Descriptions for both species involve semi-hyaline anterior abdominal segments not present on my specimens. Distribution records suggest reticulata lives nearby, so I'm going with that, but I can't confidently rule out gartrelli.
Cinygmula reticulata (Heptageniidae) (Western Ginger Quill) Mayfly Nymph from the Big Thompson River in Montana
I collected several live specimens of nymphs and reared them to the imago stage. They were C. reticulata. The interesting thing is they were collected in May and were emerging along with Rhithrogena (March Brown). This seems to be an overlooked hatch since in some rivers it emerges very early, before runoff.
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