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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 Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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 Isonychia campestris (Slate Drakes)

Where & when

Time of year : September and October

In 4 records from GBIF, adults of this species have been collected during July (50%) and August (50%).

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: 9-10? mm
Wing length: 9-11? mm

Somewhat paler than Isonychia sicca; venation dark.

Head of female pale yellowish, with traces, more or less distinct, of brown submedian stripes on the vertex and occiput. Blackish brown spot between eye and lateral ocellus, but no dark mark in corners of occiput. Thorax of male light brown, shaded somewhat with reddish and blackish brown on the scutellum. Fore leg entirely reddish black, tarsi somewhat paler. Middle and hind legs dull yellow, with slight red shading on the femora. Wings with fine veins, all dark-tinged.

Abdomen dorsally dull brown tinged with red, shaded laterally with deeper brown in the form of poorly defined dark patches; lateral margin pale brown. Light brown ventrally, rather paler than the dorsum. Penes similar in form to those of sicca.

Female Spinner

Head of female pale yellowish, with traces, more or less distinct, of brown submedian stripes on the vertex and occiput. Subanal plate of female rather deeply excavated apically, its outer angles acute.

Start a Discussion of Isonychia campestris


Mayfly Species Isonychia campestris (Slate Drakes)

Species Range
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