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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 Male Ameletus vernalis (Ameletidae) (Brown Dun) Mayfly Nymph from the Icicle River in Washington
Keying this one out using Zloty & Pritchard 1997:
-No ganglionic markings on sternites 2–8
-Posterior margins of sternits 6–8 without numerous spines
-Mesal gill extension well developed (I really don't like the "well developed" language when it's subjective, but in this case the other option doesn't lead anywhere productive)
-Tails pale in basal 1/3
-Larger species (13–16 mm) — this one is just shy of 12 mm, but closer to 13 than to <10
-Spring emergence
This all points to Ameletus vernalis.
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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Stonefly Species Sweltsa borealis (Boreal Sallflies)

I have found no references to this species in the angling literature, but it seems to be at least moderately common in the state of Washington based on the collections here.

Where & when

In 42 records from GBIF, adults of this species have been collected during July (31%), June (29%), May (24%), August (10%), and April (7%).

In 8 records from GBIF, this species has been collected at elevations ranging from 330 to 10180 ft, with an average (median) of 9098 ft.

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.


Described in Banks (1895) as Chloroperla borealis

14 mm. Yellowish.

Head a little broader than the prothorax, pale yellow, with a dark V-shaped mark connecting the ocelli, two dark spots in front, and a small reddish tubercle each side; palpi blackish, antennae fuscous, yellowish at base.

Prothorax short, twice as wide as long, a little broader in front than behind, the angles broadly rounded; pale yellow, the elevated margin blackish, each side a little ruguose and brownish; rest of thorax and the abdomen brown.

Legs brownish yellow, a transverse black line at ends of femora; setae; short, yellowish in middle, brownish at ends.

Wings greenish yellow, veins of anterior pair, except subcosta and radius, brownish (in one wing there are two crossveins (note from Troutnut: original source calls crossveins "transversals") beyond subcosta, but one is bent and appears abnormal), crossveins at end of discal cells are opposite each other, and the upper fork of radial sector is more than twice as long as the pedicel beyond these, there are five crossveins between vein Cu1 and vein Cu2.

The bove description is based on one female from Olympia, WA, collected in April. A male from Ft. Collins, Colorado, is smaller, 10 mm., and the radius is only yellowish toward base, the crossveins at end of discal cells are slightly separated, and the forks of radial sector are not quite so long, otherwise it is like the female.

Specimens of the Stonefly Species Sweltsa borealis

1 Female Adult
1 Adult
1 Nymph

Start a Discussion of Sweltsa borealis


Stonefly Species Sweltsa borealis (Boreal Sallflies)

Species Range
Common Names
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