Brown Willow Stonefly

Skwala americana

Key Identification Characters - nymph: 2

  1. Two tails at least as long as abdomen.
  2. No gills on thorax or between tails.
  3. Body light brown with dark markings on abdomen and thorax, with a distinct light mark in the shape of the letter “M” on top of the head.
  4. Body length of mature nymphs (excluding tails) 15-25mm.

Similar nymphs: 2

Two species of the genus Skwala occur in the West: S. americana and S. curvata. S. americana is the most widespread and common species of the two. Species confirmation requires viewing male or female genitalia.
The species most often confused with Skwala nymphs is Calineuria californica. The absence of any gills on the thorax of Skwala nymphs can quickly distinguish them from C. californica.

Key Identification Characters - adult: 2

  1. Two tails as long as abdomen or longer.
  2. Four wings of female fully developed, while wings of male often (not always) much reduced and non-functional.
  3. Body color uniform brown with some pale markings on head, thorax, and legs.
  4. No remnants of gills on thorax.
  5. Body length (excluding tails) 15-25mm.

Similar adults: 2

While somewhat similar in appearance to golden stone adults, Skwala adults are slightly smaller and are darker and more uniform in color. Male Skwala have the unique characteristic of having short non-functional wings. Megarcys sp, another genus of Perlodidae can also be confused with Skwala, but Megarcys adults are more yellow in color and have small single-filament gill remnants at the base of each leg.

Habitat & Distribution: 2

The brown willow fly is widely distributed throughout the Rocky Mountains and the Pacific Northwest, and is one of the most common stoneflies in the region. They are equally at home in small streams and large rivers, and often found together with golden stones. S. americana has a wider distribution than S. curvata, with S. curvata found in higher and colder streams than S. americana.
In Oregon good populations of Skwala americana occur on both sides of the Cascade Mountains, in both small streams and large rivers.

Life Cycle - Emergence 2

Unlike golden stones, the brown willow fly has a one-year life cycle. Nymphs complete their growth in late winter/early spring, and adult emergence occurs from February to July depending on elevation and latitude.
In Oregon the emergence of Skwala americana depends on winter and spring weather conditions. Typically it begins in early to mid March and extends into April and May.

Oregon Emergence Period: early March - late May.
Metolius: ?
McKenzie: Emerge early March-April.
Necanicum/Nehalem: March-April?

Sources and Credits

  1. (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
  2. (c) Jerry Schoen, some rights reserved (CC BY-SA)

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