Is this a Stonefly Nymph?

Discussion in 'Fly Fishing Entomology' started by Matt Smith, Jan 30, 2009.

  1. cabezon

    cabezon Sculpin Enterprises

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    Guestimating size from the picture, that nymph looks to be about 2cm (20mm) long. The presence of wing buds would indicate that it should be ready to emerge in the next month or so. According to Hafele and Hilton (Guide to Pacific Northwest Aquatic Invertebrates), yellow stones (Skwala spp.) are 13 - 22mm, about the size of this larva; H2 indicate that the adults are active from April to June, but we know that they emerge as early as March on the Yak. According to H2, golden stones are 25 - 38mm when they emerge after 2 - 3 years as a larva, generally May - August. A key difference between the two is the development of gills. In golden stones, there are thickly-branched filamentous gills at the base of each leg. Yellow stone species either lack these gills or have simple fingerlike gills under the head or base of the legs. Of course, we can't see any of that. Someone with more experience could do the definitive ID from the picture that Matt supplied.

    Steve
     
  2. Taxon

    Taxon Moderator Staff Member

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    Matt-

    Yes, you are right; it is a stonefly. As numerous others have already indicated, this a Skwala nymph, which is a member of stonefly family Perlodidae (Patterned Stoneflies). There are two species of this genus present in Washington, Skwala americana (American Springfly) and S. curvata (Curved Springfly), and they can not be easily distinguished from one another. However, both exhibit the rather distinctive dorsal head patterning which can be seen below.

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  3. Matt Smith

    Matt Smith On the river Noyb

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    Cool, Thanks. :thumb:
     
  4. Plecoptera

    Plecoptera Active Member

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    Just to add, small stoneflies are often confused with mayfly nymphs. Remember not all stones are large. The most abundant ones around here are similar in size to a mayfly. I've worked with invertebrate samples from several streams around here and was surprised at how small most stoneflies were. One way to really tell the difference is to look at their legs. Stoneflies will have 2 claws at the end and mayflies will only have 1. Difficult to see with the naked eye, but a dead give away with a microscope.