Is this a Stonefly Nymph?

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

  1. I found this guy under a rock along the Stilly. I think that it is a stonefly... am I right?

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  2. Mat,

    That looks like a Stonefly nymph to me. Stonefly nymphs are often confused with Mayfly nymphs. Stonefly nymphs have two long tails and Mayfly nymphs have three long tails.

  3. I would say it is but I'm still learning...if I had to guess I would say it was a Skwala
  4. Some mayflies have 2. Typically, stoneflys tend to be larger then mayflies.
  5. Send a shout out to Taxon. Roger will very likely be able to give a specific ID to the nymph in question. He's an incredible resource. :ray1:
  6. I think that's a copper john size 10, or a pheasant tail rubber legs size 12
  7. Given the size and time of year, I'd agree with Big E...looks like a Skwala
  8. That is a Hesperoperia pacifica (Golden Stonefly) Thay are smaller than a Salmonfly and usually hatch a few weeks after the Salmonflies. They are agressive eaters and use speed and ambush tactics to catch there prey. they live subsurface for three years before crawling out on the rocks to hatch.

    Here in Oregon they are everywhere in the mid. spring.

  9. Cool, thanks guys. I love to know what the little buggers are that are living in the river. It makes me think that i can match the hatch and give me a better chance at outsmarting the fishies.
  10. There isn't much of a hatch on the Stilly anytime of the year. Hitting it with dries is a hit and miss thing. I did notice some may flies in the summer at just about opening of the river that were green in body color. About a half an inch long. But I didn't see any kind of fish keying in on them. This was in the Hazel area.

    Also yellow Caddis's work well on the Upper Sauk above the N/F. I've always had good luck up there with that color. But that was when you could drive all the way around on the Mountain Loop hiway. Is this road oopen yet?????
  11. Matt, this species is a clinger, not a free drifter. When you match that nymph, fish it off riffles. You'll need to get it to the bottom quick. It's tough to successfully fish this nymph in the winter, as most fish don't hold off the faster riffles in these cold waters.

    Small mayfly nymphs dead drifted below moderate speed currents in the winter.

  12. It does appear to be a stonefly but the quality of the picture would make identification of genus or species difficult. Skwalas (Skwala Americana) are active and nearing emergence at this time of the year. Here's a picture of a Skwala nymph from the Yakima River around this time of the year. Note the well-developed wing pads indicating that this nymph is nearing its molt into the adult form.

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  13. Given the time of year, wouldn't this rule out the golden? I always thought that they appeared in late spring / early summer.
  14. Hi Big E,

    Remember, these stoneflies live under those rocks from the time they hatch out of their eggs until they crawl out of the water and molt into adults. As the time comes closer for them to emerge, they tend to become more active, hence they "appear" more common. But the same bugs, in the same number (just smaller in size), were there two, four, six, eight months earlier.

    Some of the mayflies, like BWOs, appear to have multiple generations a year (fall and spring), but stonefly larvae require 11-12 months, to as long as 2 -3 years for the larger species, to complete their development.

  15. Wouldn't the key element be size? A golden stone may live underwater for a few years, but it won't be emerging size in January, will it?
  16. 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.

  17. 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.

  18. Cool, Thanks. :thumb:
  19. 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.

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