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article by Ron Newman

The Stonefly Nymph Description:

  • 5-50mm in body length
  • 2 long antennae
  • eyes are widely separated
  • gills are usually found on the throat and the base of the legs and abdomen
  • 3 pairs of crablike legs with terminal tarsi with 2 tarsal claws
  • 2 separate pairs of wing pads
  • 2 long tails (cerci)
  • with over 460 species known to inhabit North American, colour is varied, however shades of yellow, orange, green, brown and black are the most common. Quite often stoneflies are 2 toned.
  • adult stoneflies look much the same as the nymphs with the following differences:
  • wings are folded flat over the back and often extend 10 to 20% past the tip of the abdomen
  • 2 short tails (cerci)
Life Cycle:
  • Most female stoneflies skim the surface of the stream, dipping their abdomens in the water and releasing their eggs. Others will actually crawl to the bottom of the stream and then release their eggs on submerged objects. Stoneflies are very clumsy fliers and during the egg releasing procedure will cause quite a fluttering and splashing on the water surface which immediately attracts the attention of hungry fish especially steelhead.
  • eggs hatch into a nymph stage which, depending on the species, can last up to 3 years requiring up to 25 molts to develop completely.
  • Stoneflies do not go through a pupa stage and is therefore considered an incomplete metamorphosis.
  • Nymphs will crawl out of the stream onto a stone, tree branch or log and remain long enough to dry and split it's nymphal case. The adults will emerge from the exoskeleton, looking very much like nymphs with wings added and will then fly or climb into the nearby trees. Following the emergence the adults will mate (males attract the females by drumming their abdomens on a tree branch) and then the females will start the lifecycle over again by depositing her eggs back into the stream. Depending on the species, stonefly adults may live for several weeks.

The Stonefly AdultFood:

  • nymphs will feed on organic and vegetable matter found in the stream substrate.
  • some species are carnivorus, feeding on mayfly nymphs and other insect larva.


  • Stonefly nymphs require well oxygenated water so are consequently found in rivers and streams amongst the rocks and bottom debris, a few species can also be found in the rocky shoals of cold lakes.

Fishing Techniques:

  • nymphs are available to fish year round
  • nymphs are very poor swimmers and prefer to crawl amongst the rocks on the river bottom using their clawed crablike legs
  • quite often they will loose their footing and will drift helplessly down current, thereby being at the mercy of fish
  • fish will often target nymphs as they attempt to crawl out of the water during emergence
  • in either case the fisherman must imitate this action by keeping the fly near the bottom, this can be achieved by casting upstream or up and across and letting your fly sink and tumble with the current along the stream bottom.
  • Adult female stoneflies are also highly prized by fish, they clumsily make contact with the water to deposit their eggs. This is an extremely important time for the dry fly fisherman. Imitating the fluttering, splashing movement of a female stonefly on the water surface will often provoke a violent response from trout or steelhead.

Stoneflies and Steelhead:

  • Stonefly nymphs and juvenile steelhead share the same habitat. Stoneflies, because of their size provide much of the steelhead's food requirements. This relationship of stonefly nymph and juvenile steelhead is called "juvenile habitat imprint" and is very important when trying to find the holding areas of large adult steelhead returning from the ocean to their home streams for spawning. More importantly to the dry fly fisherman is the relationship between the juvenile steelhead and the adult female stonefly. Unlike caddis flies, adult egg laying stoneflies do not swarm. Therefore, a single egg laying female upon the water surface will provoke a very competitive feeding response from many fish including the juvenile steelhead. This relationship of the single egg laying stonefly and the feeding response of the juvenile steelhead is called "the single fly juvenile feeding imprint" and explains why a non-feeding upward migrating adult steelhead will take a dry fly. Quite simply, the fluttering dryfly will trigger the "juvenile feeding imprint" in the adult and it will strike the fly. This behavior is very important to the fisherman, because when steelhead enter the river from the ocean to spawn there is usually no adult stonefly activity. Adult stoneflies usually die during the middle of late summer.

Be sure to read other articles by Ron Newman

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