Stoneflies

• Phylulm Arthropoda (Jointed Legs)
• Class Insecta (Six Legs)
• Order Plecoptera ( "Twisted or Folded Wings")

Other Names

• Salmonfly
• Giant stonefly
• Plecoptera

Nymph Appearance Range in size from very small (5 mm) to very large (6 cm). Commonly a dark, shiny black colour but also brown and yellow. Strong-looking legs. Two long tails. Two long and obvious antennae. Two claws at the end of each leg. Gills are not obvious. Have "hairy armpits" - tiny gills are located behind the base of each leg.

Nymph Behavior

• Nymphs do "push-ups" when subjected to low dissolved oxygen concentrations.
• Bouncing and rocking on their legs creates a current through their gills.
• Not as abundant as mayflies and caddisflies.
• Cling to the underside of submerged rocks.
• Appear clumsy and cumbersome.
• Hide under concrete blocks that line some riverbanks.
• Prior to emergence as adults, nymphs migrate towards the riverbanks.
• Final nymph stages crawl out of the water to shed their skin.

Nymph Feeding Types

• Some nymphs start as herbivores and become carnivores in later stages.
• Bright, active species are predators on small nymphs (mayflies).
• Most are herbivores on decomposing leaves.
• Dark, sluggish species are scrapers and shredders feeding on leaves.
• The "Peanut Butter and Cracker Hypothesis" describes a leaf falling into a river and eventually becoming colonized by decomposer fungi and bacteria. Stoneflies wait to feed until the leaves have sat in the river and gained a slimy coating of these decomposers. Scientists think that this combined meal of leaf and coating is a better source of nutrition (especially protein) than just leaf tissue. This is just like eating peanut butter spread on crackers - you get more calories, fats and protein from the peanut butter than the crackers!

Adult Appearance

• The adults look like a large nymph but with wings.
• Two long, stiff tails.
• Long, stiff antennae.

Adult Behavior

• Wings fold flat like a tabletop at rest.
• All four wings flutter busily during flight.
• Not very good flyers.

Aquatic Habitat

• Nymphs prefer fast flowing water with large stones.
• Deeper, faster water with high oxygen concentrations.
• Rapids.

Reproduction

• Incomplete metamorphosis includes egg, nymph and adult stages.
• Adult females lay eggs while flying over the river.
• Eggs are laid while fluttering, dipping and dragging along the water surface.
• Overwinter as nymphs.
• Nymphs crawl to the shore and climb rocks or bushes to emerge with wings.
• The shed, empty exoskeletons stick to rocks just above the waterline.

Predation

• Because there is no hatch on the water surface, trout don't feed in a frenzy.

Nymph Pollution Tolerance

• The least pollution tolerant of all aquatic macro invertebrates.
• Pollution intolerant.
• These nymphs can indicate good water quality and high oxygen levels.