Stoneflies


Stoneflies, order Plecoptera

About

The order Plecoptera has nine families of stoneflies that occur in North America. The name Plecoptera derives from the Greek plekein (to braid) and ptera. Adult stoneflies have two sets of wings, one of which folds under the outer set by pleating. The English name likely derives from the fact that many species crawl on or live on or under both submerged and exposed stones in or along streams. They can be traced back through fossils to the early Permian period (250 to 300 million years ago). They are closely related to cockroaches but are more advanced because of the pairs of folded wings on their backs.

Habitat

Stoneflies are generally found in the “lotic-erosional” habitat (flowing water, riffles) and for the most part prefer small cool streams that are under 25°C (77 degrees Fahrenheit) with dissolved oxygen near saturation. A few species are found in lakes and there is one species that lives its entire life cycle below 70 meters.

Diet

Stoneflies provide important functions in the aquatic ecosystem. The shredder-detritovores help break down woody and leaf debris. Some stoneflies are scrapers, generally eating algae and bacteria on the surface of the substrate. Others are predators, and some even change their feeding habits as they go through larval molts.

Carnivorous stoneflies help keep balance in populations of other aquatic organisms. They engulf prey such as midges, black flies, mayflies, and other stoneflies. Mouthparts can be used to help identify stonefly types as well as feeding habits. Feeding generally takes place at night.

Stoneflies are food for other predators, including salamanders, birds, and fish.

Morphology and Behavior

Stoneflies breathe using filamentous gills on their thorax and/or upper segments of the abdomen though a few have no gills and breath by diffusing oxygen through the surface of body parts.

Larval stoneflies have elongate bodies and vary from flattened to cylindrical forms. They range from 5 to 70 mm in length, not including tails. They have well-developed legs, each with 2 claws. Some stonefly larvae have filamentous or fingerlike gills on the thorax, legs, or abdomen. Most mature larvae have developing wing pads. Stonefly larvae have 2 primitive tails that are actually sensory.

Stoneflies are very intolerant of water quality impairment. Some stonefly families respond to low oxygen levels by creating their own local oxygen rich environments. Common stoneflies can be seen to do “push-ups,” raising and lowering themselves with their legs to create flow over the gills and body. Giant stoneflies will “wag their tails” to create the same effect.

Giant stoneflies use auto-hemhoraging (reflex bleeding), excreting blood at one joint of the leg, to repel predators. The blood is thought to have a bad taste or smell or perhaps confuse the attacker. Some species “play dead” when approached by fish that eat insects, becoming still with a curl in their bodies and heads bent down.

Life Cycle

These insects have incomplete metamorphosis, which means that during the larval stage the insect has many of the features that are also seen in the adult, and the main difference is in the development of wings. Eggs usually hatch in 3 to 4 weeks, with most stoneflies spending 10 to 11 months as larvae shedding their skin (molting) between 10 to 22 times.

All species have aquatic larvae that become terrestrial once achieving adulthood. When ready to emerge as adults, they crawl out of the water, grab a solid surface with their tarsal claws, and split the skin along lines in the head and thorax, finally crawling out as adults. Usually, this takes place at night. Stoneflies are unique among aquatics insects in that there are some insect families that emerge throughout the seasonal year. Adults live one to four weeks as terrestrial flies, hiding on branches or leaves of vegetation during the day and crawling about at night to feed. Females must eat to produce eggs, though some male adults do not feed at all.

Reproduction

Reproducing adult stoneflies find each other through vibrational “drumming” that is transmitted through the substrate (such as rock) as opposed to through the air. The male “calls” by thumping the rock with its abdomen and the female responds in kind as they close in on each other’s location. Frequency and pattern of drumming is unique to each species and different between the male and female. The female carries fertilized eggs for as short as a few hours or as long as 4 to 5 weeks before depositing on the surface of the water in an eggs mass that separates as it falls to the bottom of the stream (or lake), where individual eggs stick to the substrate. There is only one generation a year and in some species it takes two or three years to complete a generation.

References

  1. iThe Bug Guide. http://bugguide.net/node/view/52788
  2. Kondratieff, Boris C. and Richard W. Baumann (coordinators). 2000. Stoneflies of the United States. Jamestown, ND: Northern Prairie Wildlife Research Center Online. http://www.npwrc.usgs.gov/resource/distr/insects/sfly/index.htm (Version 12DEC2003).
  3. McCafferty, W. Patrick. 1983. Aquatic Entomology. Boston, MA: Jones and Bartlett Publishers.
  4. The McKenzie Stonefly Page. http://zebu.uoregon.edu/~dmason/Mckenzie/bugs/stone.html
  5. Mississippi College. American Stonefly Web Page http://www.mc.edu/campus/users/stark/american.html
  6. Ramel, Gordon. http://www.earthlife.net/insects/plecopt.html
  7. USEPA. http://www.epa.gov/bioindicators/html/stoneflies.html
  8. Voshell Jr., J. Reese. 2002. Blacksburg, VA: McDonald Woodward Publishing Company. A Guide to Common Freshwater Invertebrates of North America.

For more stonefly pictures, click here
Research conducted by StreamWatch volunteer Leslie Middleton