Wading for Bugs: Exploring Streams with the Experts

Part II: Stonefly Recoveries

22 PART II Stonefly Recoveries About Stoneflies (Plecoptera) Compared to other more numerous and diverse insect orders, Plecoptera has relatively few members; nevertheless, stoneflies are important in aquatic systems. Because they generally live in well-oxygenated, cool streams (and sometimes lakes in similarly clear circumstances), they are very useful bioindicators. Plecoptera literally means “folded wings,” describing how the hind wings tuck under the anterior wings when the adult stonefly is at rest. Their larvae (nymphs) are among the largest aquatic insects, and include the giant salmonflies (Pteronarcys) and golden stoneflies (family Perlidae) familiar to fishermen. Morphology Stoneflies have primitive morphological features that are relatively unspecialized. Body shapes include the stout, rounded peltoperlids with a roach-like appearance; thin, pencil-like winter stoneflies; flattened, marbled-colored golden stoneflies, and dark, flattened bodies of giant salmonflies that grow to two or three inches long. Herbivorous stoneflies often have larval mouthparts shaped for shredding leaves, and predacious species can stab and grasp their prey. The thorax has three distinct segments, each with a pair of crawling legs. Gills, either single or in tufts, often occur on the bottom (ventral) thoracic surface or along the bases of legs on the thorax; however some stoneflies are without gills, and a few have gills on the abdomen. Wings begin on nymphs as small pads on the second and third thoracic segments, growing longer with each molt. The abdomen has ten segments and a pair of somewhat stout tails (cerci) on the tenth segment. Adult stoneflies carry their wings folded over their backs when at rest. They are not strong fliers and characteristically flutter in About Stoneflies 23 flight. Many species of stoneflies lack mouthparts as adults, and, like mayflies, those species do not feed. Life History Stonefliescommonlyhaveone-yeargenerationcycles(univoltine), though some live from one to two years (semivoltine), and occasionally two or three years. Stoneflies are flexible in the timing of their life cycles, varying rates of development through egg or nymphal life stages. Growth is gradual, in which nymphs metamorphose through a succession of molts (ten to twentytwo depending on species and conditions) until they are ready to emerge as adults. Stoneflies do not have a pupal stage. The stonefly’s last molt is fairly unique among aquatic insects because the last instar nymph crawls out onto streamside rocks or vegetation for the final molt. Its skin splits along the top (dorsal) surface, and the soft-bodied adult basks in its new environment while its wings and body cuticle harden. Where stoneflies are abundant, gravels next to the stream may be covered in cast-off skins following the time when particular species emerge. To attract mates, males of many species “drum” by tapping on a substrate (for example, tree bark or stones) with the tips of their abdomens. Virgin females respond with signals specific to each species. After mating, females release eggs on the stream surface, or deposit them into the water, where eggs attach to the substrates in the stream. In some species, particularly under unfavorable environmental conditions like drought, eggs undergo diapause (delayed development). In another strategy to avoid adversity, some stonefly nymphs burrow into gravels below the stream surface (a region called the hyporheos). Winter stoneflies (family Capniidae) often deploy this strategy, then emerging midwinter onto the snow and other winter conditions. Bioindicators Stoneflies are some of the best indicators of good water quality as they are among the most sensitive of aquatic insect larvae. Typically, they are among the first to disappear as pollutants enter an ecosystem. However, they are not useful indicators 24 Stonefly Recoveries in warm, low-gradient streams because they are generally not present in those conditions. References Stark, B. P., S. W. Szczytko, and C. R. Nelson. 1998. American Stoneflies: A Photographic Guide to the Plecoptera. Columbus, OH: The Caddis Press. Stewart, K. W., and B. P. Stark. 1988. Nymphs of North American Stonefly Genera (Plecoptera). Lanham, MD: Entomological Society of America. Stewart, K. W., and B. P. Stark. 2008. “Plecoptera.” In An Introduction to the Aquatic Insects of North America, edited by R. W. Merritt, K. W. Cummins, and M. B. Berg, 311–13. Dubuque, IA: Kendall Hunt. ...