Stoneflies, order Plecoptera, are indeed intriguing insects. Of a very old and stable lineage, the Plecoptera have changed very little since the Tertiary (66-1.6 million years ago) (Illies 1965). Some believe that cold running water was their ancestral home, contrary to many ecological theories which assume most evolutionary diversity stems from the tropics (Williams & Feltmate 1992).

Stoneflies are aquatic in the immature nymphal stage, with aerial adults that are fairly short-lived. The word "nymph" should give you a clue - stoneflies have direct metamorphosis, meaning they have no pupal resting stage. The nymphal instars develop wing pads which enlarge with successive molts until the final molt to adult. For this final transformation, the nymph crawls up overhanging roots, vegetation or rocks protruding from the water, splits the thorax, and crawls away as an adult.

Although most adults have typical wings, they are weak fliers. Some species have reduced wings, and a few are wingless (Williams & Feltmate 1992). This "home-body" tendency has led to local speciation. It also makes stoneflies ideally suited to studies of biogeography.

Adults engage in an interesting behavior known as "drumming." When drumming, stoneflies strike their last few abdominal segments against the substrate, producing a signal of pulses and pauses. Both males and females have been observed drumming (Stewart et al. 1982). Investigators have developed methods to record stonefly drumming signals, much as ornithologists record bird songs. The number of beats and beat interval are species specific, improving the efficiency of mate-finding in an otherwise silent, drab, relatively sedentary insect.

The nymphs are, however, my favorite stage. As the name stonefly implies, they typically crawl around under rocks in streams. The mouthpart structure of nymphs directly reflects their food habits. Predatory species have developed pointed lobes extending from the paraglossae. Non-predators generally have paraglossae of about equal length to the glossae (see diagram). Since much of aquatic ecology revolves around feeding niches, this is indeed a handy trait to display so prominently.

Nymphs are pollution intolerant, needing high oxygen content in the water. They are almost always found either in flowing water or along shores of cold water lakes. Nymphs have diagnostic finger-like gills on the neck, thorax, and sometimes the abdomen to assist in oxygen uptake.

Though most stoneflies are relatively small, there is a genus of giant stonefly which frequents streams in the maritime northwest. It has a 2-3 year life cycle, the price it must pay to gain its large stature. I've seen giant stonefly nymphs in the small streams draining Cougar Mountain, in the Soleduck River, and in Mt. Rainier National Park. The best way for you to see these and other stonefly nymphs is to pick up and examine hand-size rocks whenever you're near a river or stream. Yes, even in winter!

References

Illies, J. 1965. Phylogeny and zoogeography of the Plecoptera. Annual Review of Entomology, 10: 117-140.

Stewart, K.W., S.W.Szczytko, B.P.Stark, & D.D.Zeigler. 1982. Drumming behavior of the six North American Perlidae (Plecoptera) species. Annals of the Entomological Society of America, 75(5): 549-554.

Williams, D.D., & B.W. Feltmate. 1992. Aquatic insects. CAB International, Wallingford, U.K.

This page last updated 16 June, 2005