Doc Leonard’s Lecture Notes on Plecoptera

Nearly 500 species of stoneflies occur in North America north of Mexico. Unpublished records for Michigan indicate 61 species, and over the world as a whole, a fairly close estimate would be 1700 species. On a world-wide basis, the Plecoptera compare numerically with the Ephemeroptera but make some of their species richness in other parts of the world. State lists in the North-central tend to run about twice as high for Mayflies as for stoneflies.

Almost all species of stoneflies pass their aquatic stages in cool, well-oxygenated streams with very definite current and thus are particularly important items in the diet of trout, and there is an entire fauna of “winter” stoneflies. From the standpoint of fisheries, stoneflies are perhaps somewhat less important than Mayflies but still look large in the management picture. Throughout trout and salmon stream country, flights of stoneflies stimulate heavy feeding by fish. As with Mayflies, there is a sufficient range in the emergence dates of stoneflies that one or another species is likely to be emerging or laying eggs at almost any time during the spring, summer, fall or winter.

Like Mayflies, stoneflies not only are an important food source for fish, but also, as immatures, important in pollution studies because of their requirement for waters comparatively free from contaminants.

Stoneflies lay their eggs in flowing streams (sometimes along windswept rocky lake shores) and observation indicates that most adult females deposit egg masses by dipping the tip of the abdomen beneath the water surface while in flight. In other a method reminiscent of the Libelluline dragonflies and some of the Mayflies, especially the genus Stenonema, some of the Capniidae may actually crawl under the water surface to deposit their eggs. After the eggs hatch, the early instar nymphs may remain essentially dormant during late spring and early summer months before they start feeding and growing rapidly. Most of our species and probably all of those which are essentially small in size, such as the Capniidae and Leuctridae, have an annual life cycle. Some of the larger forms, such as Pteronarcys, and the larger Perlidae, may require 2 or more seasons to mature.

Though now known not to be systematically correct, older works on stoneflies divide the order into 2 major subgroups, the Setipalpia and Filapalpia. For purposes of discussing the general ecology and biology of stoneflies, this classification is useful. In way of a tabulated summary, the following should be helpful in later decisions: 

shredders, herbivores
1st instars do not feed
adults may take a final meal
carnivores, omnivores
1st instars may feed
adults do not feed before mating
Habitat & Habits:
cool, small streams
winter and early spring emergers
day active, not attracted to lights
Habitat & Habits:
often found in warmer waters
late spring and summer emergers
night active, often attracted to lights
females mate and oviposit only once
eggs often without sculpturing
females may mate and oviposit > once
eggs may be highly sculptured

Nymphs of North American stonefly species always have two tails and there are always two claws on each tarsus. Similar appearing Mayfly nymphs have a single claw. This is one useful way to distinguish in fish stomachs between stonefly nymphs and those occasional groups of Mayflies whose nymphs also appear to have two tails. As we have mentioned in connection with the Mayflies, stonefly nymphs are characterized by the external gills which may be on the underside of the head, as with Isogenus, on the under surfaces laterally of the thorax or abdomen, or in some cases, extruding from the rectal cavity. Obviously, some species must absorb dissolved oxygen directly through the body covering.

At full maturity, nymphs crawl from the water and come to rest on some exposed surface such as a protruding rock, a stream-side tree trunk, or some other site. Apparently the activity of crawling from the water to a resting site is a necessary part of the emergence process. This appears when one is trying to rear the nymphs in captivity. As is true with almost all transforming insects, once the adult has emerged from the nymphal skin, it is “teneral” and must rest for a while until the soft wings and other parts of the body harden sufficiently to sustain flight. Whereas Mayflies tend to have their peak emergence during late afternoon or evening, stoneflies, in this part of the country, emerge during the night or during the early morning hours. perhaps the most common emergence period is from 3:30 or 4:00 in the morning until sun-up.

Many of our stoneflies seem to particularly like concrete bridge abutments as a resting site for transformation. This trait is useful to the collector because he is often able, by checking bridges, to find out quite a lot about the stonefly fauna of a stream, regardless of the season of the year at which the inspection is made. 
Some of Frison's early work on the winter stoneflies of Illinois indicated that species of Capnia and Allocapnia had to feed, generally on tree trunk-growing algae, before they could mate successfully. It is still somewhat uncertain whether this is an inflexible requirement, and it is obvious from rearing experiments that many other species do not require food in the adult stage, particularly the setipalpia. Mating may take place at almost any hour of afternoon or evening. Sometimes it seems to take place in almost any daylight hour including morning. There are statements in the literature that, unlike Mayflies, stoneflies do not mate on the wing. At least one of the common trout stream species Isoperla dicala does mate on the wing during almost all daylight and evening hours. Because of its pale yellow color, it is one of the few stoneflies which is easy to recognize in flight. Although many species do not feed as adults, the imagos generally persist much longer than Mayflies, and thus, do not have as synchronous emergence periods. Males often emerge well before females which may have some mating advantage.

Adult stoneflies vary widely in size. Some of small Allocapnia are not much over 4-5mm while the big Pteronarcys may have a body length of at least 50mm and a wing spread exceeding 100mm. Most of the North American species are drably colored gray, brown, or dark brownish black. A few species are bright yellow, and a few, as indicated by the family name Chloroperlidae, are definitely pale green. Some adults are nocturnal and phototropic just like most Mayflies. But some species are daytime and evening fliers. All are essentially weak fliers and spend most of their time close to the stream.

In several stonefly groups, the females have short wings and the extra body segments exposed are sclerotized (like other normally exposed segments), presumably conferring some kind of protection. In the winter stoneflies of the genus Allocapnia, this change has evolved independently in two lines in the opinion of Ross (1962). In one line, represented by Allocapnia vivipara, the wings of the female vary from fairly long to short pads. Also, an extra abdominal segment is variously sclerotized. In these A. vivipara females, however, the two characters are not correlated. In populations over a wide part of the range, a third of the females with the shortest wings have the most extensively sclerotized abdomens, and a third have the least sclerotized; conversely, of the females with the most extensively sclerotized abdomens, a third have unusually short wings, and a third have the longest wings. In spite of this lack of adaptive correlation, both traits are being carried in many populations and are available for selection pressures to influence. Certainly the correlation of seemingly linked adaptive characters is not nearly always perfect.

In the case of Allocapnia, it would seem that at any time and through a variety of causes, new viable genetic components or combinations might occur and become a part of the gene pool of the population. In other words, the genetic composition of a population is dynamic and constantly adding an element of variability into the relationship between gene frequency and natural selection. As an alternative to Ross, Nebeker and Gaufin (1967) feel that wing length results from environmental variables such as water temperature, altitude, and hours of daylight.

The largest stoneflies in North America are species of the genus Pteronarcys. In Michigan there presumably are two species, dorsata and pictetii. The species dorsata also has been recorded under the names nobilis and shelfordi in recent literature, in addition to several other names in early or obscure publications. Adult body lengths are about 50mm and the color essentially dark gray with some of the soft parts between the head and thoracic segments and on the underside of the belly, orange to dirty straw color. The wings are particularly notable for their extensive network of crossveins reminiscent of dragonflies and Mayflies. Most publications state that the common North Central and Eastern species of Pteronarcys essentially are an inhabitant of small headwater streams. Actually, the largest populations occur in streams of some dimension up to good sized rivers. Eastern species of Pteronarcys occur much less frequently in trout stomachs than does the Western species Pteronarcys californica. This, in part, may reflect habitat differences among the stonefly species.

Adults of the family Nemouridae are not particularly frequent in trout stomachs. Species of Leuctridae, particularly the genus Leuctra, on the other hand appear with some frequency. This Holarctic genus is fairly common in English streams where the adult serves as model or natural for the “Willow” trout fly. Capniidae adults are infrequent in trout stomachs, perhaps because these winter species rarely swarm over streams, and at that time of the year, trout may not have much of their attention focused on the surface. It should be pointed out that trout can and do surface-feed in cold weather.

Members of Perlidae (Acroneuria, Neophasganophora, Paragnetina, and Perlesta) are popular with trout when they are available as adults. In this part of the world, perlids seldom occur in large enough flights to attract a specific type of feeding behavior on the part of the trout. Perhaps the most important adults in the feeding of trout is the Perlodidae genus Isoperla. In the Northeast, Isoperla dicala would be the model for the “Yellow Sally” dry fly. Further west, some species of Alloperla would better fit as the model for this particular fly. As is often true in United States trout streams, fairly good and successful representations of natural aquatic insects may be tied by local fishing guides, but very few of these are published or gain general currency.

It may be suitable to close this portion of the discussion on Plecoptera by repeating the frequently made observation that stoneflies, both as naiads and even more as adults, look like aquatic cockroaches. The family Peltoperlidae has representatives which are particularly close in shape and action, and it may be questioned if the relationship between the stoneflies and the Orthoptera may not be quite close.

Stoneflies still have not attracted a great many workers. Only a handful have contributed to present knowledge of the systematics. In their general biology, an even smaller number have made significant contributions. Stanley Jewett made a number of observations of this kind, as well as those toward traditional taxonomy. Ricker's works have been almost entirely systematic. Arden Gaufin and his students at Utah have published a number of papers on environmental requirements and information on the eggs of stoneflies. Ken Stewart in Texas and his students presently are doing much to clear up systematic problems and to associate of immatures and adults. Hitchcock, in the only recent book on stoneflies, has summarized the distribution and ecology of the species of the Northeastern region. Abroad, Brinck of Sweden and Macan of England are perhaps the best known. Hynes, now situated in Canada, published considerably on British stoneflies beginning in the early 1940s, and it is hoped that he will continue his studies on the North American fauna.

Despite the work that has been done to date, the nymphs of many of the North American species have yet to be described. Work done in Europe indicates that stoneflies differ perhaps more in their ecology than they do morphologically. Thus, detailed information on habitat requirements and physiological differences of the various species may be quite essential to a proper understanding of stonefly systematics and evolution. Most of the ecological data published on North American stoneflies thus far consists of rather general statements regarding habitat. Frison's publications stated a simple dichotomy that certain species are found in small streams while others are found only in large streams. Back in the 1930s, Frison made some attempt to tie the type of bottom into the concentration of different species. In the 1940s Hynes (1942) and Brinck (1949) each attempted to list the factors they considered as most important in determining stonefly distribution. Hynes listed 5 major factors as controlling the distribution of stonefly nymphs:

1. movement of water.
2. altitude (which probably is a function of temperature).
3. substratum (which is to a certain extent controlled by the move - ment of water).
4. drought and its relation to length of naiad life and emergence period.
5. the “colonization factor” or proximity of habitats in which stoneflies are abundant.

In addition to these factors, Brinck considered of primary importance such factors as water temperature, the amount of certain dissolved gasses and other chemical factors of the water, and the presence of food. There is no doubt that all of these factors are important in the distribution of species. A somewhat more systematic attempt at developing a stream classification based on occurrence of stoneflies was made on the Gunnison River of Colorado by Knight and Gaufin:

Stream type, Stream width, or current Number of species
temporary, snow melt 10
     > 8000 ft.1 ft.-15ft
     < 8000 ft.1ft-15 ft. 
stony rivers 30 ft.-90ft 

constant rivers 90 ft.-180ft 24
sluggish rivers, frequent muddy bottoms with rooted aquatic plants 11

In the United States, at least, additional factors would be introduced by human activities. Pollution, construction of reservoirs, dredging, stream channelization, and water diversion perhaps alter the distribution of stoneflies more than all other factors combined. It is worthwhile to consider each of these factors here as they not only affect the distribution of stoneflies but also most other aquatic orders as well.

The movement of water is perhaps the most important single factor controlling the distribution of nymphs in that it controls the oxygen supply of the water and, to a certain extent, the temperature, the food, and the character of the substrate. Stoneflies only rarely are found in stagnant water, and thus are confined almost entirely to moving streams or waveswept shores.

Dissolved oxygen content of water during immature development is certainly an important factor in stonefly distribution. Most situations where stoneflies occur are characterized by water at or near the oxygen saturation point. Exceptions do occur. Nemoura has been collected near the outfall of a cheese factory along the Rifle River near Rose City, Michigan, where the oxygen level often dropped to less than 4ppm. Taeniopteryx also occurs in spots along the Huron River where low oxygen levels may be a fairly frequent occurrence. It must be assumed, however, that such low oxygen episodes are of short enough duration or infrequent enough that stoneflies can continue to occupy these niches. Gaufin has reported finding a species of Taeniopteryx in the Great Miami River near Dayton, Ohio, where the oxygen level was low, the flow was comparatively stagnant, and the nymphs possessed protruding anal gills. Since the species of Taeniopteryx are not supposed to possess such gills, that was a rather interesting observation. (It has been thought that these were not really gills at all but tufts of fungus.) Also at the same location, he found large numbers of Perlinella drymo, a species not common in eastern streams, and Hexagenia (Ephemeroptera), Agrion, and Argia (Odonata) which all might indicate that the water quality was not that bad over long periods of time.

Organic pollution causes a rapid drop in dissolved oxygen in streams. Brinck reported that polluted sections of streams in Sweden with an oxygen content below 40% saturation had an insignificant stonefly population or none. This may have been due, however, not only to the lowered oxygen content, but also to poisonous substances such as hydrogen sulfide, which were produced by a breakdown of sewage. The latter seems more probable because stoneflies are known to survive very low oxygen concentration for limited periods of time without mortality. Stoneflies cannot move their gills as can many of the Mayflies but have developed the habit of doing “pushups” to aid in ventilation during periods of low oxygen. This curious habit allows many stonefly species to survive, at least temporarily, when conditions become unfavorable.

In addition to its role in oxygenation, water movement is important in several other ways. In slow moving streams, emergent vegetation can develop which provides a habitat in which some species of Nemoura can live. Fast-flowing water is generally without rooted vegetation, and often deposited dead leaves and detritus are lacking. Species of stoneflies dependent on these materials for food are thus eliminated from streams where the current may be this strong. On the other hand, slender nymphs of such genera as Leuctra and Alloperla can live in rapid current because they work their way down among stones and gravel and find detritus in the interstices. Omnivorous species, as are many Isoperla, appear not to be too much affected by current and can adapt to fast streams where they feed on algae, or to slow-moving sections where they feed on detritus.

The substrate of both lakes and streams is very important to stonefly nymphs. Pure sandy bottoms, or a pure muddy bottom is seldom if ever inhabited by stoneflies. On stable substrates, consisting of large stones and boulders, moss and algae often develop on top of the stones and form a different microhabitat than is found among stones and loose gravels. Nymphs adapted for clinging, as exemplified by several species of Isoperla and Nemoura, are typical of the former habitat. Small slender species belonging to Alloperla and Leuctra are more common among stones and loose gravel in an unstable substrate.

In the arid western United States, many streams dry up during summer, and consequently support a very poor stonefly population or none at all. Even in the high Rocky Mountains where the period of drought is limited, intermittent streams support few stoneflies. By comparison, however, in Eastern and Midwestern states, stoneflies can be found abundantly in many streams that dry up regularly each year. Gaufin's work in Ohio as well as that of Frison and others in Illinois, indicates the presence of an abundant and varied stonefly population during winter and spring months in small, intermittent streams which dry up in winter. The adults of species in these streams emerge in winter or early spring and have a one-year life cycle. Species of Allocapnia and Nemoura are usually represented. When these streams are dry, the nymphs are either very small or the eggs have not yet hatched. This seems to enable the species to withstand drought conditions. The younger nymphs work their way down into the substrate to where the soil is damp and thus they escape desiccation. Large species which have two- or three-year life cycles cannot avoid desiccation in a similar manner and consequently are unable to live in this habitat.

Kinds of food and availability are ecological factors which determine the distribution of many species. In the Northeast, herbivorous nymphs rarely lack for suitable food supplies, regardless of current speed. The presence of rooted aquatic vegetation, diatoms, and other algae is quite general, and organic debris is usually available. Carnivorous species rarely find food a limiting factor at all. In general, however, the more highly productive or eutrophic streams have other characteristics which provide a generally less suitable environment for stoneflies, often resulting in more individuals but less diversity.

Because of the variety of pollutants that are being released into our streams and lakes, only brief consideration can be given to their effect on stoneflies. Organic wastes can completely eliminate most species through oxygen depletion, through the production of toxic products or decomposition, or by changing the nature of the substrate. Stoneflies are rarely happy in large growths of Sphaerotilus, the sewage bacterium. Nymphs respond adversely to most industrial wastes, to hot water from cooling processes, and to physical alteration wrought by a variety of human activities. In short, they are extremely sensitive to change.

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