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Stoneflies - Plecoptera: Perlodidae of Gunnison County, Colorado

Megarcys signata Laramide Springfly

(Hagen 1874)
Updated 20 Jan 2020
TSN 103111
Closeup of Megarcys signata nymph


Late instar Megarcys nymphs are easy to identify in the field because they´re big, active stonefly nymphs with small stubs of gills on each side of their thorax and neck. Look for the "W" on their heads as well. They become darker as they approach the time to emerge as adults.


Under rocks in the swift portions of streams and rivers throughout Gunnison county. Present across the Coastal, Cascade and Rocky Mountains of the western United States.

Local Research Results

Megarcys signata nymphs have been popular research subjects in the upper Gunnison Basin around the Rocky Mountain Biological Lab (RMBL). Common and easy to catch in the summer, these stoneflies are robust and relatively easy to handle and observe (as are Kogotus modestus). Both the Peckarsky and Allan Labs at RMBL have worked with this species. In fact Megarcys is such a popular research subject, you may want to read the annotations in the references of this webpage to enjoy them :-)

This adult was running and rushing around the rocks and spray in a steep tributary to Copper Creek on 18 July 2007.

Locations Collected

Copper Creek, East River, Benthette Brook, Cement Creek, Snodgrass Creek, Oh be Joyful Creek, Lake Fork Gunnison River and tributaries of all these streams. This author has specimens from the East River and Copper Creek near Gothic and Paradise Basin.


Hagen described this species in 1874 as Dictyopteryx signata, then it was called Perlodes signata, then renamed Arcynopteryx signata and now its called Megarcys signata. These generic name changes are common for many species as additional collecting reveals more information about the diversity or ranges of a group of insects. Some name changes occur when entomologists name species based on adults, then later manage to rear and associate nymphs and adults. Entomologists often discover traits of the nymphs that either match or are strikingly different from other genera. This causes everyone paying attention to the details to agree that the animal should be assigned to a different genera. At least the specific name (signata) stayed exactly the same through all this!

Good Links

On this website:
Photo - of Megarcys chasing an Epeorus nymph. You may need to page down once to see the photo.

Other Websites:
Photo - of Megarcys sp. nymph from the Tree of Life

A large Megarcys signata stonefly sitting in the sun on a rock in an alpine basin stream.
This adult was crawling around on a rock near a stream in an alpine basin near Crested Butte on the 19th of July 2007.


Allan, JD 1982 Feeding habits and prey consumption of three setipalpian stoneflies (Plecoptera) in a mountain stream. Ecology 63 1, 26-34. Abstract
     Allan found that Megarcys has a 1 year life cycle in Cement Creek and is present above 2900m Early instar nymphs appear by the beginning of June, grow rapidly throughout the summer, overwinter as larvae and emerge the following late June, early July. Growth was rapid until the water temperature dropped to 0°C in mid-October. Final instar females weighed 25-40 mg and males weighed 12-17 mg. M. signata guts were found to be fullest at 0400 and 0800 when animals were collected every 4 hours over a 24 hour period. Further research showed that gut clearance times were longer when the water was colder at night. Therefore, taking into account the 1 day time periods for gut clearance in starved animals, it appears that M. signata feeds continuously and not primarily at night (Allan, 1982). M. signata fed primarily on diatoms and chironomids over the 24 hours period they were examined.

Allan,JD; Flecker,AS and McClintock,NL 1987 Prey size selection by carnivorous stoneflies. Limnology and Oceanography 32 4, 864-872.
     Size selectivity varied with size of Megarcys. Percent of attacks per encounter by small stoneflies was strongly biased towards small prey. Large Megarcys attacks were weakly biased towards large prey. Capture success was greater and handling times were shorter with small prey compared to large prey.

Baumann,RW; Gaufin,AR and Surdick,RF 1977 The stoneflies (Plecoptera) of the Rocky Mountains. Memoirs of the American Entomological Society 31, 1-208.
     Quoted from page 131: "This species is common in creeks and rivers. The adults emerge from April to July"

Cather,MR and Gaufin,AR 1975 Life history and ecology of Megarcys signata (Plecoptera: Perlodidae), Mill Creek, Wasatch Mountains, Utah. Great Basin Naturalist 35 (1) PDF
     "During investigation of some of the stoneflies of Mill Creek, Wasatch Mountains, Utah, Megarcys signata, a large omnivourous stonefly was found to have a univoltine life history and a slow seasonal life cycle.
     Temperature appears to affect the growth rate of Megarcys signata. Warmer stream temperatures accompany the acceleration of growth rate, whereas cooler stream temperatures apparently retard the growth rate.
     Periods of maximum absolute growth rate correspond with maximum carnivorous feeding from August to September and March to April. Chironomidae, Ephemeroptera and Plecoptera, in that order were the most abundant prey in the foreguts. Young nymphs ingested considerable amounts of diatoms, filamentous algae, and detritus but not as much animal matter as did older nymphs.
     Megarcys signata was uniformly distributed throughout Mill Creek,except at the lowest station, where few nymphs were found.
     Emergence occurred in May and June, the peak occurring in June. The mean size of females and males decreased as emergence progressed."

Colburn,T 1982 Measurement of low levels of molybdenum in the environment by using aquatic insects. 29, 422-428.

Gill,BA; Harrington,RA; Kondratieff,BC; Zamudio,KR; Poff,NL and Funk,WC 2014 Morphological taxonomy, DNA barcoding, and species diversity in southern Rocky Mountain headwater streams. Freshwater Science 33(1) 288-301.
     Working in wadeable streams on the Front Range of Colorado, they found a cryptic species of Megarcys alongside M. signata.

Hagen,HA, 1874 Report on the Pseudo-neuroptera and Neuroptera collected by Lieut. W.L. Carpenter in 1873 in Colorado. Annual Report of the U.S. Geological and Geographical Survey of the Territories, embracing Colorado, 7: 571-577.
     Described as Dictyopteryx signata.

Klapálek, Frantisek 1912 Plécoptères. I. Fam. Perlodidae; [monographische Revision. II. Fam. Perlidae; Subfam. Perlinae, Subfam. Neoperlinae; mongraphische Revision] Series Sélys-Longchamps, Edmond de, baron, 1813-1900. Collections zoologiques; catalogue systematique et descriptif, fasc. 4, pt. 1-2.

Knight,AW; Gaufin,AR 1966 Altitudinal distribution of stoneflies (Plecoptera) in a Rocky Mountain drainage system. Journal of the Kansas Entomological Society 39 4, 668-675.
      They refer to this species as Arcynopteryx signata. They never caught any below 8,500 feet and considered Megarcys signata and five other species to be stenothermic or adapted to cold temperatures.

Kondratieff,BC and Baumann,RW 2002 A review of the stoneflies of Colorado with description of a new species of Capnia (Plecoptera: Capniidae). Transactions of American Entomological Society 128 3, 385-401.
     Quote from page 397; "Adults are active from April to September, depending on the elevation."

Molles,MC and Pietruszka,RD 1983 Mechanisms of prey selection by predaceous stoneflies: roles of prey morphology, behavior and predator hunger. Oecologia 57(1) 25-31. Abstract

Needham,JG and Claassen,PW 1925 A Monograph of the Plecoptera of North America. Entomological Society of America, Lafayette, Indiana. 397 pages.
     Described as Perlodes signata.

Nelson,CH 2009 Surface ultrastructure and evolution of tarsal attachment structures in Plecoptera (Arthropoda: Hexapoda). Aquatic Insects, (31)523-545. Html
     The author used scanning electron microscopy (SEM) to image the plantar surfaces of the stonefly tarsomeres and pretarsus of Megarcys signata and a number of other species.

Obernborfer,RY; McArthur,JV; Barnes,JR and Dixon,J 1984 The effect of invertebrate predators on leaf litter processing in an alpine stream. Ecology, 65(4), pp.1325-1331.
     Abstract: " The effect of the predators Megarcys signata (Plecoptera: Perlodidae) and Rhyacophila sp. (Trichoptera: Rhyacophilidae) on the abundance of macroinvertebrates and on the rates of leaf processing in artificial leaf packs was studied by using manipulative field experiments. Predators were confined within artificially constructed leaf packs in an alpine stream in Utah. Both predators significantly reduced the rate of breakdown of leaves in the fall, but had no effect in late winter, when the most important shredder, Zapada cinctipes, emerges, Predation on shredders caused a reduction in breakdown rates and an increase in the residence time of the leaf litter in the stream. In experimental treatments where predators significantly reduced the numbers of shredders and decreased the rate of leaf processing, leaf pack half-life increased an average of 10.3 d. These experiments demonstrate that invertebrate shredders can contribute substantially to the rate of leaf processing in streams and suggest that one of the factors limiting the abundance of natural shredder population is predation."

Peckarsky,BL 1988 Why predaceous stoneflies do not aggregate with their prey. Internationale Vereinigung für Theoretische und Angewandte Limnologie Verhandlungen 23, 2135-2140.
     Investigating optimal foraging theory, she found that contrary to theory K. modestus and Megarcys signata larvae did not hang out in high concentrations of prey.

Peckarsky,BL 1990 Habitat selection by stream-dwelling predatory stoneflies. Canadian Journal of Fisheries and Aquatic Sciences 48, 1069-1076.

Peckarsky,BL 1991a A field test of resource depression by predatory stonefly larvae. Oikos 61 1, 3-10.

Peckarsky,BL 1991b Is there a coevolutionary arms race between predators and prey? A case study with stoneflies and mayflies. Advances in Ecology 1, 167-180.

Peckarsky,BL 1991 Mechanisms of intra- and interspecific interference between larval stoneflies. Oecologia 85(4) 521-529. Abstract

Peckarsky,BL and Cowan,CA 1995 Microhabitat and activity periodicity of predatory stoneflies and their mayfly prey in a western Colorado stream. Oikos, 513-521. PDF
     Abstract: " Experiments were conducted to determine whether overlap between microhabitat preferences and activity periodicities of four mayfly species and their stonefly predators could explain species-specific differences in predator-prey encounter frequencies. Preferences for rock type (slate or granite), flow microhabitat (high or low), rock surface (top, bottom, upstream or downstream sides), and periodicity of drift and the use of rock tops were measured in a stream-side system of flow-through circular Plexiglas chambers receiving natural stream water and light levels. These parameters were compared among the predatory stoneflies, Megarcys signata or Kogotus modestus, and four species of mayflies that vary in their encounter rates with the stoneflies. Based on predator-prey encounter rates previously observed in similar chambers, we expected greater overlap between Megarcys and Ephemerella infrequens and the overwintering generation of the bivoltine mayfly, Baetis bicaudatus than with Cinygmula sp. Likewise, we expected Kogotus microhabitat use to overlap more strongly with that of summer generation Baetis than with later instars of Cinygmula and Epeorus deceptivus. Results ran counter to our predictions, indicating that microhabitats of the prey species with high predator encounter rates did not overlap more strongly with the stoneflies than did mayflies with low predator encounter rates. Most mayflies and stoneflies preferred the bottom surfaces of granite rocks, and showed few flow preferences. Most were nocturnal in their use of top rock surfaces, in drift and feeding activity periodicity. Therefore, nocturnal activity periodicities of both mayflies and stoneflies confirm that mayflies have not evolved feeding periodicity to avoid encounters with foraging stonefly predators. We conclude therefore, that neither temporal nor spatial microhabitat overlap is a reasonable explanation of differential encounter rates between predatory stoneflies and their mayfly prey. Alternative explanations for differential encounter rates are that more abundant or more mobile mayflies have higher encounter rates with predators, and effective pre-contact predator avoidance responses of other mayflies reduce their encounter rates with stoneflies."

Peckarsky, BL; Cowan,CA; Anderson,CR. 1994 Consequences and plasticity of the specialized predatory behavior of stream-dwelling stonefly larvae. Ecology 75(1) 166-181. Abstract

Peckarsky, B.L., C.A. Cowan, M.A. Penton and C. Anderson. 1993 Sublethal consequences of stream-dwelling predatory stoneflies on mayfly growth and fecundity. Ecology 74(6):1836-1846. Abstract

Peckarsky,BL; Dodson,SI 1980 Do stonefly predators influence benthic distributions in streams? Ecology 61(6) 1275-1282. Abstract

Peckarsky,BL, Kerans,B; Taylor,BW and McIntosh,AR. 2008 Predator effects on prey population dynamics in open systems. Oecologia.

Rader,Rb and Belish, TA 1999 Influence of mild to severe flow alterations on invertebrates in three mountain streams. Regulated Rivers: Research & Management. 15(4)353 - 363.
     Discussing Megarcys signata along with three other aquatic insects, they comment that "some stoneflies declined or were even locally extirpated" by severe flow alterations due to dams and water abstraction of their habitat.

Richardson,JW; Gaufin,AR 1971 Food habits of some western stonefly nymphs. Transactions of American Entomological Society 97, 91-121.
     Discussed as Arcynopteryx signata, they found that the nymphs emerge April to August. Based on their gut data, Megarcys is mostly carnivorous. They eat mostly Baetidae (small minnow mayflies) and Heptageniidae, as well as lesser amounts of Chironomidae (midges) and Simuliidae (blackflies). A few stoneflies were found in their guts indicating occasional cannibalism or intraguild predation. One specimen´s gut contained strictly plant material, indicating this species can be omnivorous and switch it´s diet when needed.

Smith,LW 1917 Studies of North American Plecoptera (Pteronarcinae and Perlodini) Transactions of the American Entomological Society 43(4):433-489. PDF
Page 472 of Lucy Smith's 1917 description of Megarcys signata
Page 473 of Lucy Smith's 1917 description of Megarcys signata
Page 474 of Lucy Smith's 1917 description of Megarcys signata

Stark,BP and Szczytko,SW 1988. Egg morphology and phylogeny in Arcynopterygini (Plecoptera: Perlodidae) Journal of the Kansas Entomological Society 61(2) 143-160.First Page
     Abstract: Comparative data are provided for eggs of nine of the eleven recognized genera in the Holarctic tribe, Arcynopterygini, and these data are used to generate a preliminary phylogeny for the group. Four generic clusters (Megarcys/Sopkalia; Frisonia/Perlinodes/Oroperla; Arcynopteryx/Neofilchneria/Skwala and Setvena/Pseudomegarcys) are established primarily from egg data, but resolution of the Frisonia and Arcynopteryx trichotomies required data from other character suites. Detailed illustrations of the epiproct complex are given for six Nearctic genera to provide additional support for the current generic classification of the group and a standardized terminology is proposed for systellognathan Plecoptera eggs.

Stewart,KW and Kondratieff,BC 2012Larvae of the Nearctic species of the stonefly genus Megarcys Klapálek (Plecoptera: Perlodidae). Illiesia 8(3):16-36. PDF
     Abstract: " Associated larvae of the five Nearctic species of Megarcys are comparatively described and illustrated. Four of them, Megarcys irregularis Banks, M. subtruncata Hanson, M. watertoni (Ricker), and M. yosemite (Needham and Claassen) are described in detail for the first time. Body size and setation, wingpad macroptery/brachyptery, and gill size were variable between populations of some species. Diagnostic differences were not found in late instar larvae of the five species in color and pigment patterns, mouthparts, body and appendage setation, gill size and structure, and developing male and female genitalia. The detailed descriptions and measurements help expand the generic diagnosis, show morphological differences between populations of different habitats, and provide an atlas of illustrations for larvae of the five species. "

A large stonefly sitting in the sun on a rock in an alpine basin stream.

Look for the orange mites on this female Megarcys nymph. The white nubs among the mites are gills. She was caught and released from the upper East River on the 17th of July 2009. Straight in front of her is a small Cinygmula nymph. There are three Chironomid or Midge larvae around the stonefly as well.

Stewart,KW and Stark,BP 2002 Nymphs of North American Stonefly Genera. 2nd edition The Caddis Press, Columbus, Ohio. 510 pages.
     Photo of nymph on page 111 figure 6.43. Illustrations of nymph on page 420-421, figures 14.35-14.36

Taylor,BW; Anderson,CR and Peckarsky,BL 1998 Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive sucess. Oecologia 114, 494-502. Abstract

Taylor,BW; Anderson,CR and Peckarsky,BL 1999 Egg diapause and semivoltinism in the Nearctic stonefly Megarcys signata (Plecoptera: Perlodidae). Aquatic Insects 21, 179-185.
     They found that M. signata was protandrous (males emerge first) and females were about twice the body mass of males in both streams studied. M. signata emerged earlier and larger from a trout stream (East River) than the stoneflies from a smaller fishless tributary (Benthette Brook). Temperature did not affect size at metamorphosis. Many females from Benthette Brook were brachypterous (short-winged), while East River Megarcys females had longer wings. In 1992 East River stoneflies emerged from mid June to mid July with the peak emergence in late June. Benthette brook Megarcys emerged from early July to late August, peaking in Late July. Adults emerged at dawn between 0600 and 088 hours until later in the season when they emerged at night (after 2200 hours). Adults became more active after dark. Manipulative experiments determined that male body mass did not affect reproductive success. Sometimes male drumming attracted females who mated immediately upon finding the male. Other times the first male to stumble on the female mated with her. Mating usually occurred within the first three days after emergence. Copulation generally lasted all night. Females who mated multiple times had lower total lifetime fecundity than females who mated once. Multiple matings reduced the lifespan of males and females. There was no evidence of parthenogenesis in unmated females. Neither sex fed on sugar water except rarely and dissections of field collected adults showed atrophied digestive tracts.

Taylor,BW; Anderson,CR; Peckarsky,BL 1999 Delayed egg hatching and semivoltinism in the Nearctic stonefly Megarcys signata (Plecoptera: Perlodidae). Aquatic Insects, 21(3), pp.179-185. PDF
     Abstract: "Delayed egg hatching can influence the survival of aquatic insects by reducing exposure to competitors, predators, parasites, or unfavorable environmental conditions. We examined egg development in a Colorado population of the stonefly, Megarcys signata (Plecoptera: Perlodidae), whose larvae inhabit high altitude streams in the Rocky Mountains of western North America. Five-thousand fertilized eggs were collected from 40 different females and incubated in the East River, Gunnison County, Colorado, until late fall, at which time unhatched eggs were transported to incubation chambers in Ithaca, New York. We used three different combinations of photoperiod and temperature (10L:14D, 4°C; 10L:14D, 8°C; 8L:16D, 8°C) in an attempt to induce hatching. Eggs in the treatments with temperatures elevated above normal winter temperatures in the East River (8°C) began developing after 6 months, whereas eggs in the treatment most closely simulating natural stream conditions (4°C) did not hatch after 10 months of incubation. Our data indicate that this population of Megarcys signata has an extended egg stage that persists for almost a year, and that it is semivoltine at these elevations of 2885 and 2895 meters."

VanWieren,BJ; Kondratieff,BC and Stark,BP 2001 A review of the North American species of Megarcys Klapálek (Plecoptera: Perlodidae). Proceedings of the Entomological Society of Washington 103 2, 409-427.
     They provide keys to identify the five North American species by male or females and eggs. There are illustrations of an adult male Megarcys signata on page 410, female subgenital plates on page 418 and four electron micrographs of an egg on page 419. Among the specimens they examined were a male and female collected by B. Armitage from the East River near Avery campground in 1986 and a male and female from tributaries to Oh-Be Joyful Creek in 1996 by B. Kondratieff and N. Lorenzon

Ward,JV, Kondratieff,BC and Zuellig,RE 2002 An Illustrated Guide to the Mountain Stream Insects of Colorado. 2nd ed. University Press of Colorado, Boulder, Colorado. 219 pages.
     Illustration of M. signata nymph on page 74, figure 34.

Wellnitz,T 2014 Can current velocity mediate trophic cascades in a mountain stream?. Freshwater Biology, 59(11) 2245-2255. PDF


Mesosternal Y-arms.
Only visible easily on mature nymphs.

Brown, WS 2004 Plecoptera or Stoneflies of Gunnison County, Colorado, USA