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Stoneflies - Plecoptera: Perlodidae of Gunnison County, Colorado

Skwala americana
American Stonefly, Little Golden Stonefly

(Klapálek) 1912
Updated 20 Jan 2020
TSN 568735

Life History

Richardson and Gaufin (1971) found nymphs in Utah were polyphagous (they ate many things), feeding primarily on Ephemeroptera (mayflies) (25%) and Chironomidae (midges) (21%).

Fuller and Stewart (1977) found S. americana's diet was over 75% animal matter in May in the Gunnison River at Lost Canyon Resort. Approximately equal volumes of Ephemeroptera (34%) and Plecoptera (34%) were present in the gut with detritus making up the remaining 24%. Chironomids and Simuliids were insignificant in the diet, suggesting S. americana, Cultus aestivalis and Isogenoides zionensis avoid direct competition through prey resource partioning. Small nymphs appeared in September and were highly carnivorous, showing positive electivity for Chironomidae. Their diet switched from carnivorous to polyphagous eating filamentous algae (30%, mostly Ulothrix) and diatoms (18% mostly Synedra, Fragilaria and Cymbella) in September and October. A similar diet switch was also noted in the Perlids Claassenia sabulosa and Hesperoperla pacifica.

Locations Collected

Gunnison River at the Lost Canyon Resort


Older publications may refer to this species confusingly as Arcynopteryx americana, Arcynopteryx parallela or in the latter part of the 20th century as Skwala parallela.

Good Links

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Baumann, RW Gaufin, AR, Surdick, RF 1977 The stoneflies (Plecoptera) of the Rocky Mountains. Memoirs of the American Entomological Society 31, 1-208.

Buchwalter,DB; Cain,DJ; Martin,CA; Xie,L; Luoma,SN; Garland,JT 2008 Aquatic insect ecophysiological traits reveal phylogenetically based differences in dissolved cadmium susceptibility. Proceedings of the National Academy of Sciences 105 24, 8321-8326.

Frison,TH 1937 II. Descriptions of Plecoptera with special reference to the Illinois species. Illinois Natural History Survey Bulletin 21 3, 78-99.
     Discussed as Hydroperla paralella.

Fuller,RL; Stewart,K,W 1977 The food habits of stoneflies (Plecoptera) in the Upper Gunnison River, Colorado. Environmental Entomology 6, 293-302.
     Discussed as Skwala parallela

Gaufin,AR; Clubb,R and Newell,R 1974 Studies on the tolerance of aquatic insects to low oxygen concentrations. Great Basin Naturalist 34:45-59. PDF
      The authors studied the acute short term tolerance of aquatic insects to low oxygen. They used the 96 hour Median Tolerance Limit. They discussed Skwala americana as Arcynopteryx parallela. They did not find a TLm96 for S. americana using their experimental apparatus. Survival was 100% from 2-5mg/l of oxygen.

Hassage,RL and Stewart,KW 1990 Growth and voltinism of five stonefly species in a New Mexico mountain stream. The Southwestern Naturalist, 35 (2)130-134. Abstract and first page
     Discusses as Skwala parallela.

Kiffney,PM; Clements,WH 1993 Bioaccumulation of heavy metals by benthic invertebrates at the Arkansas River, Colorado. Environmental Toxicology and Chemistry 12, 1507-1517.
     Quote from page 1512: "Variation among taxa: Metal concentrations in organisms collected from station AR-5 [impacted by heavy metal pollution from California Gulch] (fall, spring, summer) varied significantly among taxa (Fig 7). The highest concentrations were generally found in the mayfly Baetis spp., the stonefly Pteronarcella badia, and the caddisfly Arctopsyche grandis, whereas the lowest levels were measured in the two predators, Skwala americana, and Rhyacophila spp."

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.
     Described as Arcynopteryx americana.

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.

Mutch,RA and Pritchard,G 1986 Development rates of eggs of some Canadian stoneflies (Plecoptera) in relation to temperature. Journal of the North American Benthological Society 5(4)272-277. First page
     DIscussed as Skwala parallela. Abstract: The eggs of eight species of Plecoptera from Alberta were reared at constant temperatures between 2 and 25°C. All species' eggs hatched at 2°C but none hatched at 25°C. The relationships between temperature and number of days required for development and hatching were fitted to power equations. Slope values (b) ranged from -1.31 for Skwala parallela to 0.03 for Amphinemura banksi; the remainder fell between -0.94 and -0.68. All of these values, except for that for S. parallela, imply that stonefly eggs develop most efficiently at the lowest temperature within the favorable range or develop with equal efficiency throughout the favorable range.

Richardson,JW; Gaufin,AR 1971 Food habits of some western stonefly nymphs. Transactions of American Entomological Society 97, 91-121.
     Discussed as Arcynopteryx parallela

Ricker,WE 1992 Origin of stonefly names proposed by Ricker and collaborators. Perla, 18(1) 12 pages. PDF
      Quote from page 9: "Skwala Ricker 1943 (as sg. of Arcynopteryx). The name of a clan of Salish Indians living near Sardis, British Columbia. "

Shepard, WD. and Stewart KW 1983 Comparative Study of Nymphal Gills in North American Stonefly Genera and a New, Proposed Paradigm of Plecoptera Gill Evolution. Miscellaneous Publications of the Entomological Society of America 13:1-57
     Illustration of nymphal osmobranchiae (gills) on page 52 as Skwala parallela.

Short,RA and Ward,JV 1980 Life cycle and production of Skwala parallela (Frison) (Plecoptera: Perlodidae) in a Colorado montane stream. Hydrobiologia 69(3), 273-275.
     Abstract: " The life cycle and production of Skwala parallela, a perlodid stonefly, was investigated in a third-order Colorado montane stream. The species exhibited a univoltine life cycle with a distinct cohort. Small nymphs appeared in May. Rapid growth was exhibited throughout summer and autumn. During winter, growth slowed somewhat but was continuous until April. Maximum density of 34 nymphs/m2 occurred in July. Based upon the instantaneous growth method, annual production was 395.3 mg/m2 or 3.95 kg/ha dry weight with a P/B ratio of 4.4. "

Smith,LW 1917 Studies of North American Plecoptera (Pteronarcinae and Perlodini) Transactions of the American Entomological Society 43(4):433-489. PDF
     Discussed as Arcynopteryx americana.
Page 481 of Lucy Smith's 1917 description of Arcynopteryx, now Skwala americana Page 482 of Lucy Smith's 1917 description of Arcynopteryx, now Skwala americana

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 Stark,BP 2002 Nymphs of North American Stonefly Genera. 2nd edition The Caddis Press, Columbus, Ohio. 510 pages.
     Photo of mature nymph pattern on page 111 figure 6.45. Illustrations of nymph on page 449-450, figures 14.55-14.56

Zwick,P 1989 Notes on Plecoptera (18) Skwala americana (Klaplek, 1912), comb.n., the valid name for Skwala parallela (Frison, 1936). Aquatic Insects 11(3), 181-182.


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


Notice the serrated edge on the big tooth. Compare to Arcynopteryx.