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Updated 6 Oct 2011

Side by Side Comparison
of the Stoneflies:
Alloperla pilosa Alloperla thalia

Both are bright green as live adults, but bleach to white in ethanol.
Adult abdomen striped Adult abdomen plain

epiproct
epiproct

subgenital plate
subgenital plate

References

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

Baumann,RW and Kondratieff, BC 2008 The Alloperla severa complex (Plecoptera: Chloroperlidae) of western North America. Illiesia 2008 4(6):66-75. PDF
     The authors split A. severa into three species A. severa, A. elevata and A. thalia. A. severa is restricted to northern North America. Alloperla thalia is present in Colorado and may be the species we have been calling A. severa.

Lyon,ML; Stark,BP 1997 Alloperla (Plecoptera: Chloroperlidae) of western North America. Entomological News 108 5, 321-334.
      Quoted from page 333: "Although A. severa and A. pilosa are usually identified on the basis of size and abdominal striping, their epiprocts are difficult to distinguish with light microscopy. The dorsal shapes of the epiprocts are very similar, and it may be difficult to detect the shallow apical crenulations of A. severa (Figs 5, 13). The epiproct of A. severa is shorter and narrower than in A. pilosa, but the most conspicuous difference is in the setation. A. pilosa has only a tiny bare, button-like apex to the epiproct (Fig 5), whereas A. severa has at least the apical third bare (Figs 5, 13). The eggs of these species have similar collars, and the chorionic surfaces of both are pitted throughout with aeropyles (Figs 28, 34). In A. pilosa the aeropyles are larger and more widely spaced in the equatorial area than in the polar areas, but in A. severa the aeropyles are rather uniform in size and spacing." .

Brown, Wendy S. 2004 Stoneflies or Plecoptera of Gunnison County, Colorado
www.gunnisoninsects.org