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Montana Field Guide

Montana Field Guides

Giant Snowfly - Isocapnia grandis

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Native Species

Global Rank: G5
State Rank: SNR

Agency Status


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General Description
Species within the Capniidae are small winter stoneflies. This family is one of the largest families in the order Plecoptera, containing some 300 species distributed throughout the Holarctic region, 25 species occur in Montana. As their name implies these species are generally cold-water stenotherms and hatch in late-winter through early spring. The adults emerging in the winter are often found walking around on the snow. Their closest relatives are the rolled-winged stoneflies (Leuctridae). Many species are endemic to small ranges, perhaps due to the family's tendency to evolve tolerance for cold (isolating populations in mountain valleys) and winglessness (inhibiting dispersal).

Observations in Montana Natural Heritage Program Database
Number of Observations: 1

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Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Very little information exists about the ecology of this species (Baumann, Gaufin, and Surdick 1977). Merritt and Cummins (1996) report that members of the genus Isocapnia are found in both creeks and river, and are trophically shredder-detritivores.

Food Habits
Merritt and Cummins (1996) report that members of this family are trophically shredder-detritivores; eating large particulate organic materials such as detritus, leaves and plants.

Probably cold water stenotherms

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Citation for data on this website:
Giant Snowfly — Isocapnia grandis.  Montana Field Guide.  .  Retrieved on , from