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Montana Field Guide

Montana Field Guides

Hooked Snowfly - Isocapnia crinita

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Species of Concern
Native Species

Global Rank: G5
State Rank: S2
(see State Rank Reason below)

Agency Status

External Links

State Rank Reason (see State Rank above)
The Hooked Snowfly is currently ranked "S2" in Montana because it was thought to be at risk due to very limited and/or potentially declining population numbers, range and/or habitat, making it vulnerable to extirpation in the state. But, recent range extensions due to newly reported collections may warrant re-evaluating this SOC rank.
  • Details on Status Ranking and Review
    Hooked Snowfly (Isocapnia crinita) Conservation Status Review
    Review Date = 09/18/2008
    Population Size

    ScoreU - Unknown


    Range Extent

    ScoreU - Unknown

    Area of Occupancy

    CommentLB = 4-40 km (about 2.5-25 miles)

    Length of Occupancy

    ScoreLB - 4-40 km (about 2.5-25 miles)

    Long-term Trend

    ScoreE - Relatively Stable (±25% change)

    Short-term Trend

    ScoreE - Stable. Population, range, area occupied, and/or number or condition of occurrences unchanged or remaining within ±10% fluctuation


    ScoreF - Widespread, low-severity threat. Threat is of low severity but affects (or would affect) most or a significant portion of the population or area.

    SeverityLow - Low but nontrivial reduction of species population or reversible degradation or reduction of habitat in area affected, with recovery expected in 10-50 years.

    ScopeModerate - 20-60% of total population or area affected

    ImmediacyLow - Threat is likely to be operational within 5-20 years.

    Intrinsic Vulnerability

    ScoreB - Moderately Vulnerable. Species exhibits moderate age of maturity, frequency of reproduction, and/or fecundity such that populations generally tend to recover from decreases in abundance over a period of several years (on the order of 5-20 years or 2-5 generations); or species has moderate dispersal capability such that extirpated populations generally become reestablished through natural recolonization (unaided by humans).

    Environmental Specificity

    ScoreA - Very Narrow. Specialist. Specific habitat(s), substrate(s), food type(s), hosts, breeding/nonbreeding microhabitats, or other abiotic and/or biotic factor(s) are used or required by the Element in the area of interest, with these habitat(s) and/or other requirements furthermore being scarce within the generalized range of the species within the area of interest, and, the population (or the number of breeding attempts) expected to decline significantly if any of these key requirements become unavailable.

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).

Adults are emerging in March-May.

Diagnostic Characteristics
Larvae cannot be identified to species and adults are rarely collected during summer sampling.

Species Range
Montana Range Range Descriptions


Range Comments
Rangewide, the species is known from Alaska (SNR), Montana (S2), Idaho, New Mexico (SNR), Colorado, Utah (S1S2), Manitoba (SNR), Saskatchewan (SNR), and the Yukon Territory (SNR).

Observations in Montana Natural Heritage Program Database
Number of Observations: 22

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



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

Very little information exists about the ecology of this species (Baumann et al. 1977). Merritt and Cummins (1996) report that members of the genus Isocapnia are found in both creeks and rivers 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.

These stoneflies are probably cold water stenotherms.

Reproductive Characteristics
Adults are emerging in March-May.

No management plan is in place, but more targeted sampling in early spring is recommended to fully understand this species' distribution, as it may be more widespread than previously thought.

Threats or Limiting Factors
Listed as a rare species when found; never abundant. Adults are rarely collected by biomonitoring agencies and few records exist.

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