ONH

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    1 - Thatching Ant, Formica obscuripes.

    04/28/2012 Dungeness National Wildlife Refuge, Washington

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    2 - Thatching Ants.

    05/18/2017 Heart O' the Hills Area, Olympic National Park, Washington

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    3 - Thatching Ants.

    05/25/2016 Heart O' the Hills Area, Olympic National Park, Washington

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    4 - Thatching Ant.

    09/28/2016 Sunrise Ridge Trail, Olympic National Park, Washington

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    5 - Thatching Ants.

    04/01/2017 Heart O' the Hills Area, Olympic National Park, Washington

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    6 - Thatching Ants.

    06/05/2017 Heart O' the Hills Area, Olympic National Park, Washington

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    7 - Thatching Ants scavenging a Checkerspot caterpillar.

    06/26/2016 Blue Mountain/Deer Park, Olympic National Park

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    8 - During the sunniest part of the day, Thatching Ants gather on side of their anthill away from the sun.

    04/29/2016 Heart O' the Hills Area, Olympic National Park, Washington

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    9 - The act of building anthills causes plants to flourish in a ring around the hill.

    04/01/2006 Dungeness National Wildlife Refuge, Washington

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    10 - Vegetation growing around a Thatching Ant hill.

    11/07/2014 Blue Mountain/Deer Park, Olympic National Park

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    11 - In the dead of winter the hill may be entirely covered in snow and frost.

    01/15/2017 Heart O' the Hills Area, Olympic National Park, Washington

Among the many ant species in northern Washington State are these Thatching Ants, Formica obscuripes. They build large anthills of bits of grass and other stems. The hills can be three or more feet in diameter and reach a foot or more above ground level. They scavenge insects for food (slide 8). We’ve photographed Thatching Ants from near sea level to around 6,000 feet elevation.

When the sun is out, Thatching Ants congregate in a crescent-shaped area on the side of the anthill opposite to the sun (slide 7).

In some cases, the activity of the ants causes plants to flourish in a ring around the hill. The effects on plants near ant hills may arise from several factors, including seed dispersal by the ants, a process called myrmecochory (slides 9 and 10).

Thanks to James C. Trager for the species ID on bugguide.net.