ONH

Cairn-building Running Crab Spider

Rhysodromus alascensis

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    1 - Cairn-building Running Crab Spider, Rhysodromus alascensis, female before laying eggs, same individual in slides 1-5.

    08/16/2011 Obstruction Point Area, Olympic National Park, Washington

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    2 - Cairn-building Running Crab Spider, female before laying eggs, same individual in slides 1-5.

    08/16/2011 Obstruction Point Area, Olympic National Park, Washington

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    3 - Cairn-building Running Crab Spider, female before laying eggs, same individual in slides 1-5.

    08/16/2011 Obstruction Point Area, Olympic National Park, Washington

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    4 - Cairn-building Running Crab Spider, female before laying eggs, same individual in slides 1-5.

    08/16/2011 Obstruction Point Area, Olympic National Park, Washington

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    5 - Cairn-building Running Crab Spider, female before laying eggs, same individual in slides 1-5.

    08/16/2011 Obstruction Point Area, Olympic National Park, Washington

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    6 - Cairn-building Running Crab Spider, male, same individual in slides 6-8.

    07/08/2013 Obstruction Point Road, Olympic National Park, Washington

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    7 - Cairn-building Running Crab Spider, male, same individual in slides 6-8.

    07/08/2013 Obstruction Point Road, Olympic National Park, Washington

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    8 - Cairn-building Running Crab Spider, male, same individual in slides 6-8.

    07/08/2013 Obstruction Point Road, Olympic National Park, Washington

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    9 - Cairn-building Running Crab Spider, male.

    08/09/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    10 - Cairn-building Running Crab Spider, female on stonecrop.

    08/19/2011 Blue Mountain/Deer Park, Olympic National Park

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    11 - Cairn-building Running Crab Spider, female.

    07/08/2013 Obstruction Point Road, Olympic National Park, Washington

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    12 - Cairn-building Running Crab Spider, female.

    07/18/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    13 - Cairn-building Running Crab Spider, female.

    07/11/2014 Upper Wolf Creek Trail, Hurricane Ridge, Olympic National Park, Washington

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    14 - Typical location for Cairn-building Running Crab Spiders: sunny, bare rock face. Two cairns are visible in this image.

    07/28/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    15 - Typical location for Cairn-building Running Crab Spiders, closeup showing the two cairns visible in slide 14.

    07/28/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    16 - Cairn-building Running Crab Spider females build cairns around their egg sac using very small objects from their environment. Bits of rock are typical building material.

    09/27/2009 Obstruction Point Road, Olympic National Park, Washington

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    17 - Two adjacent cairns of rock pieces.

    09/05/2011 Sunrise Ridge Trail, Olympic National Park, Washington

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    18 - Cairn of rock pieces.

    07/28/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    19 - Cairn of rock pieces.

    08/09/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    20 - Cairn of rock pieces built into crack in rock face.

    08/17/2014 Sunrise Ridge Trail, Olympic National Park, Washington

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    21 - Cairn of rock pieces and at least one seed.

    09/14/2008 Sunrise Ridge Trail, Olympic National Park, Washington

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    22 - Cairn of rock pieces and needles.

    09/14/2008 Sunrise Ridge Trail, Olympic National Park, Washington

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    23 - Cairn of rock pieces and needles.

    09/14/2008 Sunrise Ridge Trail, Olympic National Park, Washington

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    24 - Cairn of rock pieces and needles, with a covering silk layer.

    08/19/2011 Blue Mountain/Deer Park, Olympic National Park

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    25 - Cairn built mostly of needles.

    09/02/2010 Hurricane Hill Area, Olympic National Park, Washington

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    26 - Cairn with needles and evergreen seeds.

    08/26/2014 Obstruction Point Area, Olympic National Park, Washington

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    27 - Cairn with grass seeds.

    09/06/2014 Blue Mountain/Deer Park, Olympic National Park

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    28 - Cairn-building Running Crab Spider, female with cairn. This is one of the earliest photos we have clearly associating the spider with the cairn.

    09/11/2008 Blue Mountain/Deer Park, Olympic National Park

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    29 - Female with cairn. This is the same individual as in slide 28.

    09/11/2008 Blue Mountain/Deer Park, Olympic National Park

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    30 - Female with cairn.

    07/11/2009 Blue Mountain/Deer Park, Olympic National Park

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    31 - Female with cairn.

    08/07/2011 Obstruction Point Area, Olympic National Park, Washington

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    32 - Female with cairn.

    07/17/2012 Hurricane Hill Area, Olympic National Park, Washington

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    33 - Female with cairn. This cairn has a covering of silk.

    07/06/2013 Obstruction Point Road, Olympic National Park, Washington

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    34 - Female with cairn. Same individual as in slide 32.

    07/06/2013 Obstruction Point Road, Olympic National Park, Washington

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    35 - Female on silk strands.

    07/17/2013 Obstruction Point Road, Olympic National Park, Washington

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    36 - Female on silk covering cairn.

    08/25/2013 Upper Wolf Creek Trail, Hurricane Ridge, Olympic National Park, Washington

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    37 - Female with cairn.

    07/18/2013 Sunrise Ridge Trail, Olympic National Park, Washington

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    38 - Female with cairn.

    07/26/2014 Sunrise Ridge Trail, Olympic National Park, Washington

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    39 - Female with cairn.

    08/26/2014 Obstruction Point Area, Olympic National Park, Washington

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    40 - Cairn with needles and rock pieces, and the exoskeleton of a Fire-colored Beetle larva (see Insects > Beetles in menu). The same cairn is shown in slides 39-42.

    09/29/2008 Blue Mountain/Deer Park, Olympic National Park

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    41 - The same cairn is shown in slides 39-42. Beginning to pull the cairn away from the egg sac.

    09/29/2008 Blue Mountain/Deer Park, Olympic National Park

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    42 - The same cairn is shown in slides 39-42. The cairn is pulled away, revealing the egg sac.

    09/29/2008 Blue Mountain/Deer Park, Olympic National Park

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    43 - The same cairn is shown in slides 39-42. The egg sac opened, revealing the eggs.

    09/29/2008 Blue Mountain/Deer Park, Olympic National Park

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    44 - In 2014, we opened another cairn and egg sac, revealing spiderlings. Spiderlings after hatching are about 2.5 mm long.

    09/06/2014 Blue Mountain/Deer Park, Olympic National Park

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    45 - Cairn-building Running Crab Spider spiderlings in opened egg sac closeup

    09/06/2014 Blue Mountain/Deer Park, Olympic National Park

In summer of 2008 as we walked at Blue Mountain and at Hurricane Ridge we noticed that in crevices of rocky outcroppings we would occasionally find small collections of tiny rock bits bound together and anchored by silk. They looked like rock cairns to us.

We didn’t know what organism built these cairns but suspected it was a spider, because we observed and photographed female spiders standing on or near cairns as well as running away from cairns and returning. We pulled apart a cairn and found eggs.

Photographs of the spider posted to bugguide.net in 2008 led to an ID as Philodromus alascensis (now called Rhysodromus alascensis), though some commenters doubted the spiders were associated with the cairns. The confirming evidence to us that these spiders build the cairns came in 2014 when we saw and photographed a cairn that contained spiderlings. We informally call these spiders cairn-building spiders. Our Rhysodromus alascensis images on bugguide.net

In 2018 we found this mention of the cairn-building behavior: In Field Guide to the Spiders of California and the Pacific Coast States, Richard J. Adams notes this behavior: “At least one species, Philodromus alascensis, places its egg sac in a small, rocky recess, where it is covered with grains of sand, wood flakes, and other debris.”

The slides show cairns made from various materials. The egg sac is under the cairn. In some cases females cover the cairn with a layer of silk. Cairns are typically located on sunny rock faces.

Running Crab Spiders, family Philodromidae, are free-living ground hunters. They spin silk, but not to catch prey.

 

Note on the genus name: In 1965, Robert X. Schick introduced a new genus, Rhysodromus for the former Philodromus species alascensis and virescens. Bulletin of the American Museum of Natural History. The new genus was challenged, but then ressurected in 2016 in “The spider genus Rhysodromus Schick, 1965 in the Crimea” Kastrygina1 and Kovblyuk