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