This species occurs on sandy, silty, and gravelly banks of medium to large streams all over western North America. Before the Glines Canyon dam was removed in 2011, the Elwha River formed a delta as it flowed into Lake Mills in Olympic National Park. Robbie Hart discovered this population on a sand bank at the far southern end of Lake Mills. Thanks to David H. Kavanaugh, Senior Curator of Entomology at the California Academy of Sciences, for the identification from a photo.
While Opisthius, with its large eyes, looks somewhat like a small tiger beetle (both are members of the family Carabidae), its jaws are much smaller. These beetles move somewhat like Tiger Beetles, but fly less and are easier to approach. On the same sandbank, I observed and photographed two species of tiger beetles.
On one occasion, I photographed a mating pair (6).