This semiaquatic spider is one of eight species of fishing spiders in the Dolomedes genus found in the US and Canada. It is greenish brown with distinct white lines and spots and a body length of up to ½ inch for males and ¾ inch for females (not including legs). The common name is not derived from the white spots, rather six dark spots on the underside of the cephalothorax (on the sternum).

Fishing spiders use the water much like a web to sense insects struggling in the surface film. They will rest on something floating or sticking out of the water, but keep their front two to three pairs of legs on the surface. Ripples caused by potential prey send out vibrations and trigger their predatory response–a quick chase and pounce on the water. They can also dive into the water, either to escape predation, or to prey on submergent animals like small fish or tadpoles. Waterproof hairs on the spider’s body create a layer of air, allowing it to breathe underwater for up to 30 minutes.
Photo by Kristi DuBois on 3/6/2020 in Missoula.