There are many different kinds of spiders that live throughout the Pacific Northwest---both native and introduced. There are a few kinds that are dangerous to humans, but most are relatively harmless and will never bite a human (if they are even physically capable of doing so) unless seriously provoked. A combination of web design, habitat and body design can be used to identify most Northwest spiders. However, some types of must be examined under a microscope by a professional.
Black Widow Spider
One of the few dangerous spiders that can be found in the Pacific Northwest is the black widow. Adult female widows have a black body, 3 to 4 cm in diameter, and a red marking on their underbelly that usually resembles an hourglass. Male and immature widows have white or yellow stripes and are less venomous than adult females; males do not develop the full hourglass shape. Black widow webs are fairly shapeless and thin. According to Washington State University's (WSU) entomology department, widows are found mainly in "dry, undisturbed piles of firewood, old lumber, dry crawl spaces, outbuildings, rock piles, or bales of hay."
According to WSU, the hobo, or funnel-web, spider is "one of the most common spiders found in houses in the Pacific Northwest." While not nearly as venomous as black widows, their bites may cause moderate epidermal damage and flu-like symptoms. They are also quite aggressive. 4 to 5 cm in diameter, hobos are a brownish color with a light vertical stripe along their sternum. They live in funnel shaped webs, usually in dank, dark places, like under rocks. Hobos are very similar in design to some harmless Northwest spider species, with only microscopic differences.
Crab spiders are harmless to humans. They are either white or yellow with reddish marks on the side of the abdomen and two extra long pairs of front legs that resemble crab claws. Crab spiders do not spin webs, but live inside flowers where they wait to ambush bees and other prey.
European Cross Spider
Cross spiders are quite common outdoors throughout the Pacific Northwest. They are brown or orange with white dots on their backs that come together to resemble a cross and can grow to be quite large. Cross spiders often build large, fairly organized webs. According to Spiders of the Pacific Northwest Lowlands, "there may be more than a hundred [cross spider] webs in a typical suburban yard."
Wolf spiders are large and hairy ground dwelling spiders. They are usually dark brown in color. Spiders of the Pacific Northwest Lowlands identifies "meadows, forests, and shorelines" as common dwelling places for wolf spiders. They do not build webs or nests.
Washington State University provides some Northwest spider identification services. See their website for more information and detailed submission instructions.