Spiders in the home can be startling or even frightening, but not every spider is dangerous. Some of these arachnids are non-toxic to humans and help keep household insect pest populations down. It can be difficult to tell the difference between the venomous hobo spider, whose bite can cause a significant skin injury, and the harmless -- and more common -- giant house spider. Learning these creatures' identifying marks can help homeowners stay safe without killing the harmless species.

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Use a magnifying glass to identify spiders.

Species

Hobo spiders (Tegenaria agrestis) are a member of the funnel web spider family, and were originally native to Europe. These venomous spiders were introduced to the Pacific Northwest and other parts of the northwestern United States in the early 20th century. Giant house spiders (Tegenaria duellica or Tegenaria gigantea), close relatives of the hobo spider, were also introduced from Europe, and resemble their more dangerous cousins. However, these spiders don't pose a threat to pets or humans.

Identification

Many spider guides use color as an identifying feature. However, spiders can exhibit a wide range of colors in the wild. Other physical characteristics, such as shape and spotting, are much more reliable ways of telling the harmless giant house spider from the harmful hobo spider. Washington State University recommends using a hand lens or magnifying glass to identify these creatures.

Marking Patterns

Hobo spiders have a light-colored stripe running down the center of their sternum, on the underside of the spider. Giant house spiders also have a light stripe, but it's surrounded by 6 to 8 light spots. On some individuals, these spots are very faint and may be difficult to see. Hobo spiders also have uniformly colored legs, with no spotting, striping or dark rings.

Size

Hobo spiders are medium to large spiders with bodies between 8 and 15 mm long, or about 1/4 to 2/3 inch. Males are generally smaller than females, and have narrower abdomens. According to Washington State University, giant house spiders are larger and about three times more common. These spiders range from about 2/5 to 3/4 inch long. Always measure the body of the spider, rather than its legs, since the legs can vary significantly between individuals.