However, many insect/spider bites reported in the PNW are, unfortunately, attributed to a spider that does not even occur here, namely the brown recluse spider. While the bite of the aggressive house spider is similar (see below), the brown recluse spider does not naturally occur in the PNW. The only known cases of this spider have been linked to people moving into the PNW from areas of known distribution.

The following characteristics should help to separate these spiders. The black widow spider (Latrodoctus sp.) occurs in the PNW generally in dry, undisturbed areas such as firewood piles, old lumber, dry crawl spaces, bales of hay, etc. The adult female is 1-1/2 inch in diameter, including legs, jet black with red markings on the underside of her abdomen. Males and immatures may be striped white or yellow. This spider is secretive, not aggressive, and rarely causes bite injuries. The venom is neurotoxic and may lead to systemic effects, sometimes even death.

Aggressive house spider (Tegenaria agrestis) is common in the PNW in and around houses, barns and sheds. It is a member of the funnel-web spider group. This is a relatively large spider (body size, excluding legs 3/8" - 5/8"; 1" - 1-3/4" including legs) with long, hairy legs and chevron-shaped abdominal markings. The venom is necrotic, causing open, localized wounds that may be slow to heal. Males may be more venomous than females and become quite common in mid-summer through the fall during the mating season. Despite its aggressive nature, however, reported bites from this spider are relatively rare. There are several closely related species that look very similar but whose venom is not as toxic.

For treatment of spider bites the medical community is referred to "Wasserman, G.S. and P.C. Anderson. 1984. Loxoscelism and necrotic arachnidism. J. Tox. Clin. Tox. (1983-1984). 21:451-472".