Spiders lurking beneath toilet seats, awaiting human prey, are the subject of urban legends. However, there is one situation where a spider may actually be underneath toilet seats. A spider can conceivably find its way to your toilet, but it is unlikely for it to be inside the toilet. The spider will have to enter the bathroom using the pipe of another fixture, as it can't come up through the toilet's plumbing.
When venturing indoors, spiders naturally seek out cluttered, secluded spots that are away from humans. They seek out moist environments, which a bathroom toilet provides. However, if a toilet is regularly cleaned, the chemicals keep spiders from its surface. A spider is unable to come up through the drain of a toilet because of the water that stays inside the toilet's trap, which is the curved section of the toilet drain's passageway. It seals out small intruders like spiders, and also protects against sewer gases entering through the toilet pipes.
The one exception to a spider possibly lying in wait underneath a seemingly safe toilet seat is an outhouse toilet. An outhouse is a small structure that is separate from a main residence. The toilet that is inside does not have plumbing, nor is it attached to any sewer system. The female black widow spider, which is known to bite humans more than the male black widow, builds her web in the hidden confines underneath the seat on an outhouse toilet. These spiders, according to the Centers for Disease Control and Prevention, or CDC, choose outhouses because the flies that the spiders eat are plentiful in these spaces.
U.S. Venomous Spiders
In addition to the black widow, which is found mostly in southern and western states, the CDC also lists two other venomous spiders that are found in the United States, and also may venture indoors. The brown recluse spider, most often found in southern and Midwestern states, normally seek out secluded, dry areas, like a closet or inside a shoe. Inspecting, and shaking out, any shoes before you wear them can help prevent brown recluse bites. The hobo spider, of the Pacific Northwest, normally seeks out window sills, spots behind furniture and closets. This spider, which runs fast, is also more likely to attack if it feels threatened.
Spiders Entering Bathrooms
Spiders can wander into a home in search of insect prey, and when they do, they normally settle into undisturbed areas like garages and basements. If a home has a crawl space underneath it, a spider can enter directly into the bathroom from below. If a lavatory drainpipe goes down through the floor, instead of through the wall behind it, the spider will scale the pipe up through the floor hole around it. Prevent this by making sure that the hole around the pipe is totally sealed.