A misconception commonly held by many homeowners, according to Iowa State University Entomologist Donald Lewis, is that bathrooms are more prone to insect invasion since so many insects live in sewers and can find their way into bathrooms through sink and tub drains and faucets. In actuality, insects are no more common in bathrooms than in any other areas of the house. Still, there are insects that you are more likely to find in the bathroom than in other parts of the house.
House centipedes have long, flattened, segmented bodies and can measure up to 1 1/2 inches long. They have 15 pairs of legs that grow out of their 15 body segments. Centipedes are typically found in bathrooms because they tend to prefer damp and dark areas in relative seclusion. House centipedes are considered beneficial since they feed on spiders and other bothersome insects, so they should be tolerated when possible. To remove them, clean and vacuum that areas they are likely to frequent such as under shelves, behind stored boxes and behind appliances.
Like house centipedes, pseudoscorpions usually find their way into bathrooms because of their proclivity for damp conditions. Pseudoscorpions are arachnids like spiders; they have eight legs and giant pincers that give them a close resemblance to scorpions. Pseudoscorpions do not bite or sting and cannot cause structural damage, so they almost never present a problematic pest problem.
Anyone who has ever been in a bathroom has more than likely seen a silverfish scurry across the bathroom floor. Silverfish rarely grow to larger than half an inch. They do not cause structural damage and feed on many different food sources. Problematic infestations are quite rare but they are possible. Silverfish can be captured on sticky tape or using other sticky traps. Because they favour damp conditions, you can often compel silverfish to leave a given area by removing sources of excess moisture.
Other Common Insects
Any insects that reside in your home can find their way into the bathroom, and in fact, insect infestations can often go unnoticed until an insect accidentally becomes trapped in a sink or bathtub and cannot climb out. Cockroaches, like the other insects mentioned, tend to favour damp conditions so may be more likely to be in bathrooms than in other rooms in the house with fewer sources of excess moisture.