Tiny pseudoscorpions (about 4mm) live inside old books, effectively protecting them by eating booklice and dustmites
Pseudoscorpions are generally beneficial to humans since they prey on clothes moth larvae, carpet beetle larvae, booklice, ants, mites, and small flies. They are tiny and inoffensive, and are rarely seen due to their size.
There are more than 3,300 species of pseudoscorpions recorded in more than 430 genera, with more being discovered on a regular basis. They range worldwide, even in temperate to cold regions like Northern Ontario and above timberline in Wyoming’s Rocky Mountains in the United States and the Jenolan Caves of Australia, but have their most dense and diverse populations in the tropics and subtropics, where they spread even to island territories like the Canary Islands, where have been found around 25 endemic species. Species have been found under tree bark, in leaf and pine litter, in soil, in tree hollows, under stones, in caves, at the seashore in the intertidal zone, and within fractured rocks.
Chelifer cancroides is the species most commonly found in homes, where they are often observed in rooms with dusty books. There the tiny animals (2.5–4.5 mm or 0.10–0.18 in) can find their food like booklice and house dust mites. They enter homes by “riding along” attached to insects (known as phoresy), the insects employed are necessarily larger than the pseudoscorpion, or they are brought in with firewood.