Crossopriza lyoni is a widespread species of cellar spiders that prefer to live in or around human structures. They are commonly known as tailed cellar spiders, tailed daddy longlegs spiders, and sometimes box spiders. They all possess extremely long fragile legs that can reach up to 6 cm (2.4 in) long and a body length of that ranges from 2.5 to 7 mm (0.098 to 0.276 in). Their abdomens are distinctly squarish when viewed from the side and their carapace is more or less circular when viewed from above. They also possess two kinds of sound-producing organs and have six eyes.

The original range of C. lyoni is unknown. They have been introduced into other parts of the world accidentally and are now pantropical in distribution. They are a regulated species in some countries and are often regarded as pests due to the large amounts of unsightly webs they construct inside human homes. Some people, however, regard them as beneficial, as they are efficient predators of mosquitoes and other arthropods. They are harmless to humans.

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