Uro (ウロ) are mushi who are usually found in closed spaces, including silkworm cocoons. They are capable of opening portals called uro passageways.
When uro are young they usually occupy empty cocoons. When they occupy a double cocoon, it can be split into two separate cocoons. This restricts the uro's movements, only allowing them to pass between the two. These cocoons can be used to send letters between people possessing either of the pair. When a letter is inserted into one of the cocoons, the person possessing its partner will eventually receive the letter via the uro.
Cocoons have to replaced regularly. Over the course of several years, the uro expand the uro passageway and penetrate into another passageway. When this starts happening, the letters are not delivered properly, and may arrive tattered or at the wrong addressee.
In areas where uro are commonly found, doors should never be completely shut, and if shut, they should never be opened again. When closed in, uro will multiply rapidly in the small space, but since they cannot survive in open areas, they will flee into uro passageways when a door is opened, sucking any humans in along with them.
Uro cannot live long outside an enclosed space. When sealed in the separated cocoons, they can only travel between the two.