In nature, termites help to convert dead wood and other organic materials containing cellulose to humus. From this standpoint, termites are very beneficial animals. Only when people began building with wood in the natural home of the termite did the termites start feeding on buildings.
These termites remove only the soft layers (spring wood) within the annual rings of the wood grain, penetrating the hard layers only to get from one soft layer to another. This frequently leaves a damaged piece of wood looking very much like the pages of a book.
The most distinctive feature of subterranean termite damage is the presence of a brown mud like material that lines the galleries in an irregular pattern. Subterranean termites travel constantly from their nests in the ground to the wood, or to the other cellulose-bearing material upon which they feed.
This mixture of soil, feces, and saliva is also frequently used to cover the crack between two boards so that the termites can move about in a protected environment.