If you've ever run across bugs in old, damp books or in your stored flour, you are seeing psocids. These small bugs, also known as booklice, are common household pests. In fact, their dead bodies, when found in household dust, are thought to contribute to asthma attacks.
Psocids like damp areas with high humidity, since they have trouble controlling water loss. The high humidity also promotes mold growth, which is the primary food for psocids. Oftentimes, they can be found in very large numbers, due to the fact that they go unnoticed until they reach adult size.
Outdoors, psocids like loose, damp bark and sometimes go by the name of barklice. Moisture is what they seek and psocids will migrate to areas with higher relative humidity if their environments begin to dry out.