Booklice (Psocoptera) are very small (less than 1.5mm in long) but not true lice. While they resemble lice in size and shape, booklice feed only on fungi. mold, together with dried or decaying plant and animal materials.
They become particularly abundant in dark, damp places such as basements, storerooms, homes closed for the summer, and closets during the warmer periods of the year. If you find them in grain or other stored food products, it is an indication of high humidity which encourages mold growth.