Flour or grain mites are pale, pearly or grayish white, with legs varying in color from pale yellow to reddish-brown. Each leg has one claw at the end. As with all mites, they are smooth, wingless, soft-bodied creatures. The males are from 0.3 to 0.4 mm long, and the female is from 0.3 to 0.6 mm. The males have enlarged forelegs which bear a thick spine on the ventral side. These two characters can be used to separate Acarus sp. from other genera.
Juvenile mites are similar in appearance to the adults. The first or larval stage has only six legs. However, when they molt into the nymphal stage, they have eight legs like the adults. Mite eggs are oval, smooth, white, and are 0.12 mm long.
Grain mites are pests that can feed on a variety of processed or finely ground grains, wheat germ, yeast, cheese, powdered milk, flour, or mold spores. Grain mites primarily attack the germ. However, they will feed on other parts of the kernel, as well as mold growing on the grain. These mites are responsible for the spread of various fungal spores throughout a grain mass and into adjoining bins. When present in large numbers, the flour or grain mites promote sweating and impart a disagreeable odor to the grain.