Arrenurus (αρρεν - male, ουρά - tail) is a genus of water mites within the family Arrenuridae, and was first described by Antoine Louis Dugès in 1834.

It has a cosmopolitan distribution in lentic waters, even on remote Pacific islands,[1] and is found on every continent, with the possible exception of Antarctica.

Studies also show that parasitism by the mites affect the host's longevity and fecundity by draining its tissue fluids, hampering copulation and interfering with sperm transfer.

They often select the ventral parts of the thorax or abdomen, and to a lesser extent are found on the back or on the head, or on the major wing veins.

Most stylostomes are able to resist assault by the host's immune system, such action consisting of initial hemolymph clotting, and deposition of melanin around the sac and its encapsulation.