Hypopus

The hypopus (plural hypopi or hypopodes) is a modified juvenile stage of acaridid mites. It corresponds to the deutonymph, the second of up to three nymphal stages. This stage is a facultative and may or may not be present in the life-cycle of the mites, depending on environmental conditions (food availability, humidity) and density-dependent factors (overcrowding).

The hypopus is morphologically and behaviourly very different from other instars: there are no functional mouthparts, the body is sucker-shaped, smooth, with a thick cuticle. They are more apt to resist bad environmental conditions, such as desiccation. Also, in many cases they are used for dispersal. This is achieved through phoresy, the hypopus attaching to a passing animal, often an insect, with suckers or grasping organs.