Adults have a genital opening and either chelicerae modified for sperm transfer (if male) or a sperm-receiving structure (if female).

There are two nymphal stages (protonymph, deutonymph) that usually have lightly sclerotized dorsal, intercoxal and ventral plates.

[2][3] The soil-dwelling Mesostigmata are not as abundant as oribatids or prostigmatids that also occur in this habitat, but they are still ubiquitous in soil and may be important predators.

Size of these mesostigmatans decreases with soil depth: plant litter and humus have large species such as Veigaia (Veigaiidae), the humus-soil interface has smaller species like Dendrolaelaps (Digamasellidae) and the mineral soil has the tiny Rhodacarellus (Rhodacaridae).

[6] A few species are known from freshwater habitats, such as wet soil, phytotelmata, waterside vegetation and sewage filter-beds.

These include parasites of invertebrates (e.g. Varroidae) and of vertebrates (other families), as well as both ectoparasites (external) and endoparasites (internal).

It harms bees both directly by feeding on fat body tissue, and indirectly by transmitting viruses.

[4] The oldest known record of the group is an indeterminate Sejidae deutonymph from the mid-Cretaceous (Albian-Cenomanian) aged Burmese amber of Myanmar.