Water vapour uptake and drought resistance in immobile instars of Parasitengona (Acari : Prostigmata)

Eggs and protonymphs of a number of terrestrial Parasitengona were tested f or drought resistance and their ability to take up atmospheric moisture. Sp ecies of the Erythraeoidea display higher drought resistance in both instar s than species of the Trombidioidea. This is achieved by evolutionary modif ications of the ground plan of the Parasitengona, including reduction of bo dy openings and additional sealing of others with lipids, resulting in redu ced water loss. Water vapour uptake is not evident in erythraeoid eggs or p rotonymphs but has been observed in the Trombidioidea and probably constitu tes a characteristic of at least the stem species of the terrestrial Parasi tengona. The resulting increase in fresh mass is about 50% before the proto nymph stage begins; its role is probably not related to drought resistance or osmotic regulation. Instead, it presumably serves to extend the cuticle and thus provides more space for the development of the subsequent instar.