Eggs and protonymphs of a number of terrestrial Parasitengona were tested for drought resistance and their ability to take up atmospheric moisture. Species of the Erythraeoidea display higher drought resistance in both instars than species of the Trombidioidea. This is achieved by evolutionary modifications of the ground plan of the Parasitengona, including reduction of body openings and additional sealing of others with lipids, resulting in reduced water loss. Water vapour uptake is not evident in erythraeoid eggs or protonymphs but has been observed in the Trombidioidea and probably constitutes a characteristic of at least the stem species of the terrestrial Parasitengona. The resulting increase in fresh mass is about 50% before the protonymph 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.
