Larvae of an African chironomid, Polypedilum vanderplanki, which live in temporal rock pools, are completely dehydrated when the pools dry up and undergo anhydrobiosis until the next rain comes. During the dehydration process, larvae accumulate large amounts of trehalose, which provides effective protection against desiccation because of its high capacity for water replacement and vitrification. As the occurrence of rapid trehalose synthesis coincides with loss of body water, changes of osmolarity in the body are thought to be a cue for trehalose synthesis. Indeed, exposure to high salinity triggers rapid and efficient accumulation of trehalose even without desiccation treatment. As this rapid production occurs mainly in high concentrations of salt solutions, we conclude that an increase in internal ion concentration triggers trehalose synthesis associated with cryptobiosis in this species.