Cold or drought – the lesser of two evils for terrestrial arthropods?
Water is an essential part of all living organisms and terrestrial arthropods are no exception. They have to balance water loss and water gain to and from their bodies, often over a wide range of environmental conditions. Water exists in the arthropod body in two main forms: free, bulk or freezable water (usually forming 65-75% of fresh weight), and unfreezable water (often inappropriately termed bound water) associated with membranes and other macromolecular structures. Many factors influence the body water content of arthropods from the physical environment and climatic effects to feeding activity, metabolism and life stage. Some species are adapted to withstand dehydration (loss of 17-89% of their body weight), whilst others enter a state of anhydrobiosis or dormancy induced by partial desiccation. Body water plays an important role in arthropod cold hardiness. Partial dehydration may significantly affect the cold resistance of both freezing tolerant and freezing susceptible forms. An increase in the quantity of unfreezable water in arthropods of both strategies may occur through the production of low molecular weight sugar alcohols (polyols), and desiccation promotes polyol (e.g. glycerol) synthesis in some species.
Hibernating insects and other terrestrial arthropods may be threatened by lethal desiccation in their natural habitats, and in cold environments this may be in addition to the possibility of freezing, which could result in death. Whilst a 50% reduction in body water content may double the haemolymph osmolality in an arthropod, the effect of this on its supercooling ability will depend largely on the cryoprotectant concentration and the initial haemolymph osmolality. On the other hand, partial dehydration in winter may increase the supercooling capacity of micro-arthropods by reducing ice nucleator activity.
Whilst the biochemical and physiological similarities of the effects of cold and drought may be obvious, it should not be overlooked that desiccation tolerant arthropods may be pre-adapted for freezing tolerance, Thus cold and drought should be viewed as not necessarily competing mortality factors in overwintering arthropods, but as producing complementary adaptations for aiding survival in some species.