Water balance

Definition

The physiological equilibrium between water gain and water loss in an organism, maintained through behavioral, morphological, and physiological mechanisms. In arthropods, water balance is governed by intake (drinking, food moisture, metabolic water), storage (hemolymph, fat body), and loss (cuticular transpiration, respiratory evaporation, excretion). The balance shifts with ambient humidity, temperature, and activity level, and its regulation is especially critical for small, terrestrial ectotherms with high surface-area-to-volume ratios.

Etymology

Example

Desert tenebrionid beetles (family Tenebrionidae) achieve positive water balance by extracting metabolic water from dry food, possessing highly impermeable cuticles with wax layers, and producing excreta nearly dry enough to qualify as uric acid paste—allowing survival without drinking in habitats with <10% relative humidity.

Synonyms

• water economy
• hydric balance

Related Terms

• cuticular transpiration
• spiracular control
• Cryptobiosis
• Osmoregulation
• Excretion
• metabolic water
• critical equilibrium humidity

Usage Notes

Distinguished from ecological or hydrological water balance (ecosystem-level water budgets). In arthropod physiology, 'positive water balance' indicates net water gain; 'negative water balance' indicates net loss leading to dehydration. The term is often paired with 'critical equilibrium humidity'—the ambient humidity at which water gain equals loss for a given species. Specialists may specify 'desiccation resistance' or 'dehydration tolerance' when discussing the organism's capacity to withstand negative water balance.