GIBBS, Allen G.; JOHNSON, Robert A.; Univ. of Arizona; Arizona State Univ.: Water balance in seed-harvester ants: effects of mating and a test of the chthonic hypothesis for discontinuous gas exchange

The two major routes for water loss from most insects are transpiration through the cuticle and respiratory water loss when the spiracles are open. A major technical challenge is discriminating between these two routes, particularly in insects that do not perform discontinuous gas-exchange cycles (DGC). The ability to partition cuticular and respiratory water loss is also crucial for testing hypotheses regarding the function of different ventilatory patterns, and for understanding the physiological consequences of variation in cuticular hydrocarbons. We developed a new analytical approach that allowed us to quantify respiratory and cuticular water loss in seed-harvester ants, Pogonomyrmex barbatus, using different ventilatory modes, including continuous breathing. Water-loss rates of alate females increased 68% after mating, and another 40% after these queens had excavated their new colonies. In contrast, metabolic rates were unaffected by mating status. Using our analytical technique, we found that increased cuticular permeability accounted for 97% of the increased water loss. Higher transpiration was associated with increased methyl-branching of cuticular hydrocarbons, which tends to reduce the melting point of surface lipids. Our data also allowed us to test the chthonic hypothesis, which suggests that DGC originated as a mechanism to minimize the ratio of respiratory water loss (RWL) to CO₂ release, relative to other ventilatory patterns. We found that the RWL:CO₂ release ratio was similar for ants using cyclic and continuous ventilation, suggesting that the chthonic hypothesis is incorrect.