How do grasshoppers maintain metabolic rates with reduced oxygen availability?

When grasshoppers are exposed to atmospheric oxygen levels as low as 5%, ventilatory air flow and metabolic rate do not change. What is the mechanism for constant oxygen consumption despite reduced oxygen availability? Two possible explanations are: 1) in normal air the cellular P_O2 is at least four times the critical cellular P_O2, or 2) during hypoxia grasshoppers are increasing respiratory conductance in some way other than increasing convective air flow. The insect respiratory system can be looked at as two steps in series: the spiracular step characterized by the exchange of oxygen and carbon dioxide between the large trachea and ambient air via the spiracles and the tracheolar step in which gas exchange occurs between the cells and the large trachea via the tracheoles. We conducted an experiment to test the possibility that grasshoppers vary the conductances of the spiracular and tracheolar steps in response to hypoxia. We measured the effect of decreasing atmospheric oxygen (21, 10, 5 and 2%) on the CO₂ gradients between the hemolymph and the large trachea and across the spiracle. As atmospheric P_O2 drops from 21% to 2% both tracheal and hemolymph PCO₂ decrease by roughly half, and hemolymph pH becomes increasingly more alkaline. These data suggest that grasshoppers compensate for decreasing levels of oxygen in atmospheric air by doubling the conductance of both the spiracular and tracheolar steps to gas exchange. Spiracular conductance may be increased during hypoxia simply by the changing the degree or duration of spiracular opening. Tracheolar conductance may be increased during hypoxia by removal of fluids from the tracheoles as suggested by Wigglesworth.