Acid-base and respiratory responses to hypoxia in the grasshopper Schistocerca americana

How do quiescent insects maintain constant rates of oxygen consumption at ambient P(O₂) values as low as 2-5 kPa? To address this question, we examined the response of the American locust Schistocerca americana to hypoxia by measuring the effect of decreasing ambient P(O₂) on haemolymph acid-base status, tracheal P(CO₂) and CO₂ emission. We also tested the effect of hypoxia on convective ventilation using a new optical technique which measured the changes in abdominal volume during ventilation. Hypoxia caused a progressive increase in haemolymph pH and a decrease in haemolymph P(CO₂). A Davenport analysis suggests that hypoxia is accompanied by a net transfer of base to the haemolymph, perhaps as a result of intracellular pH regulation. Hypoxia caused a progressive increase in convective ventilation which was mostly attributable to a rise in ventilatory frequency. Carbon dioxide conductance (μmol h^-1 kPa^-1) across the spiracles increased more than threefold, while conductance between the haemolymph and primary trachea nearly doubled in 2 kPa O₂ relative to room air. The rise in trans-spiracular conductance is completely attributable to the elevations in convective ventilation. The rise in tracheal conductance in response to hypoxia may reflect the removal of fluid from the tracheoles described by Wigglesworth. The low critical P(O₂) of quiescent insects can be attributed (1) to their relatively low resting metabolic rates, (2) to the possession of tracheal systems adapted for the exchange of gases at much higher rates during activity and (3) to the ability of insects to rapidly modulate tracheal conductance.