Haemolymph acid-base status, tracheal gas levels and the control of post-exercise ventilation rate in grasshoppers

In grasshoppers, ventilation rate increases after jumping, in association with decreases in haemolymph pH and tracheal PO2 and increases in haemolymph and tracheal PCO2. Are these changes in haemolymph acid-base status or tracheal gas composition causally responsible for the increases in post-locomotion ventilation rate? To answer this question, we manipulated haemolymph acid-base status with injections into the haemocoel and independently manipulated tracheal PO2 and PCO2 with tracheal perfusions. Using a new technique, we continuously monitored ventilation rate and ventilatory pressures on virtually unrestrained insects. Changes in haemolymph acid-base status or tracheal PCO2 did not affect post-exercise ventilation rate, clearly demonstrating that the ventilatory stimulus associated with locomotion is not dependent on negative feedback from these variables. Post-exercise ventilation rate varied with tracheal PO2, with the lowest ventilation rates observed at the lowest tracheal PO2 values, a result opposite to that expected if negative feedback from internal PO2 levels were to drive the increase in ventilation rate. Particularly after activity, there was considerable heterogeneity in unperfused animals between tracheal and haemolymph PCO2, and between tracheal PCO2 in the thorax and leg, consistent with unidirectional airflow and a considerable role for diffusion gradients in the gas exchange of grasshoppers.