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 P_O2 and increases in haemolymph and tracheal P_CO2. 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 P_O2 and P_CO2 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 P_CO2 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 P_O2, with the lowest ventilation rates observed at the lowest tracheal P_O2 values, a result opposite to that expected if negative feedback from internal P_O2 levels were to drive the increase in ventilation rate. Particularly after activity, there was considerable heterogeneity in unperfused animals between tracheal and haemolymph P_CO2, and between tracheal P_CO2 in the thorax and leg, consistent with unidirectional airflow and a considerable role for diffusion gradients in the gas exchange of grasshoppers.