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