Control of resting ventilation rate in grasshoppers

We examined the effect of extracellular acid-base status and tracheal gas levels on the ventilation rate of resting Romalea guttata and Schistocerca americana grasshoppers. We manipulated haemolymph pH and [HCO₃^-] within normal physiological ranges using injections of HCl, NaOH, NaHCO₃ and NaCl into the haemocoel. In contrast to terrestrial vertebrates, there was no evidence that extracellular acidification increases ventilation rate in grasshoppers. Elevation of haemolymph bicarbonate levels (by NaHCO₃ injection) increased ventilation rate, while depression of haemolymph bicarbonate levels (HCl injection) had no effect. Injection of NaHCO₃ also increased tracheal P_CO2, suggesting that the effect of the NaHCO₃ injection might be mediated by a sensitivity of the ventilatory system to tracheal gases. We tested for effects of tracheal gases on ventilation rate by independently manipulating tracheal P_CO2 and P_O2 using tracheal perfusions. Ventilation rate was positively correlated with tracheal P_CO2 and negatively correlated with tracheal P_O2. Increasing tracheal P_O2 above normal resting levels or decreasing tracheal P_CO2 below normal levels decreased ventilation rate. We conclude that quiescent grasshoppers regulate tracheal P_CO2 and P_O2 by varying ventilation rate and that both P_CO2 and P_O2 in the trachea stimulate ventilation in normal, resting grasshoppers.