1. 1.

    Malpighian tubule fluid from Schistocerca gregaria adults, starved for 1 day, was collected in situ by cannulation of the gut, both before and after injecting 10 μmol of HCl or NaCl into the haemocoel.

  2. 2.

    Haemolymph pH at the neck remained depressed by 0.3 units for at least 6 h in HCl- as compared to NaCl-injected locusts. A lower haemolymph pH persisted near the acid injection site for several hours.

  3. 3.

    The pH of tubule fluid remained about 0.5 units more acid than haemolymph under all conditions. Thus, net tubular acid secretion was proportional to haemolymph acid-base status.

  4. 4.

    The greater acidity of tubular fluid after acid injection was associated with lower estimated bicarbonate concentrations and higher Pcoco2 without any change in total CO2 when compared to controls.

  5. 5.

    The combined contribution of bicarbonate, phosphate and urate to total buffering capacity of tubular fluid was estimated to be 75°, with bicarbonate responsible for 55° of the total.

  6. 6.

    The maximum rate of acid removal by all Malpighian tubules of starved locusts, including H+ trapped in ammonium ions, was calculated to be very small in relation to the acid load injected into the haemocoel

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