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Organismal homeostasis requires regulation of extracellular acid-base status, however, the mechanisms by which insects regulate haemolymph pH are poorly known. We evaluated the recovery of desert locusts Schistocerca gregaria Forskal from acute acid loads, initiated by HCI injections into the haemolymph (0.5 pH unit decrease). Haemolymph pH, P(CO2), and [HCO3-] recovered in 8-24h, providing the first unequivocal evidence that insects regulate extracellular pH. There were no changes in the concentrations of the primary haemolymph buffer compounds (protein, inorganic phosphate) during recovery. Within 1 h, the tracheal system effectively eliminated the carbon dioxide derived from bicarbonate buffering. During the remainder of the recovery, haemolymph P(CO2) was similar to control values; there was no respiratory compensation for decreased haemolymph pH. Approximately 75% of the acid equivalents removed from the haemolymph during the recovery process were transferred to the lumens of the crop and midgut. Transfer of acid equivalents to the alimentary lumen provides unfed locusts with a mechanism of haemolymph pH regulation that does not compromise intracellular acid-base status or increase ventilatory water loss.