It is well known that quiescent insects can maintain constant rates of oxygen consumption in extremely low oxygen concentrations. For example, the critical PO∑ values (the PO∑ below which oxygen consumption begins to fall) for some resting insects are as follows: Tenebrio molitor pupae, <5 kPa (Gaarder, 1918); adult Aedes aegypti mosquitoes, 3–4 kPa (Galun, 1960); adult Phormia regina flies, 2–5 kPa (Keister and Buck, 1961); adult Termopsis navidensis termites, 2–5 kPa (Cook, 1932) and adult Locusta migratoria, 3–4 kPa (Arieli and Lehrer, 1988). While these data demonstrate that the safety margin for oxygen delivery in resting insects is large, the physiological mechanisms responsible for this remain unclear. One hypothesis is that the conductance (the quantity of gas transferred divided by the partial pressure gradient) of the tracheal system is so high at rest that no response is needed to hypoxia. Alternatively, insects may need to increase the conductance of the tracheal system in proportion with the fall in atmospheric oxygen. If tracheal conductance does increase, by what mechanisms is this accomplished? We investigated these questions by testing the effects of hypoxia on the tracheal physiology of the grasshopper Schistocerca americana. Together, the tracheal morphology, mechanisms of gas exchange and the neural control of the ventilatory system have been better studied in grasshoppers than in any other insect. Large longitudinal trunks run along each side of the animal connecting all ipsilateral spiracles and branching into a system of air sacs and secondary and tertiary tracheae, further branching into tracheoles which are the sites of gas exchange in the tissues (Weis-Fogh, 1964, 1967). Convective gas exchange in non- flying grasshoppers is accomplished mostly by abdominal pumping, which includes both dorso-ventral contractions and longitudinal telescoping movements (Miller, 1960a; Weis-Fogh, 1967). Abdominal pumping is initiated by a pacemaker in the metathoracic ganglion (Miller, 1960a; Hoyle, 1959) and is synchronized with spiracular opening so that inspiration occurs through the first four pairs of spiracles and expiration through the last six pairs of abdominal spiracles, producing a largely unidirectional flow of air through the grasshopper (McCutcheon, 1940; Weis-Fogh, 1967). Abdominal pumping is stimulated by hypoxia (Miller, 1960a), but ventilatory frequency is reported not to be stimulated by hypoxia until the PO∑ falls below the point at which the rate of oxygen consumption drops (Arieli and 2843 The Journal of Experimental Biology 201, 2843–2855 (1998) Printed in Great Britain © The Company of Biologists Limited 1998 JEB1469 How do quiescent insects maintain constant rates of oxygen consumption at ambient PO∑ values as low as 2–5 kPa? To address this question, we examined the response of the American locust Schistocerca americana to hypoxia by measuring the effect of decreasing ambient PO∑ on haemolymph acid–base status, tracheal PCO∑ and CO 2 emission. We also tested the effect of hypoxia on convective ventilation using a new optical technique which measured the changes in abdominal volume during ventilation. Hypoxia caused a progressive increase in haemolymph pH and a decrease in haemolymph PCO∑. A Davenport analysis suggests that hypoxia is accompanied by a net transfer of base to the haemolymph, perhaps as a result of intracellular pH regulation. Hypoxia caused a progressive increase in convective ventilation which was mostly attributable to a rise in ventilatory frequency. Carbon dioxide conductance (μmol h -1 kPa -1 ) across the spiracles increased more than threefold, while conductance between the haemolymph and primary trachea nearly doubled in 2 kPa O 2 relative to room air. The rise in trans-spiracular conductance is completely attributable to the elevations in convective ventilation. The rise in tracheal conductance in response to hypoxia may reflect the removal of fluid from the tracheoles described by Wigglesworth. The low critical PO∑ of quiescent insects can be attributed (1) to their relatively low resting metabolic rates, (2) to the possession of tracheal systems adapted for the exchange of gases at much higher rates during activity and (3) to the ability of insects to rapidly modulate tracheal conductance. Key words: acid–base, ventilation, tracheal system, hypoxia, gas exchange, grasshopper, Schistocerca americana. Summary Introduction ACID–BASE AND RESPIRATORY RESPONSES TO HYPOXIA IN THE GRASSHOPPER SCHISTOCERCA AMERICANA KENDRA J. GREENLEE AND JON F. HARRISON* Department of Biology, Arizona State University, Tempe, AZ 85287-1501, USA *Author for correspondence (e-mail: j.harrison@asu.edu) Accepted 16 July; published on WWW 22 September 1998