After the four spiracle pairs of Opistophthalmus pugnax were stepwise occluded, the oxygen consumption rate (MO₂) and the accumulation of d-lactate in the haemolymph were measured. D-lactate and MO₂ was also measured during a 15-minute treadmill exercise at a run speed of 0.025 m/sec. The highest d-lactate concentration of 15 mmol/1 was obtained after a 15-minute exercise followed by 11 mmol/1 d-lactate when all eight spiracles were closed off for one hour. The MO₂ value of 1.93 mmol/1 obtained from four closed spiracles does not differ significantly (P < 0.05, t-test) compared with the MO₂ value of eight open spiracles. Possible mechanisms of metabolic rate depression and metabolic down-regulation are discussed. Low haemolymph pH (7.38 ± 0.08) in O. latimanus and other scorpions compared with other terrestrial arthropods are a hallmark inherent to scorpions living in dry xeric conditions.