| Summary: | In this study, I examine the effect of temperature on haemolymph acid-base status in vivo and in vitro in the two-striped grasshopper Melanoplus bivittatus . Melanoplus bivittatus experience wide (up to 40 °C) diurnal body temperature fluctuations in the field, but maintain body temperature relatively constant during sunny days by behavioural thermoregulation. Haemolymph pH was statistically constant (7.12) between 10 and 25°C, but decreased by -0.017 units °C- from 25 to 40°C. Relative alkalinity and fractional protein dissociation were conserved only at body temperatures at which feeding and locomotory activity occur, above 20°C. Haemolymph total CO 2 (C tot ) increased from 10 to 20°C and decreased from 20 to 40°C. Haemolymph P co 2 increased from 10 to 20°C and was statistically constant between 20 and 40°C. Carbonic acid pK app in haemolymph was 6.122 at 35°C, and decreased with temperature by -0.0081 units°C-1. Haemolymph buffer value averaged -35mequivl-1pHunit-1. Haemolymph pH changes with temperature were small (less than -0.004 units°C-1) in vitro at constant P co 2 . Therefore, passive physicochemical effects cannot account for the pattern of acid-base regulation in vivo . The temperature shift from 10 to 20°C was accompanied by a net addition of 4.2-6.2 mmoll-1 of bicarbonate equivalents to the haemolymph. The temperature shift from 20 to 40°C was accompanied by a net removal of 10-14 mmoll-1 of bicarbonate equivalents from the haemolymph. Haemolymph acid-base regulation in vivo during temperature changes is dominated by active variation of bicarbonate equivalents rather than by changes in Pco 2 as observed for most other air-breathers.
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