Fig. 2. The pH/[HCO₃^–] diagram for acid–base variables measured in the arterial haemolymph of Discoplax hirtipes sampled in the field during the dry season (open square, free-ranging; open circle, 5 min exercise) and the wet, migration, season (triangles, free-ranging; inverted triangles, 5 min exercise), compared with laboratory values for resting blue crabs and those exercised for 5 min at either 40% or 90% relative humidity (filled squares). The acid–base perturbations after 5 min of exercise are indicated by the broken lines with arrowheads (without implying acid–base state at times within the 5 min exercise period). The non-bicarbonate buffer line is provided (Dela-Cruz and Morris, 1997a). The P_CO2 isopleths were calculated according to the Henderson–Hasselbalch equation using CO₂= 0.3068 µmol l^–1 Pa^–1 and pK_a=6.08.